Thread: Running Postgres Daemons with same data files

Dear All,

I am working in Linux 8.0 and running postgresql 7.2. I am trying to
access same data files from the two postgres daemons. I mean there are
two PC's running with postgres and one NAS server where data files are
kept. To run postgres on the machines I first mount the NAS file system
and then run it like

shell>postmaster -D $(PATH_TO_DATA_FILE)

Daemons are running well but there is problems with synchronization of
the data files i.e when i insert some tuples in the tables its not
immediately writing then to file its kept in the cache only and when I
try to see from the other machine its displaying the old tuples.

I want after every transaction or query the database update the data
files, how can I do this? And always read from the data files for select
operations. My main aim is no cache operation to use the postgres as
file system(for every operation file operations only), I know it will
degrade the database performance but its the requirement OR if there is
any other way by which I can achieve this, please tell me.

regards
bhartendu

Don't do that!!!! This absolutely cannot be done in any sane way.

Instead, consider locating the database itself on one server, and then
having multiple other servers running the client application.

On Tue, 9 Dec 2003, Bhartendu Maheshwari wrote:

> Dear All,
>
> I am working in Linux 8.0 and running postgresql 7.2. I am trying to
> access same data files from the two postgres daemons. I mean there are
> two PC's running with postgres and one NAS server where data files are
> kept. To run postgres on the machines I first mount the NAS file system
> and then run it like
>
> shell>postmaster -D $(PATH_TO_DATA_FILE)
>
> Daemons are running well but there is problems with synchronization of
> the data files i.e when i insert some tuples in the tables its not
> immediately writing then to file its kept in the cache only and when I
> try to see from the other machine its displaying the old tuples.
>
> I want after every transaction or query the database update the data
> files, how can I do this? And always read from the data files for select
> operations. My main aim is no cache operation to use the postgres as
> file system(for every operation file operations only), I know it will
> degrade the database performance but its the requirement OR if there is
> any other way by which I can achieve this, please tell me.
>
> regards
> bhartendu
>
>
>
>
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: the planner will ignore your desire to choose an index scan if your
>       joining column's datatypes do not match
>

--

Sam Barnett-Cormack
Software Developer                           |  Student of Physics & Maths
UK Mirror Service (http://www.mirror.ac.uk)  |  Lancaster University

Dear Sam,

Thank you for the quick response.

Can I you tell me why its not possible, it is possible with mysql then
why not with postgres. Actually I am working on High Avaibility
framework, and its our need, we can't make a separate database server. I
want to read/write and then close the file, very simple isn't? So how
can I achieve in postgres? Please help me.

regards
bhartendu

On Tue, 2003-12-09 at 17:13, Sam Barnett-Cormack wrote:
> Don't do that!!!! This absolutely cannot be done in any sane way.
>
> Instead, consider locating the database itself on one server, and then
> having multiple other servers running the client application.
>
> On Tue, 9 Dec 2003, Bhartendu Maheshwari wrote:
>
> > Dear All,
> >
> > I am working in Linux 8.0 and running postgresql 7.2. I am trying to
> > access same data files from the two postgres daemons. I mean there are
> > two PC's running with postgres and one NAS server where data files are
> > kept. To run postgres on the machines I first mount the NAS file system
> > and then run it like
> >
> > shell>postmaster -D $(PATH_TO_DATA_FILE)
> >
> > Daemons are running well but there is problems with synchronization of
> > the data files i.e when i insert some tuples in the tables its not
> > immediately writing then to file its kept in the cache only and when I
> > try to see from the other machine its displaying the old tuples.
> >
> > I want after every transaction or query the database update the data
> > files, how can I do this? And always read from the data files for select
> > operations. My main aim is no cache operation to use the postgres as
> > file system(for every operation file operations only), I know it will
> > degrade the database performance but its the requirement OR if there is
> > any other way by which I can achieve this, please tell me.
> >
> > regards
> > bhartendu
> >
> >
> >
> >
> >
> >
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 9: the planner will ignore your desire to choose an index scan if your
> >       joining column's datatypes do not match
> >
>
> --
>
> Sam Barnett-Cormack
> Software Developer                           |  Student of Physics & Maths
> UK Mirror Service (http://www.mirror.ac.uk)  |  Lancaster University
>
> ---------------------------(end of broadcast)---------------------------
> TIP 8: explain analyze is your friend

On Tue, 9 Dec 2003 18:03:31 +0530
pgsql-admin-owner@postgresql.org wrote:

> Dear Sam,
>
> Thank you for the quick response.
>
> Can I you tell me why its not possible, it is possible with mysql then
> why not with postgres. Actually I am working on High Avaibility
> framework, and its our need, we can't make a separate database server.
> I want to read/write and then close the file, very simple isn't? So
> how can I achieve in postgres? Please help me.
>
> regards
> bhartendu
>

It seems MySQL tries every attempt to crash a database.

Seriously: What do you think will happen to the database files, if you
try to insert/update on the same files at the same time from two
different engines? Exactly: This would not only crash a database, this
would shatter it to thousands of pieces, not recoverable.

Even if you do only SELECTS from one engine, this cannot be synchronized
in any way. And by the way: What reason do you have for another engine
just doing selects? Why not use a client connecting to the one and only
engine, which can do everything (INSERTs, UPDATEs, SELECTs) at (nearly)
the same time? This is not XBase, where you have to look after locking
yourself - let the engine do the dirty work like file locking and
determine, who´s next to do an operation. Just connect to it with
several clients and work with these.

Regards, Frank.

Hello Bhartendu,

It happens that I was just talking to Sam on irc, and he's gone to lunch,
so I'll have a shot at this.

This should never work for any respectable DBMS.  The DBMS is what
manages access to the data files.  The DBMS does the locking and
concurrency control, and state information about transactions in progress
is held within the DBMS.  Since PostgreSQL uses far more sophisticated
transaction mechanisms than table level locking, it's not as simple as
locking files.  You're pretty much guaranteeding yourself serious data
corruption problems if you try this, since two DBMS instances will try to
maintain independent transaction state information, and end up mangling
each other's data.  Seriously.

Further, since you're relying on a single storage point, you're not
actually implementing HA at all.  You're also going to have nasty issues
with write synchronisation with NAS.  It's strongly recommended that DBMS
servers run databases only on physically local storage, otherwise there
are too many layers of data shuffling between the DBMS server and the
physical disk.  Data will get lost and corrupted sooner or later.

I'd suggest that you take a serious look at what your actual availability
requirements are.  What are the potential costs of downtime?  What will
you do if the NAS switch fails for instance, in the case you're trying to
construct?  It happens.  And most organisations don't carry spare ones
lying around, because they're expensive things to have sitting idle.

General rules with almost any proper RDBMS you care to name:  Use local
storage, not NAS.  You get a lot more bang for the buck in the
availability stakes by using good-quality, well maintained hardware and
software than by trying to do exotic things with replication (more about
this below).  You can consider using disk mirroring (RAID 1) or RAID 5 in
order to reduce the probability of having to do time-consuming restores.

Why do you need sophisticated HA?  IMVHO the only people who _really_ need
it are people like nuclear power stations, air traffic control (if only!),
hospitals and the like.  It's nice for global businesses too, which have
to provide global business services 24/7.  How were you planning to do the
failover switching?

In terms of replication, this can be done (with difficulty still) but
always (always!) between two database servers each of which keeps a local
copy of the data, with something erserv sitting between them synchronising
transactions.  You might want to look at that.

But seriously -- most applications don't need HA solutions.  PostgreSQL
running on decent, well-maintained hardware and software is perfectly
capable of achieving 99%+ uptime, which is more than most applicaitons
need.  (And I don't say that idly, we're running it on antique, creaky SGI
Challenges and achieving that kind of uptime.  If we were to put it on
really good new boxes we'd exceed that easily).

If you really, really do need an HA solution, then I'd hunt around for
someone to add to your team who has extensive experience in this kind of
thing, since it's all too easy otherwise to unwittingly leave in lots of
single points of failure.  (Have you considered multiple independent
UPSes?  Communications lines? NAS switches like I said (and you shouldn't
be using NAS for PG data!), application servers (whatever your application
may be) etc.?)

Good luck!

Hal


On Tue, 9 Dec 2003, Bhartendu Maheshwari wrote:

> Dear Sam,
>
> Thank you for the quick response.
>
> Can I you tell me why its not possible, it is possible with mysql then
> why not with postgres. Actually I am working on High Avaibility
> framework, and its our need, we can't make a separate database server. I
> want to read/write and then close the file, very simple isn't? So how
> can I achieve in postgres? Please help me.
>
> regards
> bhartendu
>

Case in point:

MS Access was designed to have multiple database managers manipulating
the files themselves directly and uses another file for locking
information.  However-- as anyone who has ever worked with the process
will tell you: Don't do it.  Data corruption (often unrecoverable) will
result.

The lessons we have learned from MS Access are:
1) Don't have 2 unrelated backends trying to access the same data and
2) Don't do it across a network.

IMO, this shows a fundamental design flaw in MS Access at least given
how it is marketed.

Don't try to do the same with PostgreSQL.

Best Wishes,
Chris Travers

On Tue, 9 Dec 2003, Chris Travers wrote:

> Case in point:
>
> MS Access was designed to have multiple database managers manipulating
> the files themselves directly and uses another file for locking
> information.  However-- as anyone who has ever worked with the process
> will tell you: Don't do it.  Data corruption (often unrecoverable) will
> result.
>
> The lessons we have learned from MS Access are:
> 1) Don't have 2 unrelated backends trying to access the same data and
> 2) Don't do it across a network.
>
> IMO, this shows a fundamental design flaw in MS Access at least given
> how it is marketed.

That all said, anyone who tells you MS Access is really an RDBMS should
lose their job...

--

Sam Barnett-Cormack
Software Developer                           |  Student of Physics & Maths
UK Mirror Service (http://www.mirror.ac.uk)  |  Lancaster University

Dear Hal, Frank, Oli and all,

I understand what you all trying to say, I know this is not good way of
designing, but we are planning for using the database for the keeping
mobile transactions and at the same time we need to provided the HA
solutions. The one solution i derive from the discussion that uses one
server and multiple clients but the issue in this if the system in which
database server was running get down the its all the way no use of HA
and load balancing, since without data the other one can't do anything.

What I have in mind is the following implementation:-

Step 1 :- Mount the data files from NAS server.
Step 2 :- start the postgres with the mounted data.
Step 3 :- Lock the data files by one server.
Step 4 :- do the database operation.
Step 5 :- Commit in the database files.
Step 6 :- Unlock the database files
Step 7 :- Now the other one can do the same way.

Or if anybody have other solution for this, please suggest. How can I
commit the data into database files and flushing cache with the latest
data files contents? Is there any command to refresh both.

Thank you Hal for
*************************************************************************
If you really, really do need an HA solution, then I'd hunt around for
someone to add to your team who has extensive experience in this kind of
thing, since it's all too easy otherwise to unwittingly leave in lots of
single points of failure.
*************************************************************************

If you really have someone to help me in this regards, please I need his
help in this issue and want to derive a common techniques so that
everyone can use this.

