Thread: pg_xlog question
Does anybody know if pg_xlog are essential for postgres to start? What would
happen if those files are lost, is the whole database corrupted or simply
some transactions lost?
I want to know if the following cluster setup would work:
* /dbase - contains the database itself, is on a DRBD mirror device and spread
across 2 machines.
* /dbase/pg_xlog is a symlink to /pg_log and is a local partition, not a DRBD
device
* what would happen if a failover happens, and the pg_xlog on the other
machine is empty. will postgres simply recreate those files (maybe some tx
are lost) or cant this setup work correctly?
Best regards,
Mario Weilguni
On 6 Aug 2003 at 8:36, Mario Weilguni wrote: > Does anybody know if pg_xlog are essential for postgres to start? What would > happen if those files are lost, is the whole database corrupted or simply > some transactions lost? > > I want to know if the following cluster setup would work: > * /dbase - contains the database itself, is on a DRBD mirror device and spread > across 2 machines. > * /dbase/pg_xlog is a symlink to /pg_log and is a local partition, not a DRBD > device > * what would happen if a failover happens, and the pg_xlog on the other > machine is empty. will postgres simply recreate those files (maybe some tx > are lost) or cant this setup work correctly? I doubt. It might end up with some transaction uncommittied in files. If you could replicate/rsync pg_xlog to another machine, you could get it correctly in case of failover. Testing the proper failover under load is recommended and you need to have a journallig file system to make sure that the transaction logs are in sane state. PG can not recover from corrupted transaction logs.. HTH Bye Shridhar -- We Klingons believe as you do -- the sick should die. Only the strongshould live. -- Kras, "Friday's Child", stardate 3497.2
On 6 Aug 2003 at 12:52, Shridhar Daithankar wrote: > On 6 Aug 2003 at 8:36, Mario Weilguni wrote: > > > Does anybody know if pg_xlog are essential for postgres to start? What would > > happen if those files are lost, is the whole database corrupted or simply > > some transactions lost? > > > > I want to know if the following cluster setup would work: > > * /dbase - contains the database itself, is on a DRBD mirror device and spread > > across 2 machines. > > * /dbase/pg_xlog is a symlink to /pg_log and is a local partition, not a DRBD > > device > > * what would happen if a failover happens, and the pg_xlog on the other > > machine is empty. will postgres simply recreate those files (maybe some tx > > are lost) or cant this setup work correctly? > > I doubt. It might end up with some transaction uncommittied in files. > > If you could replicate/rsync pg_xlog to another machine, you could get it > correctly in case of failover. Argh.. I forgot.. Simple option is to sync the pg_xlog while doing failover. It might add some 10 odd sec to failover process but should be bullet proof.. I don't know if PG is nose poking about permissions of those files. otheriwse it might refuse to start.. Check out.. Worth adding to failover HOWTO. I doubt if one exists for postgresql. Bye Shridhar -- Do you know about being with somebody? Wanting to be? If I had thewhole universe, I'd give it to you, Janice. When I see you, I feellike I'm hungry all over. Do you know how that feels? -- Charlie Evans, "Charlie X", stardate 1535.8
> > If you could replicate/rsync pg_xlog to another machine, you could get
it
> > correctly in case of failover.
>
> Argh.. I forgot.. Simple option is to sync the pg_xlog while doing
failover. It
> might add some 10 odd sec to failover process but should be bullet proof..
>
> I don't know if PG is nose poking about permissions of those files.
otheriwse
> it might refuse to start.. Check out..
>
> Worth adding to failover HOWTO. I doubt if one exists for postgresql.
Thanks for the response, but syncing pg_xlog separatly (without DRBD device)
is useless, because a failover will take place when there's an error, so the
extra sync of pg_xlog is very likely to fail.
Regards,
Mario Weilguni
On 6 Aug 2003 at 9:56, Mario Weilguni wrote:
> Thanks for the response, but syncing pg_xlog separatly (without DRBD device)
> is useless, because a failover will take place when there's an error, so the
> extra sync of pg_xlog is very likely to fail.
Well, I googled for DRBD device. I was unfamiliar with the device.
I would say it should work if data and transaction logs both of them are
mirrored. But of course you need to test.
If you could write a HOWTO, that would be real great..
Bye
Shridhar
--
"We have the right to survive!" "Not by killing others." -- Deela and Kirk,
"Wink of An Eye", stardate 5710.5
Why not
DRDB the whole Postgres file system (possibly multiple disks) and let WAL recovery take care, of uncommitted transactions at db startup time.
Shridhar Daithankar wrote:
DRDB the whole Postgres file system (possibly multiple disks) and let WAL recovery take care, of uncommitted transactions at db startup time.
Shridhar Daithankar wrote:
On 6 Aug 2003 at 9:56, Mario Weilguni wrote:Thanks for the response, but syncing pg_xlog separatly (without DRBD device) is useless, because a failover will take place when there's an error, so the extra sync of pg_xlog is very likely to fail.Well, I googled for DRBD device. I was unfamiliar with the device. I would say it should work if data and transaction logs both of them are mirrored. But of course you need to test. If you could write a HOWTO, that would be real great.. ByeShridhar --"We have the right to survive!" "Not by killing others." -- Deela and Kirk, "Wink of An Eye", stardate 5710.5 ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings