Thread: unlogged tables

* Anibal David Acosta (aa@devshock.com) wrote:
> Unlogged tables are not memory tables don't?

Unlogged tables are not memory tables.

> If we stop postgres server (normal stop) and start again, all information in
> unlogged tables still remain?

Yes.

> So, can I expect a data loss just in case of crash, power failure or SO
> crash don't?

Yes.

> In case of crash, is possible that data corruption happened in a unlogged
> tables?

In a crash, unlogged tables are automatically truncated.

> For performance purpose can I use async commit and unlogged tables?

I'm not aware of any issues (beyond those already documented for async
commit..) with having async commit and unlogged tables.

    THanks,

        Stephen

Hi,

On 12 November 2011 00:18, Stephen Frost <sfrost@snowman.net> wrote:
> In a crash, unlogged tables are automatically truncated.

BTW I wonder what for they are truncated in a crash case?

--
Sergey Konoplev

Blog: http://gray-hemp.blogspot.com
LinkedIn: http://ru.linkedin.com/in/grayhemp
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On 14/11/11 08:10, Sergey Konoplev wrote:
> Hi,
>
> On 12 November 2011 00:18, Stephen Frost<sfrost@snowman.net>  wrote:
>> In a crash, unlogged tables are automatically truncated.
>
> BTW I wonder what for they are truncated in a crash case?

Because they bypass the transaction-log (WAL), hence unlogged.

There's no way to know whether there were partial updates applied when
the system restarts.

--
   Richard Huxton
   Archonet Ltd

On 14 November 2011 12:58, Richard Huxton <dev@archonet.com> wrote:
> Because they bypass the transaction-log (WAL), hence unlogged.
> There's no way to know whether there were partial updates applied when the
> system restarts.

I probably did not understand the "truncate" meaning correct. It
truncates all the records of the table or several recent records only?

>
> --
>  Richard Huxton
>  Archonet Ltd
>



--
Sergey Konoplev

Blog: http://gray-hemp.blogspot.com
LinkedIn: http://ru.linkedin.com/in/grayhemp
JID/GTalk: gray.ru@gmail.com Skype: gray-hemp

On 14/11/11 10:08, Sergey Konoplev wrote:
> On 14 November 2011 12:58, Richard Huxton<dev@archonet.com>  wrote:
>> Because they bypass the transaction-log (WAL), hence unlogged.
>> There's no way to know whether there were partial updates applied when the
>> system restarts.
>
> I probably did not understand the "truncate" meaning correct. It
> truncates all the records of the table or several recent records only?

All.

Let's say you were doing something like "UPDATE unlogged_table SET x=1
WHERE y=2". If a crash occurs during this command, there's no guarantee
that the affected disk pages were all updated. Worse, a single page
might be partially updated or even have rubbish in it (depending on the
nature of the crash).

Without the WAL there's no way to check whether the table is good or
not, or even to know what the last updates were. So - the only safe
thing to do is truncate the unlogged tables.

In the event of a normal shutdown, we can flush all the writes to disk
so we know all the data has been written, so there is no need to truncate.

--
   Richard Huxton
   Archonet Ltd

On 14 November 2011 14:17, Richard Huxton <dev@archonet.com> wrote:
> On 14/11/11 10:08, Sergey Konoplev wrote:
>>
>> On 14 November 2011 12:58, Richard Huxton<dev@archonet.com>  wrote:
> Let's say you were doing something like "UPDATE unlogged_table SET x=1 WHERE
> y=2". If a crash occurs during this command, there's no guarantee that the
> affected disk pages were all updated. Worse, a single page might be
> partially updated or even have rubbish in it (depending on the nature of the
> crash).
>
> Without the WAL there's no way to check whether the table is good or not, or
> even to know what the last updates were. So - the only safe thing to do is
> truncate the unlogged tables.
>
> In the event of a normal shutdown, we can flush all the writes to disk so we
> know all the data has been written, so there is no need to truncate.

Thank you for the explanation. Now I understand it.

>
> --
>  Richard Huxton
>  Archonet Ltd
>



--
Sergey Konoplev

Blog: http://gray-hemp.blogspot.com
LinkedIn: http://ru.linkedin.com/in/grayhemp
JID/GTalk: gray.ru@gmail.com Skype: gray-hemp

Maybe an option like "Recover from file " will be useful
So, for example, daily some process do a COPY of entire table to a file

In case of crash postgres recover content from the file.

:)



-----Mensaje original-----
De: Sergey Konoplev [mailto:gray.ru@gmail.com]
Enviado el: lunes, 14 de noviembre de 2011 07:39 a.m.
Para: Richard Huxton
CC: Stephen Frost; Anibal David Acosta; pgsql-performance@postgresql.org
Asunto: Re: [PERFORM] unlogged tables

On 14 November 2011 14:17, Richard Huxton <dev@archonet.com> wrote:
> On 14/11/11 10:08, Sergey Konoplev wrote:
>>
>> On 14 November 2011 12:58, Richard Huxton<dev@archonet.com>  wrote:
> Let's say you were doing something like "UPDATE unlogged_table SET x=1
> WHERE y=2". If a crash occurs during this command, there's no
> guarantee that the affected disk pages were all updated. Worse, a
> single page might be partially updated or even have rubbish in it
> (depending on the nature of the crash).
>
> Without the WAL there's no way to check whether the table is good or
> not, or even to know what the last updates were. So - the only safe
> thing to do is truncate the unlogged tables.
>
> In the event of a normal shutdown, we can flush all the writes to disk
> so we know all the data has been written, so there is no need to truncate.

Thank you for the explanation. Now I understand it.

>
> --
>  Richard Huxton
>  Archonet Ltd
>



--
Sergey Konoplev

Blog: http://gray-hemp.blogspot.com
LinkedIn: http://ru.linkedin.com/in/grayhemp
JID/GTalk: gray.ru@gmail.com Skype: gray-hemp

"Anibal David Acosta" <aa@devshock.com> wrote:

> Maybe an option like "Recover from file " will be useful
> So, for example, daily some process do a COPY of entire table to a
> file
>
> In case of crash postgres recover content from the file.

If you need to recover file contents on a crash, then an unlogged
table is probably not the right choice.  There is always
asynchronous commit.

-Kevin

I am doing asynchronous commit but sometimes I think that there are so  many
"things" in an insert/update transaction, for a table that has not too much
important information.

My table is a statistics counters table, so I can live with a partial data
loss, but not with a full data loss because many counters are weekly and
monthly.

Unlogged table can increase speed, this table has about 1.6 millions of
update per hour, but unlogged with a chance of loss all information on a
crash are not a good idea for this.

Anyway, thanks Kevin!







-----Mensaje original-----
De: Kevin Grittner [mailto:Kevin.Grittner@wicourts.gov]
Enviado el: lunes, 14 de noviembre de 2011 02:27 p.m.
Para: 'Richard Huxton'; Anibal David Acosta; 'Sergey Konoplev'
CC: pgsql-performance@postgresql.org; 'Stephen Frost'
Asunto: Re: [PERFORM] unlogged tables

"Anibal David Acosta" <aa@devshock.com> wrote:

> Maybe an option like "Recover from file " will be useful So, for
> example, daily some process do a COPY of entire table to a file
>
> In case of crash postgres recover content from the file.

If you need to recover file contents on a crash, then an unlogged table is
probably not the right choice.  There is always asynchronous commit.

-Kevin

"Anibal David Acosta" <aa@devshock.com> wrote:

> I am doing asynchronous commit but sometimes I think that there
> are so  many "things" in an insert/update transaction, for a table
> that has not too much important information.
>
> My table is a statistics counters table, so I can live with a
> partial data loss, but not with a full data loss because many
> counters are weekly and monthly.
>
> Unlogged table can increase speed, this table has about 1.6
> millions of update per hour, but unlogged with a chance of loss
> all information on a crash are not a good idea for this.

pg_dump -t 'tablename' from a cron job?  (Make sure to rotate dump
file names, maybe with day of week or some such.)