regards
bhartendu

On Tue, 2003-12-09 at 18:20, Halford Dace wrote:
> Hello Bhartendu,
>
> It happens that I was just talking to Sam on irc, and he's gone to
lunch,
> so I'll have a shot at this.
>
> This should never work for any respectable DBMS.  The DBMS is what
> manages access to the data files.  The DBMS does the locking and
> concurrency control, and state information about transactions in
progress
> is held within the DBMS.  Since PostgreSQL uses far more sophisticated
> transaction mechanisms than table level locking, it's not as simple as
> locking files.  You're pretty much guaranteeding yourself serious data
> corruption problems if you try this, since two DBMS instances will try
to
> maintain independent transaction state information, and end up
mangling
> each other's data.  Seriously.
>
> Further, since you're relying on a single storage point, you're not
> actually implementing HA at all.  You're also going to have nasty
issues
> with write synchronisation with NAS.  It's strongly recommended that
DBMS
> servers run databases only on physically local storage, otherwise
there
> are too many layers of data shuffling between the DBMS server and the
> physical disk.  Data will get lost and corrupted sooner or later.
>
> I'd suggest that you take a serious look at what your actual
availability
> requirements are.  What are the potential costs of downtime?  What
will
> you do if the NAS switch fails for instance, in the case you're trying
to
> construct?  It happens.  And most organisations don't carry spare ones
> lying around, because they're expensive things to have sitting idle.
>
> General rules with almost any proper RDBMS you care to name:  Use
local
> storage, not NAS.  You get a lot more bang for the buck in the
> availability stakes by using good-quality, well maintained hardware
and
> software than by trying to do exotic things with replication (more
about
> this below).  You can consider using disk mirroring (RAID 1) or RAID 5
in
> order to reduce the probability of having to do time-consuming
restores.
>
> Why do you need sophisticated HA?  IMVHO the only people who _really_
need
> it are people like nuclear power stations, air traffic control (if
only!),
> hospitals and the like.  It's nice for global businesses too, which
have
> to provide global business services 24/7.  How were you planning to do
the
> failover switching?
>
> In terms of replication, this can be done (with difficulty still) but
> always (always!) between two database servers each of which keeps a
local
> copy of the data, with something erserv sitting between them
synchronising
> transactions.  You might want to look at that.
>
> But seriously -- most applications don't need HA solutions.
PostgreSQL
> running on decent, well-maintained hardware and software is perfectly
> capable of achieving 99%+ uptime, which is more than most applicaitons
> need.  (And I don't say that idly, we're running it on antique, creaky
SGI
> Challenges and achieving that kind of uptime.  If we were to put it on
> really good new boxes we'd exceed that easily).
>
> If you really, really do need an HA solution, then I'd hunt around for
> someone to add to your team who has extensive experience in this kind
of
> thing, since it's all too easy otherwise to unwittingly leave in lots
of
> single points of failure.  (Have you considered multiple independent
> UPSes?  Communications lines? NAS switches like I said (and you
shouldn't
> be using NAS for PG data!), application servers (whatever your
application
> may be) etc.?)
>
> Good luck!
>
> Hal
>
>
> On Tue, 9 Dec 2003, Bhartendu Maheshwari wrote:
>
> > Dear Sam,
> >
> > Thank you for the quick response.
> >
> > Can I you tell me why its not possible, it is possible with mysql
then
> > why not with postgres. Actually I am working on High Avaibility
> > framework, and its our need, we can't make a separate database
server. I
> > want to read/write and then close the file, very simple isn't? So
how
> > can I achieve in postgres? Please help me.
> >
> > regards
> > bhartendu
> >
>
> ---------------------------(end of
broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On Tuesday 09 December 2003 08:21 pm, Bhartendu Maheshwari wrote:
> Dear Hal, Frank, Oli and all,
>
> I understand what you all trying to say, I know this is not good way of
> designing, but we are planning for using the database for the keeping
> mobile transactions and at the same time we need to provided the HA
> solutions. The one solution i derive from the discussion that uses one
> server and multiple clients but the issue in this if the system in which
> database server was running get down the its all the way no use of HA
> and load balancing, since without data the other one can't do anything.

Is the NAS server redundant ? If not it's not HA anyways.
If there is a problem with the NAS or the network itself (say someone
accidentially cuts a bunch of network wires) - what do you do ?
I don't see a big difference between the one server or the other server or the
network going down. Unless ALL components in your network are redundant and
have failover capabilities (for example one NAS automatically replacing the
other one if it fails) you don't have high availibility.

What exactly do you mean by "mobile transactions" ?
The easiest way that probably comes close to what you intend to do is to have
a second server stand by and take over (i.e. mount the NAS storage and start
a postmaster as well as taking over the IP address of the original machine)
the moment the primary server fails. That will still disrupt all queries
currently in progress, but at least things can be used immediately after the
failure.
Still, the NAS storage is a huge point of failure. What you failed to realize
in the list below is that via the network to a remote storage a lot of caches
and buffers are involved. I bet you won't be able to tell exactly when what
piece of data has been physically written to the disk. Even if you close the
files some information could still hang around in some buffer until the
storage array feels it's time to actually write that stuff.

What you are trying to achive is the classic "replication" approach. Replicate
the database to a second server and have that one take over if the first one
fails. Look into the replication projects on gborg - that's more likely to
give you a workable solution.



>
> What I have in mind is the following implementation:-
>
> Step 1 :- Mount the data files from NAS server.
> Step 2 :- start the postgres with the mounted data.
> Step 3 :- Lock the data files by one server.
> Step 4 :- do the database operation.
> Step 5 :- Commit in the database files.
> Step 6 :- Unlock the database files
> Step 7 :- Now the other one can do the same way.
>
> Or if anybody have other solution for this, please suggest. How can I
> commit the data into database files and flushing cache with the latest
> data files contents? Is there any command to refresh both.
>
> Thank you Hal for
> *************************************************************************
> If you really, really do need an HA solution, then I'd hunt around for
> someone to add to your team who has extensive experience in this kind of
> thing, since it's all too easy otherwise to unwittingly leave in lots of
> single points of failure.
> *************************************************************************
>
> If you really have someone to help me in this regards, please I need his
> help in this issue and want to derive a common techniques so that
> everyone can use this.
>
> regards
> bhartendu
>
> On Tue, 2003-12-09 at 18:20, Halford Dace wrote:
> > Hello Bhartendu,
> >
> > It happens that I was just talking to Sam on irc, and he's gone to
>
> lunch,
>
> > so I'll have a shot at this.
> >
> > This should never work for any respectable DBMS.  The DBMS is what
> > manages access to the data files.  The DBMS does the locking and
> > concurrency control, and state information about transactions in
>
> progress
>
> > is held within the DBMS.  Since PostgreSQL uses far more sophisticated
> > transaction mechanisms than table level locking, it's not as simple as
> > locking files.  You're pretty much guaranteeding yourself serious data
> > corruption problems if you try this, since two DBMS instances will try
>
> to
>
> > maintain independent transaction state information, and end up
>
> mangling
>
> > each other's data.  Seriously.
> >
> > Further, since you're relying on a single storage point, you're not
> > actually implementing HA at all.  You're also going to have nasty
>
> issues
>
> > with write synchronisation with NAS.  It's strongly recommended that
>
> DBMS
>
> > servers run databases only on physically local storage, otherwise
>
> there
>
> > are too many layers of data shuffling between the DBMS server and the
> > physical disk.  Data will get lost and corrupted sooner or later.
> >
> > I'd suggest that you take a serious look at what your actual
>
> availability
>
> > requirements are.  What are the potential costs of downtime?  What
>
> will
>
> > you do if the NAS switch fails for instance, in the case you're trying
>
> to
>
> > construct?  It happens.  And most organisations don't carry spare ones
> > lying around, because they're expensive things to have sitting idle.
> >
> > General rules with almost any proper RDBMS you care to name:  Use
>
> local
>
> > storage, not NAS.  You get a lot more bang for the buck in the
> > availability stakes by using good-quality, well maintained hardware
>
> and
>
> > software than by trying to do exotic things with replication (more
>
> about
>
> > this below).  You can consider using disk mirroring (RAID 1) or RAID 5
>
> in
>
> > order to reduce the probability of having to do time-consuming
>
> restores.
>
> > Why do you need sophisticated HA?  IMVHO the only people who _really_
>
> need
>
> > it are people like nuclear power stations, air traffic control (if
>
> only!),
>
> > hospitals and the like.  It's nice for global businesses too, which
>
> have
>
> > to provide global business services 24/7.  How were you planning to do
>
> the
>
> > failover switching?
> >
> > In terms of replication, this can be done (with difficulty still) but
> > always (always!) between two database servers each of which keeps a
>
> local
>
> > copy of the data, with something erserv sitting between them
>
> synchronising
>
> > transactions.  You might want to look at that.
> >
> > But seriously -- most applications don't need HA solutions.
>
> PostgreSQL
>
> > running on decent, well-maintained hardware and software is perfectly
> > capable of achieving 99%+ uptime, which is more than most applicaitons
> > need.  (And I don't say that idly, we're running it on antique, creaky
>
> SGI
>
> > Challenges and achieving that kind of uptime.  If we were to put it on
> > really good new boxes we'd exceed that easily).
> >
> > If you really, really do need an HA solution, then I'd hunt around for
> > someone to add to your team who has extensive experience in this kind
>
> of
>
> > thing, since it's all too easy otherwise to unwittingly leave in lots
>
> of
>
> > single points of failure.  (Have you considered multiple independent
> > UPSes?  Communications lines? NAS switches like I said (and you
>
> shouldn't
>
> > be using NAS for PG data!), application servers (whatever your
>
> application
>
> > may be) etc.?)
> >
> > Good luck!
> >
> > Hal
> >
> > On Tue, 9 Dec 2003, Bhartendu Maheshwari wrote:
> > > Dear Sam,
> > >
> > > Thank you for the quick response.
> > >
> > > Can I you tell me why its not possible, it is possible with mysql
>
> then
>
> > > why not with postgres. Actually I am working on High Avaibility
> > > framework, and its our need, we can't make a separate database
>
> server. I
>
> > > want to read/write and then close the file, very simple isn't? So
>
> how
>
> > > can I achieve in postgres? Please help me.
> > >
> > > regards
> > > bhartendu
> >
> > ---------------------------(end of
>
> broadcast)---------------------------
>
> > TIP 7: don't forget to increase your free space map settings
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster

- --
    UC

- --
Open Source Solutions 4U, LLC    2570 Fleetwood Drive
Phone:  +1 650 872 2425        San Bruno, CA 94066
Cell:   +1 650 302 2405        United States
Fax:    +1 650 872 2417
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Dear UC,

You are right for the HA solution but at the same time we are also
implementing the load balancing solution so we can't have for one Node 2
different processing entity and database as well. We try to provide
solution for HA, load balancing both and in that there are 2 different
processing machine but sharing the common database so that both get the
latest and synchronized data files.

You are right if the NAS is down then everything get down but the
probability for the NAS is down is very less and by this we are able to
provide service for 99% cases and if you are 99% handle cases then you
are providing good service, isn't?

About the cache to file write :- If the database is writting all the
stuff to the files after each transaction then both have one
synchronized set of data file whoever want can acquire the lock and use
and then unlock it. The MySQL have command "flush tables" to enforce the
database to write all the cache contents to the files, Is there anything
similar in postgres? This will definitely degrade the performance of my
system but its much more fast since I have 2 processing unit.

Anyway if somebody have some other solution for the same please help me.
One I got have one common postmaster running on one PC and the two nodes
connect to that server to get the data. Any other please let me know.

regards
bhartendu

On Wed, 2003-12-10 at 10:38, Uwe C. Schroeder wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On Tuesday 09 December 2003 08:21 pm, Bhartendu Maheshwari wrote:
> > Dear Hal, Frank, Oli and all,
> >
> > I understand what you all trying to say, I know this is not good way of
> > designing, but we are planning for using the database for the keeping
> > mobile transactions and at the same time we need to provided the HA
> > solutions. The one solution i derive from the discussion that uses one
> > server and multiple clients but the issue in this if the system in which
> > database server was running get down the its all the way no use of HA
> > and load balancing, since without data the other one can't do anything.
>
> Is the NAS server redundant ? If not it's not HA anyways.
> If there is a problem with the NAS or the network itself (say someone
> accidentially cuts a bunch of network wires) - what do you do ?
> I don't see a big difference between the one server or the other server or the
> network going down. Unless ALL components in your network are redundant and
> have failover capabilities (for example one NAS automatically replacing the
> other one if it fails) you don't have high availibility.
>
> What exactly do you mean by "mobile transactions" ?
> The easiest way that probably comes close to what you intend to do is to have
> a second server stand by and take over (i.e. mount the NAS storage and start
> a postmaster as well as taking over the IP address of the original machine)
> the moment the primary server fails. That will still disrupt all queries
> currently in progress, but at least things can be used immediately after the
> failure.
> Still, the NAS storage is a huge point of failure. What you failed to realize
> in the list below is that via the network to a remote storage a lot of caches
> and buffers are involved. I bet you won't be able to tell exactly when what
> piece of data has been physically written to the disk. Even if you close the
> files some information could still hang around in some buffer until the
> storage array feels it's time to actually write that stuff.
>
> What you are trying to achive is the classic "replication" approach. Replicate
> the database to a second server and have that one take over if the first one
> fails. Look into the replication projects on gborg - that's more likely to
> give you a workable solution.
>
>
>
> >
> > What I have in mind is the following implementation:-
> >
> > Step 1 :- Mount the data files from NAS server.
> > Step 2 :- start the postgres with the mounted data.
> > Step 3 :- Lock the data files by one server.
> > Step 4 :- do the database operation.
> > Step 5 :- Commit in the database files.
> > Step 6 :- Unlock the database files
> > Step 7 :- Now the other one can do the same way.
> >
> > Or if anybody have other solution for this, please suggest. How can I
> > commit the data into database files and flushing cache with the latest
> > data files contents? Is there any command to refresh both.
> >
> > Thank you Hal for
> > *************************************************************************
> > If you really, really do need an HA solution, then I'd hunt around for
> > someone to add to your team who has extensive experience in this kind of
> > thing, since it's all too easy otherwise to unwittingly leave in lots of
> > single points of failure.
> > *************************************************************************
> >
> > If you really have someone to help me in this regards, please I need his
> > help in this issue and want to derive a common techniques so that
> > everyone can use this.
> >
> > regards
> > bhartendu
> >
> > On Tue, 2003-12-09 at 18:20, Halford Dace wrote:
> > > Hello Bhartendu,
> > >
> > > It happens that I was just talking to Sam on irc, and he's gone to
> >
> > lunch,
> >
> > > so I'll have a shot at this.
> > >
> > > This should never work for any respectable DBMS.  The DBMS is what
> > > manages access to the data files.  The DBMS does the locking and
> > > concurrency control, and state information about transactions in
> >
> > progress
> >
> > > is held within the DBMS.  Since PostgreSQL uses far more sophisticated
> > > transaction mechanisms than table level locking, it's not as simple as
> > > locking files.  You're pretty much guaranteeding yourself serious data
> > > corruption problems if you try this, since two DBMS instances will try
> >
> > to
> >
> > > maintain independent transaction state information, and end up
> >
> > mangling
> >
> > > each other's data.  Seriously.
> > >
> > > Further, since you're relying on a single storage point, you're not
> > > actually implementing HA at all.  You're also going to have nasty
> >
> > issues
> >
> > > with write synchronisation with NAS.  It's strongly recommended that
> >
> > DBMS
> >
> > > servers run databases only on physically local storage, otherwise
> >
> > there
> >
> > > are too many layers of data shuffling between the DBMS server and the
> > > physical disk.  Data will get lost and corrupted sooner or later.
> > >
> > > I'd suggest that you take a serious look at what your actual
> >
> > availability
> >
> > > requirements are.  What are the potential costs of downtime?  What
> >
> > will
> >
> > > you do if the NAS switch fails for instance, in the case you're trying
> >
> > to
> >
> > > construct?  It happens.  And most organisations don't carry spare ones
> > > lying around, because they're expensive things to have sitting idle.
> > >
> > > General rules with almost any proper RDBMS you care to name:  Use
> >
> > local
> >
> > > storage, not NAS.  You get a lot more bang for the buck in the
> > > availability stakes by using good-quality, well maintained hardware
> >
> > and
> >
> > > software than by trying to do exotic things with replication (more
> >
> > about
> >
> > > this below).  You can consider using disk mirroring (RAID 1) or RAID 5
> >
> > in
> >
> > > order to reduce the probability of having to do time-consuming
> >
> > restores.
> >
> > > Why do you need sophisticated HA?  IMVHO the only people who _really_
> >
> > need
> >
> > > it are people like nuclear power stations, air traffic control (if
> >
> > only!),
> >
> > > hospitals and the like.  It's nice for global businesses too, which
> >
> > have
> >
> > > to provide global business services 24/7.  How were you planning to do
> >
> > the
> >
> > > failover switching?
> > >
> > > In terms of replication, this can be done (with difficulty still) but
> > > always (always!) between two database servers each of which keeps a
> >
> > local
> >
> > > copy of the data, with something erserv sitting between them
> >
> > synchronising
> >
> > > transactions.  You might want to look at that.
> > >
> > > But seriously -- most applications don't need HA solutions.
> >
> > PostgreSQL
> >
> > > running on decent, well-maintained hardware and software is perfectly
> > > capable of achieving 99%+ uptime, which is more than most applicaitons
> > > need.  (And I don't say that idly, we're running it on antique, creaky
> >
> > SGI
> >
> > > Challenges and achieving that kind of uptime.  If we were to put it on
> > > really good new boxes we'd exceed that easily).
> > >
> > > If you really, really do need an HA solution, then I'd hunt around for
> > > someone to add to your team who has extensive experience in this kind
> >
> > of
> >
> > > thing, since it's all too easy otherwise to unwittingly leave in lots
> >
> > of
> >
> > > single points of failure.  (Have you considered multiple independent
> > > UPSes?  Communications lines? NAS switches like I said (and you
> >
> > shouldn't
> >
> > > be using NAS for PG data!), application servers (whatever your
> >
> > application
> >
> > > may be) etc.?)
> > >
> > > Good luck!
> > >
> > > Hal
> > >
> > > On Tue, 9 Dec 2003, Bhartendu Maheshwari wrote:
> > > > Dear Sam,
> > > >
> > > > Thank you for the quick response.
> > > >
> > > > Can I you tell me why its not possible, it is possible with mysql
> >
> > then
> >
> > > > why not with postgres. Actually I am working on High Avaibility
> > > > framework, and its our need, we can't make a separate database
> >
> > server. I
> >
> > > > want to read/write and then close the file, very simple isn't? So
> >
> > how
> >
> > > > can I achieve in postgres? Please help me.
> > > >
> > > > regards
> > > > bhartendu
> > >
> > > ---------------------------(end of
> >
> > broadcast)---------------------------
> >
> > > TIP 7: don't forget to increase your free space map settings
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 4: Don't 'kill -9' the postmaster
>
> - --
>     UC
>
> - --
> Open Source Solutions 4U, LLC    2570 Fleetwood Drive
> Phone:  +1 650 872 2425        San Bruno, CA 94066
> Cell:   +1 650 302 2405        United States
> Fax:    +1 650 872 2417
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.2.1 (GNU/Linux)
>
> iD8DBQE/1qo8jqGXBvRToM4RAil5AJ9SbQ3oCdH7WhVMzsJSEGNFgyGO/gCgwdGj
> 3+Tp56/pgQz4gLxlGTO0M4k=
> =fyS0
> -----END PGP SIGNATURE-----
>

Bhartendu Maheshwari wrote:

> Dear Hal, Frank, Oli and all,
>
> I understand what you all trying to say, I know this is not good way of
> designing, but we are planning for using the database for the keeping
> mobile transactions and at the same time we need to provided the HA
> solutions. The one solution i derive from the discussion that uses one
> server and multiple clients but the issue in this if the system in which
> database server was running get down the its all the way no use of HA
> and load balancing, since without data the other one can't do anything.

Here's an important question. What exactly is the thinking behind your
load balancing and HA requirements? The reason I'm asking this question
is because there's nuances to what high available means.

As an example, you've got redundant servers but they're all in the same
server room. A fire breaks out and kills everything. Not really HA IMO.
Or you have redundant servers in different rooms/buildings hooked up to
a NAS unit someplace else. A mover knocks the head off the ceiling fire
extinguisher and floods the place (I've seen this happen) killing the
NAS device. Again, not very HA. On the otherhand, if all your users are
housed in same building as the servers where a fire kills the servers
and also stops your users from doing any work, then it's not a problem.

The situation my company is in is we have users all over the U.S.
connecting to our app so to do HA, we needed to put duplicate servers
thousands of miles away from each other. That way, an earthquake in SF
or a terrorist attack in D.C. doesn't bring down our app. And since
traffic was load balanced between both locations, we needed
master-master replication which we had to code in at the app level.

On Dec 10, 2003, at 12:25 PM, William Yu wrote:

> Bhartendu Maheshwari wrote:
>
>> Dear Hal, Frank, Oli and all,
>> I understand what you all trying to say, I know this is not good way
>> of
>> designing, but we are planning for using the database for the keeping
>> mobile transactions and at the same time we need to provided the HA
>> solutions. The one solution i derive from the discussion that uses one
>> server and multiple clients but the issue in this if the system in
>> which
>> database server was running get down the its all the way no use of HA
>> and load balancing, since without data the other one can't do
>> anything.
>
> Here's an important question. What exactly is the thinking behind your
> load balancing and HA requirements? The reason I'm asking this
> question is because there's nuances to what high available means.
>
> As an example, you've got redundant servers but they're all in the
> same server room. A fire breaks out and kills everything. Not really
> HA IMO. Or you have redundant servers in different rooms/buildings
> hooked up to a NAS unit someplace else. A mover knocks the head off
> the ceiling fire extinguisher and floods the place (I've seen this
> happen) killing the NAS device. Again, not very HA. On the otherhand,
> if all your users are housed in same building as the servers where a
> fire kills the servers and also stops your users from doing any work,
> then it's not a problem.
>
> The situation my company is in is we have users all over the U.S.
> connecting to our app so to do HA, we needed to put duplicate servers
> thousands of miles away from each other. That way, an earthquake in SF
> or a terrorist attack in D.C. doesn't bring down our app. And since
> traffic was load balanced between both locations, we needed
> master-master replication which we had to code in at the app level.
>
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 2: you can get off all lists at once with the unregister command
>    (send "unregister YourEmailAddressHere" to majordomo@postgresql.org)
>

There are 2 basic things to remember about proper HA, by any definition:

1. It is hard.
2. It is expensive.

This is true even for systems which were built with HA/redundancy in
mind - Postgres wasn't. Depending on what NAS device you're talking
about, it may not
have been either (call EMC and see what the price tag is on a
redundant, HA SAN setup. If you can afford that, you can afford the
right tools).

This topic has floated around on the erserver mailing list - either you
have the budget to do HA right, or you don't. If you don't, you have to
be clear
in your specs about what is possible with the tools that you can afford
without trying to jigger inappropriate technology to do something it
really can't
do.  Trying to do something that the developers of a product say is not
possible, and selling it as HA, is not going to make you many friends
when
problems arise.


--------------------

Andrew Rawnsley
President
The Ravensfield Digital Resource Group, Ltd.
(740) 587-0114
www.ravensfield.com

Bhartendu,

In my humble opinion, you would be well served if you listened to all
the nice people on this list.

Use a local disk subsystems with RAID-type storage, use replication to
have a  second "standby" system available if the first one fails.

The path you seem anxious to trod will get very muddy and slippery.

Good Luck.

...john

Bhartendu Maheshwari wrote:

>Dear UC,
>
>You are right for the HA solution but at the same time we are also
>implementing the load balancing solution so we can't have for one Node 2
>different processing entity and database as well. We try to provide
>solution for HA, load balancing both and in that there are 2 different
>processing machine but sharing the common database so that both get the
>latest and synchronized data files.
>
>You are right if the NAS is down then everything get down but the
>probability for the NAS is down is very less and by this we are able to
>provide service for 99% cases and if you are 99% handle cases then you
>are providing good service, isn't?
>
>About the cache to file write :- If the database is writting all the
>stuff to the files after each transaction then both have one
>synchronized set of data file whoever want can acquire the lock and use
>and then unlock it. The MySQL have command "flush tables" to enforce the
>database to write all the cache contents to the files, Is there anything
>similar in postgres? This will definitely degrade the performance of my
>system but its much more fast since I have 2 processing unit.
>
>Anyway if somebody have some other solution for the same please help me.
>One I got have one common postmaster running on one PC and the two nodes
>connect to that server to get the data. Any other please let me know.
>
>regards
>bhartendu
>
>
>

Dear All,

I got all your points, thanks for such a great discussion, Now the last
thing I want is how can I close the data files and flush the cache into
the data files. How can I do this in postgresql????

I will also try with RAID and other suggested way you all suggested.

regards
bhartendu

On Thu, 2003-12-11 at 00:46, John Gibson wrote:
> Bhartendu,
>
> In my humble opinion, you would be well served if you listened to all
> the nice people on this list.
>
> Use a local disk subsystems with RAID-type storage, use replication to
> have a  second "standby" system available if the first one fails.
>
> The path you seem anxious to trod will get very muddy and slippery.
>
> Good Luck.
>
> ...john
>
> Bhartendu Maheshwari wrote:
>
> >Dear UC,
> >
> >You are right for the HA solution but at the same time we are also
> >implementing the load balancing solution so we can't have for one Node 2
> >different processing entity and database as well. We try to provide
> >solution for HA, load balancing both and in that there are 2 different
> >processing machine but sharing the common database so that both get the
> >latest and synchronized data files.
> >
> >You are right if the NAS is down then everything get down but the
> >probability for the NAS is down is very less and by this we are able to
> >provide service for 99% cases and if you are 99% handle cases then you
> >are providing good service, isn't?
> >
> >About the cache to file write :- If the database is writting all the
> >stuff to the files after each transaction then both have one
> >synchronized set of data file whoever want can acquire the lock and use
> >and then unlock it. The MySQL have command "flush tables" to enforce the
> >database to write all the cache contents to the files, Is there anything
> >similar in postgres? This will definitely degrade the performance of my
> >system but its much more fast since I have 2 processing unit.
> >
> >Anyway if somebody have some other solution for the same please help me.
> >One I got have one common postmaster running on one PC and the two nodes
> >connect to that server to get the data. Any other please let me know.
> >
> >regards
> >bhartendu
> >
> >
> >
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org

> I got all your points, thanks for such a great discussion, Now the last
> thing I want is how can I close the data files and flush the cache into
> the data files. How can I do this in postgresql????

pg_ctl stop

Dear matt

Don't joke, this for stopping the Postgres Server. I want close the
files in between the database operations. I am looking for some SQL
command like in Mysql command is 'flush tables'. what's in postgres?

regards
bhartendu

On Thu, 2003-12-11 at 16:56, Matt Clark wrote:
> > I got all your points, thanks for such a great discussion, Now the last
> > thing I want is how can I close the data files and flush the cache into
> > the data files. How can I do this in postgresql????
>
> pg_ctl stop
>
>
>
>
>

Well I wasn't joking.

