Re: hanging for 30sec when checkpointing

Hi

Thanks to you all for your help! I'm continually impressed with how
responsive and knowledgeable y'all are!

To clarify; it's an IDE drive with a reiserfs filesystem. DMA is
definately enabled, sequential reads pull 35Mb/sec sustained.

The I/O caused by the checkpoint just seems to be too much while other
transactions are running. As it's a managed server at our ISP throwing
more hardware at it isn't an option at the moment unfortunately, so I
think I'm left with optimising the app to reduce the number of
INSERTs/UPDATEs.

Is what Iain said correct about [committed] transactions only being
written to WAL, and actual table data files are only updated at
checkpoint?

I guess really it's something I hadn't thought of - in effect, the
database is able to handle _bursts_ of high load, but sustaining it is
hard (because checkpoint happens sooner or later).

Hmm that gives me an idea, for bulk processing, is there a way of
detecting from a client when a checkpoint is about to happen so it can
wait until it's finished? Some way that's easier than -z `ps fax |
grep post | grep checkpoint` that is ;)

Cheers

Shane

On 3 Feb 2004, at 22:35, Shane Wright wrote:

> 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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Shane Wright
Technical Manager
eDigitalResearch.com
2 Berrywood Business Village
Hedge End
Hampshire
SO30 2UN
T +44 (0) 1489 772920
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