Re: hanging for 30sec when checkpointing

If I understand checkpoints correctly, data that is already written to the
WAL (and flushed to disk) is being written to the DB (flushing to disk).
Meanwhile, other writer transactions are continuing to busily write to the
WAL. In which case a disk bandwidth problem (other than kernal config
issues) may be helped by placing the WAL files on a disk (and maybe even
controller) seperate from the DB.

Something to think about anyway.

regards
Iain
----- Original Message -----
From: "Shane Wright" <me@shanewright.co.uk>
To: <pgsql-admin@postgresql.org>
Sent: Wednesday, February 04, 2004 7:35 AM
Subject: [ADMIN] hanging for 30sec when checkpointing


> Hi,
>
> I'm running a reasonable sized (~30Gb) 7.3.4 database on Linux and I'm
> getting some weird performance at times.
>
> When the db is under medium-heavy load, it periodically spawns a
> 'checkpoint subprocess' which runs for between 15 seconds and a minute.
> Ok, fair enough, the only problem is the whole box becomes pretty much
> unresponsive during this time - from what I can gather it's because it
> writes out roughly 1Mb (vmstat says ~1034 blocks) per second until its
done.
>
> Other processes can continue to run (e.g. vmstat) but other things do
> not (other queries, mostly running 'ps fax', etc).  So everything gets
> stacked up till the checkpoint finishes and all is well again, untill
> the next time...
>
> This only really happens under medium/high load, but doesn't seem
> related to the length/complexity of transactions done.
>
> The box isn't doing a lot else at the same time - most queries some in
> from separate web server boxes.
>
> The disks, although IDE, can definately handle more than 1Mb/sec - even
> with multiple concurrent writes.  The box is powerful (2.6Ghz Xeon, 2Gb
> RAM).  Its a clean compile from source of 7.3.4, although I can't really
> upgrade to 7.4.x at this time as I can't afford the 18 hours downtime to
> dump/restore the database.  Fsync is on.  Most other settings at their
> defaults.
>
> I've looked at the documentation and various bits about adjusting
> checkpoint segments and timings - but it seems reducing segments/timeout
> is implied to be bad, but it seems to me that increasing either will
> just make the same thing happen less often but more severely.
>
> If it makes any odds, this seems to happen much more often when doing
> bulk UPDATEs and INSERTs - athough these are in transactions grouping
> them together - and they don't affect the same tables as other queries
> that still get stalled (no lock contention causing the problem).
>
> What am I missing?  I'm sure I'm missing something blatantly obvious,
> but as it's only really happening on production systems (only place with
> the load and the volume of data) I'm loathe to experiment.
>
> Any help appreciated,
>
> Cheers,
>
> Shane
>
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