If you want to get the files on disk into a state where they can be modified by a different postmaster, then you need
toshut the 
current postmaster down.  In theory, maybe, if you only want the second postmaster to have read access, then you could
disconnect
all users from the current postmaster and stop accepting any more connections, but I wouldn't do it if I were you.

Maybe I'm missing the point.  The only other answer is:

commit;

Because for postgres commit *means* commit the transaction to disk.  Of course if you're using a NAS then who knows
whetherthe 
files have _really_ been written to disk after an fsync.  Postgres just has to believe what the operating system tells
it...

Matt




> Don't joke, this for stopping the Postgres Server. I want close the
> files in between the database operations. I am looking for some SQL
> command like in Mysql command is 'flush tables'. what's in postgres?
>
> regards
> bhartendu
>
> On Thu, 2003-12-11 at 16:56, Matt Clark wrote:
> > > I got all your points, thanks for such a great discussion, Now the last
> > > thing I want is how can I close the data files and flush the cache into
> > > the data files. How can I do this in postgresql????
> >
> > pg_ctl stop
> >
> >
> >
> >
> >
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster

On Thu, 11 Dec 2003, Matt Clark wrote:

> Well I wasn't joking.
>
> If you want to get the files on disk into a state where they can be modified by a different postmaster, then you need
toshut the 
> current postmaster down.  In theory, maybe, if you only want the second postmaster to have read access, then you
coulddisconnect 
> all users from the current postmaster and stop accepting any more connections, but I wouldn't do it if I were you.
>
> Maybe I'm missing the point.  The only other answer is:
>
> commit;
>
> Because for postgres commit *means* commit the transaction to disk.  Of course if you're using a NAS then who knows
whetherthe 
> files have _really_ been written to disk after an fsync.  Postgres just has to believe what the operating system
tellsit... 

Of course, they might just get written to the WAL... the *only* way to
get the files into a consistent, safe state is to stop the server.
Sorry.

> > Don't joke, this for stopping the Postgres Server. I want close the
> > files in between the database operations. I am looking for some SQL
> > command like in Mysql command is 'flush tables'. what's in postgres?
> >
> > regards
> > bhartendu
> >
> > On Thu, 2003-12-11 at 16:56, Matt Clark wrote:
> > > > I got all your points, thanks for such a great discussion, Now the last
> > > > thing I want is how can I close the data files and flush the cache into
> > > > the data files. How can I do this in postgresql????
> > >
> > > pg_ctl stop
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 4: Don't 'kill -9' the postmaster
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 5: Have you checked our extensive FAQ?
>
>                http://www.postgresql.org/docs/faqs/FAQ.html
>

--

Sam Barnett-Cormack
Software Developer                           |  Student of Physics & Maths
UK Mirror Service (http://www.mirror.ac.uk)  |  Lancaster University

On 11 Dec 2003 18:13:07 +0530
Bhartendu Maheshwari <bhartendum@jataayusoft.com> wrote:

> Dear matt
>
> Don't joke, this for stopping the Postgres Server. I want close the
> files in between the database operations. I am looking for some SQL
> command like in Mysql command is 'flush tables'. what's in postgres?
>
> regards
> bhartendu
>


forcing a checkpoint or using commit may get those tables flushed out.
(or at least the wal). but the problem is other postmasters on other
machines may not know that data has changed - it could be sitting in a
shared buffer.

Machine A reads block 123, stored in a shared buffer.
Machine B reads block 123, modifies it, writes it back out
Machine A needs block 123, finds it in its shared buffer and uses that
data [which is no longer valid]

Thus you can see the problems.

or another one

Machine A locks row 123 for update
Machine B also wants to lock row 123.  How does it know A already has
the lock? (locks are stored in shared memory)

What you really want is multi-master replication.  Currently this
doesn't exist on PG (that I know of).  There's a JDBC app that simulates
it. (And you could too, to varying degrees of success).

If mysql works, then use it, but I'd be suprised if it didn't suffer a
lot of these same problems as well.

If you seriously need this feature, then you'll either have to A. buy
oracle/whatever or B. pay to fund development of it for PG

--
Jeff Trout <jeff@jefftrout.com>
http://www.jefftrout.com/
http://www.stuarthamm.net/

Bhartendu Maheshwari <bhartendum@jataayusoft.com> writes:
> I got all your points, thanks for such a great discussion, Now the last
> thing I want is how can I close the data files and flush the cache into
> the data files. How can I do this in postgresql????

Shut down the postmaster.

> I will also try with RAID and other suggested way you all suggested.

I don't think what people have been telling you has penetrated at all.
You CAN NOT run multiple postmasters concurrently on the same set of
database files.  It will not work.  It will corrupt your data.  There
is no hack that will make it work.

            regards, tom lane

Just to add a tidbit on top of this, there are commercial RDBMS's that
do, or at least say they do, clustered database servers.  (cough cough
oracle, cough cough ingres)

The products are expensives as all get out, a bastard to set up, and
require truely kickass SAN storage and clustered hardware to be
meaningful.  Not, in general, something to be set up on a lark.

You can get an awful lot of what you seem to want with replication.
Have you considered doing so ?  For instance, I expect there are
replication products for postgres that would allow a disconnected client
to push transactions up to a master server, then resyncronise itself
with the master.

In terms of HA, in addition to replication solutions, you can also do a
fair to middling job of failover by having a primary and hot backup
database server on a heartbeat/STONITH setup, using a shared, redundant
SAN as a data store.  The hot backup would mount the SAN drives on
primary failure, takeover the IP, and start the postmaster.