-Kevin

>> Unlogged table can increase speed, this table has about 1.6
>> millions of update per hour, but unlogged with a chance of loss
>> all information on a crash are not a good idea for this.
>
> pg_dump -t 'tablename' from a cron job?  (Make sure to rotate dump
> file names, maybe with day of week or some such.)

Or just "CREATE TABLE AS" copy the table every hour to a second, backup
table.  Then it would be much easier to script automated restore of the
data.

--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com

> My table is a statistics counters table, so I can live with a partial
> data
> loss, but not with a full data loss because many counters are weekly and
> monthly.
>
> Unlogged table can increase speed, this table has about 1.6 millions of
> update per hour, but unlogged with a chance of loss all information on a
> crash are not a good idea for this.

You could use an unlogged table for hourly updates, and periodically,
accumulate those counters to a (logged) daily/weekly table...

The hourly table could be rebuilt by examining only 1 hour's worth of
data, so it isn't too much of a problem if it's lost. The other tables
would get much less updates.

"In the event of a normal shutdown, we can flush all the writes to disk
so we know all the data has been written, so there is no need to truncate."

Isn't possible to periodically flush data to disk and in case of crush
postgres to load only the data that existed at last flush? The periodic
flush could be configurable, for example every 30 minutes or after x rows
updated/inserted.



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That will be a very useful feature. I don'
t care if i loss 1-2 hours of data. I know we could have some cron jobs to
dump the table periodically but  the table could be big,  so this operation
could be expensive. Also i have to detect when postgres crush, i have no
idea how i can detect if postgres crushed. Then i have somehow to attache a
script at postgres start, to restore the dumps...the dump solution is very
complicate and  unreliable. A periodic flush feature will be amazing!

How is the procedure for feature request on postgres, github?

Thanks!



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On 2015-04-13 14:16, dgabriel wrote:
> That will be a very useful feature.

I agree, unlogged tables would be a lot more useful if they didn't "disappear"
on re-start.


> could be expensive. Also i have to detect when postgres crush, i have no
> idea how i can detect if postgres crushed. Then i have somehow to attache a
> script at postgres start, to restore the dumps...the dump solution is very
> complicate and  unreliable. A periodic flush feature will be amazing!

In my experience postgres is very aggressive in getting rid of unlogged
tables, it does get rid of them from shutdowns that seem perfectly fine (no
crash). A lot of people get surprised by this.

--
http://yves.zioup.com
gpg: 4096R/32B0F416

On Mon, Apr 13, 2015 at 4:31 PM, dgabriel <gabriel.dodan@gmail.com> wrote:

"In the event of a normal shutdown, we can flush all the writes to disk
so we know all the data has been written, so there is no need to truncate."

Isn't possible to periodically flush data to disk and in case of crush
postgres to load only the data that existed at last flush? The periodic
flush could be configurable, for example every 30 minutes or after x rows
updated/inserted.

There is no such facility implemented for UNLOGGED TABLEs. That could be a feature request though.

