Re: hanging for 30sec when checkpointing

I've seen similar behaviour with other disk-intensive apps, and in every
case it transpired that DMA was not enabled on the relevant disk/s -
something to check, certainly.

On Tue, 3 Feb 2004, 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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--

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