Of course, just the SAN setup for such a beast is going to run to a
minimum of 5 figures, and any transaction that might be in progres will
be borked. Oh, and this complicates the mobile operations requirement
quite a bit too. :-(

If you feel really really really masocistic, uh, I mean ambitious, you
could start looking at what it would take to port and run a RDBMS server
in a truely clustered, high availablibility environment.  It will not be
simple.  You have issues of distributed cache coherency, distributed
locking, distributed filesystems/datastory, load balancing on a
by-transaction basis (wouldn't want to send the second part of the same
transaction to a different server ....), recovery from server node
failure, adding server nodes to a running instance, and lots of other
things I haven't thought of.

This is complicated by the fact that there really isn't any mature linux
cluster solution that gives decent support for everything you've asked
for yet.  The closest project I'm aware of is SSIC-Linux.  Look it up on
sourceforge if you're curious.

Good luck,
-- Pat

p.s.  Yes, I was brought up and spoiled on VMS clusters.  Still nothing
beats good 'ol RDB for a clustered database solution.

Bhartendu Maheshwari wrote:
> Dear Hal, Frank, Oli and all,
>
> I understand what you all trying to say, I know this is not good way of
> designing, but we are planning for using the database for the keeping
> mobile transactions and at the same time we need to provided the HA
> solutions. The one solution i derive from the discussion that uses one
> server and multiple clients but the issue in this if the system in which
> database server was running get down the its all the way no use of HA
> and load balancing, since without data the other one can't do anything.
>
> What I have in mind is the following implementation:-
>
> Step 1 :- Mount the data files from NAS server.
> Step 2 :- start the postgres with the mounted data.
> Step 3 :- Lock the data files by one server.
> Step 4 :- do the database operation.
> Step 5 :- Commit in the database files.
> Step 6 :- Unlock the database files
> Step 7 :- Now the other one can do the same way.
>
> Or if anybody have other solution for this, please suggest. How can I
> commit the data into database files and flushing cache with the latest
> data files contents? Is there any command to refresh both.
>
> Thank you Hal for
> *************************************************************************
> If you really, really do need an HA solution, then I'd hunt around for
> someone to add to your team who has extensive experience in this kind of
> thing, since it's all too easy otherwise to unwittingly leave in lots of
> single points of failure.
> *************************************************************************
>
> If you really have someone to help me in this regards, please I need his
> help in this issue and want to derive a common techniques so that
> everyone can use this.
>
> regards
> bhartendu
>

He might also want to take a look at SUN's grid -  the developement kit and
most of the components are free.   Is also supports Linux.  I bet with two
Postgres servers in sync via replication and the GRID setup to do the switch
over should one fail it would be as low cost HA he could get.


----- Original Message -----
From: "Patrick Spinler" <pspinler@yahoo.com>
To: "Postgres Mailing List" <pgsql-admin@postgresql.org>
Cc: "Bhartendu Maheshwari" <bhartendum@jataayusoft.com>
Sent: Saturday, December 13, 2003 11:19 PM
Subject: Re: [ADMIN] Running Postgres Daemons with same data files


>
> Just to add a tidbit on top of this, there are commercial RDBMS's that
> do, or at least say they do, clustered database servers.  (cough cough
> oracle, cough cough ingres)
>
> The products are expensives as all get out, a bastard to set up, and
> require truely kickass SAN storage and clustered hardware to be
> meaningful.  Not, in general, something to be set up on a lark.
>
> You can get an awful lot of what you seem to want with replication.
> Have you considered doing so ?  For instance, I expect there are
> replication products for postgres that would allow a disconnected client
> to push transactions up to a master server, then resyncronise itself
> with the master.
>
> In terms of HA, in addition to replication solutions, you can also do a
> fair to middling job of failover by having a primary and hot backup
> database server on a heartbeat/STONITH setup, using a shared, redundant
> SAN as a data store.  The hot backup would mount the SAN drives on
> primary failure, takeover the IP, and start the postmaster.
>
> Of course, just the SAN setup for such a beast is going to run to a
> minimum of 5 figures, and any transaction that might be in progres will
> be borked. Oh, and this complicates the mobile operations requirement
> quite a bit too. :-(
>
> If you feel really really really masocistic, uh, I mean ambitious, you
> could start looking at what it would take to port and run a RDBMS server
> in a truely clustered, high availablibility environment.  It will not be
> simple.  You have issues of distributed cache coherency, distributed
> locking, distributed filesystems/datastory, load balancing on a
> by-transaction basis (wouldn't want to send the second part of the same
> transaction to a different server ....), recovery from server node
> failure, adding server nodes to a running instance, and lots of other
> things I haven't thought of.
>
> This is complicated by the fact that there really isn't any mature linux
> cluster solution that gives decent support for everything you've asked
> for yet.  The closest project I'm aware of is SSIC-Linux.  Look it up on
> sourceforge if you're curious.
>
> Good luck,
> -- Pat
>
> p.s.  Yes, I was brought up and spoiled on VMS clusters.  Still nothing
> beats good 'ol RDB for a clustered database solution.
>
> Bhartendu Maheshwari wrote:
> > Dear Hal, Frank, Oli and all,
> >
> > I understand what you all trying to say, I know this is not good way of
> > designing, but we are planning for using the database for the keeping
> > mobile transactions and at the same time we need to provided the HA
> > solutions. The one solution i derive from the discussion that uses one
> > server and multiple clients but the issue in this if the system in which
> > database server was running get down the its all the way no use of HA
> > and load balancing, since without data the other one can't do anything.
> >
> > What I have in mind is the following implementation:-
> >
> > Step 1 :- Mount the data files from NAS server.
> > Step 2 :- start the postgres with the mounted data.
> > Step 3 :- Lock the data files by one server.
> > Step 4 :- do the database operation.
> > Step 5 :- Commit in the database files.
> > Step 6 :- Unlock the database files
> > Step 7 :- Now the other one can do the same way.
> >
> > Or if anybody have other solution for this, please suggest. How can I
> > commit the data into database files and flushing cache with the latest
> > data files contents? Is there any command to refresh both.
> >
> > Thank you Hal for
> >
*************************************************************************
> > If you really, really do need an HA solution, then I'd hunt around for
> > someone to add to your team who has extensive experience in this kind of
> > thing, since it's all too easy otherwise to unwittingly leave in lots of
> > single points of failure.
> >
*************************************************************************
> >
> > If you really have someone to help me in this regards, please I need his
> > help in this issue and want to derive a common techniques so that
> > everyone can use this.
> >
> > regards
> > bhartendu
> >
>
>
>
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