On 4/13/15 3:49 PM, David G. Johnston wrote:
> On Monday, April 13, 2015, Matheus de Oliveira
> <matioli.matheus@gmail.com <mailto:matioli.matheus@gmail.com>> wrote:
>     On Mon, Apr 13, 2015 at 4:31 PM, dgabriel <gabriel.dodan@gmail.com
>     <javascript:_e(%7B%7D,'cvml','gabriel.dodan@gmail.com');>> wrote:
>
>         "In the event of a normal shutdown, we can flush all the writes
>         to disk
>         so we know all the data has been written, so there is no need to
>         truncate."
>
>         Isn't possible to periodically flush data to disk and in case of
>         crush
>         postgres to load only the data that existed at last flush? The
>         periodic
>         flush could be configurable, for example every 30 minutes or
>         after x rows
>         updated/inserted.
>
>     There is no such facility implemented for UNLOGGED TABLEs. That
>     could be a feature request though.
>
> Well, that is half right anyway.  UNLOGGED tables obey checkpoints just
> like any other table.  The missing feature is an option to
> leaved restored the last checkpoint.  Instead, not knowing whether there
> were changes since the last checkpoint, the system truncated the relation.
>
> What use case is there for a behavior that the last checkpoint data is
> left on the relation upon restarting - not knowing whether it was
> possible the other data could have been written subsequent?

Yeah, this is not something that would be very easy to accomplish,
because a buffer can get evicted and written to disk at any point. It
wouldn't be too hard to read every unlogged table during recovery and
see if there are any pages that were written after the last checkpoint,
but that obviously won't be very fast.

Actually, I suppose we could dedicate a fork for unlogged tables and use
that to record the newest LSN of any page that's been written out. But
if you have much of any write activity on the table that's probably
going to be completely useless.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com

Jim Nasby wrote:

> Yeah, this is not something that would be very easy to accomplish, because a
> buffer can get evicted and written to disk at any point. It wouldn't be too
> hard to read every unlogged table during recovery and see if there are any
> pages that were written after the last checkpoint, but that obviously won't
> be very fast.

If you consider only tables, then yeah perhaps this is easy to
accomplish (not really convinced myself).  But if you consider indexes,
things are not so easy anymore.


In the thread from 2011 (which this started as a reply to) the OP was
doing frequent UPDATEs to keep track of counts of something.  I think
that would be better served by using INSERTs of deltas and periodic
accumulation of grouped values, as suggested in
http://www.postgresql.org/message-id/20150305211601.GW3291@alvh.no-ip.org
This has actually been suggested many times over the years.

--
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On 4/13/15 4:13 PM, Alvaro Herrera wrote:
> Jim Nasby wrote:
>
>> Yeah, this is not something that would be very easy to accomplish, because a
>> buffer can get evicted and written to disk at any point. It wouldn't be too
>> hard to read every unlogged table during recovery and see if there are any
>> pages that were written after the last checkpoint, but that obviously won't
>> be very fast.
>
> If you consider only tables, then yeah perhaps this is easy to
> accomplish (not really convinced myself).  But if you consider indexes,
> things are not so easy anymore.

Are indexes not guaranteed to have LSNs? I thought they basically
followed the same write rules as heap pages in regard to WAL first.

Though, if you have an index that doesn't support logging (like hash)
you're still hosed...

> In the thread from 2011 (which this started as a reply to) the OP was

I don't keep PGSQL emails from that far back... ;)

> doing frequent UPDATEs to keep track of counts of something.  I think
> that would be better served by using INSERTs of deltas and periodic
> accumulation of grouped values, as suggested in
> http://www.postgresql.org/message-id/20150305211601.GW3291@alvh.no-ip.org
> This has actually been suggested many times over the years.

What I was suggesting certainly wouldn't help you if you were getting
any serious amount of changes to the count.

I am wondering though what the bottleneck in HEAD is with doing an
UPDATE instead of an INSERT, at least where unlogged would help
significantly. I didn't think we logged all that much more for an
UPDATE. Heck, with HOT you might even be able to log less.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com

On 4/13/15 7:32 PM, David G. Johnston wrote:
>         The missing feature is an option to leaved restored the last
>         checkpoint.  Instead, not knowing whether there were changes
>         since the last checkpoint, the system truncated the relation.
>
>         What use case is there for a behavior that the last checkpoint
>         data is left on the relation upon restarting - not knowing
>         whether it was possible the other data could have been written
>         subsequent?
>
>
>     I would like a way to have unlogged tables be available on a replica
>     provided that no changes were made to them between the pg_basebackup
>     and the recovery point.
>
>
>     My use case is that I mark certain read-only-after-bulk-loading
>     tables as unlogged solely to avoid blowing out the log archive
>     during the loading phase and refresh phase.  This is stuff like
>     vendor catalogs, NCBI datasets, ChEMBL datasets, etc, which can
>     simply be re-derived from the reference.  It would be nice if these
>     were still available (without having to repeat the ETL) after
>     crashes provided they were not written to since a checkpoint, and
>     available on cloned test servers without having to repeat the ETL on
>     those as well.
>
>
> ​My gut reaction is that those should be in their own clusters and
> accessed via postgres_fdw...

Likely to produce really crappy plans if the tables are of any real size...

> That particular use-case would probably best be served with a separate
> replication channel which pushes data files from the primary to the
> slaves and allows for the slave to basically "rewrite" its existing
> table by pointing to the newly supplied version.  Some kind of "CREATE
> STATIC TABLE" and "PUSH STATIC TABLE TO {all | replica name}" command
> combo...though ideally with less manual intervention...

You still have the same problem of knowing if someone has scribbled on
the data since the last checkpoint.

There's been recent discussion of adding support for read-only tables.
If we had those, we might be able to support something like...

INSERT INTO unlogged;
ALTER TABLE unlogged SET READ ONLY;
CHECKPOINT;
/* take backup */

This should be safe as long as we WAL log changes to read-only status
(which presumably we would).

How much work that would entail though, I don't know.

Ultimately you still have to get the data over to the other machine
anyway. ISTM it'd be a LOT more useful to look at ways to make the WAL
logging of bulk inserts (and especially COPY into a known empty table) a
lot more efficient.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com

On 2015-04-13 17:49, Jeff Janes wrote:
>
> One way would be to lock dirty buffers from unlogged relations into
> shared_buffers (which hardly seems like a good thing) until the start of a
> "super-checkpoint" and then write them all out as fast as possible (which kind
> of defeats checkpoint_completion_target).  And then if the crash happened
> during a super-checkpoint, the data would still be inconsistent and need to be
> truncated.
>

What do you call a "super-checkpoint"?

--
http://yves.zioup.com
gpg: 4096R/32B0F416

David G Johnston wrote
> Well, that is half right anyway.  UNLOGGED tables obey checkpoints just
> like any other table.  The missing feature is an option to leaved restored
> the last checkpoint.  Instead, not knowing whether there were changes
> since the last checkpoint, the system truncated the relation.
>
> What use case is there for a behavior that the last checkpoint data is
> left on the relation upon restarting - not knowing whether it was possible
> the other data could have been written subsequent?

If is possible to restore the table at last checkpoint state that will be
more than enough. I don't care about the changes since last checkpoint, I am
willing to lose those changes.  There are use cases where is acceptable to
lose some data, for example in a cache system, it is not a big issue if we
lose some cached data.



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On 4/14/15 10:56 AM, dgabriel wrote:
> David G Johnston wrote
>> Well, that is half right anyway.  UNLOGGED tables obey checkpoints just
>> like any other table.  The missing feature is an option to leaved restored
>> the last checkpoint.  Instead, not knowing whether there were changes
>> since the last checkpoint, the system truncated the relation.
>>
>> What use case is there for a behavior that the last checkpoint data is
>> left on the relation upon restarting - not knowing whether it was possible
>> the other data could have been written subsequent?
>
> If is possible to restore the table at last checkpoint state that will be
> more than enough. I don't care about the changes since last checkpoint, I am
> willing to lose those changes.  There are use cases where is acceptable to
> lose some data, for example in a cache system, it is not a big issue if we
> lose some cached data.

It is not. Unless you ensure that data is written to WAL (on disk)
BEFORE it is written to the data pages, you will probably have
corruption after a crash, and have no way to prevent or possibly even
detect the corruption.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com