Re: pb with big volumes

Wouldn't IO contention make for additive timings instead of exponential?

On 8/10/23 20:41, Adam Scott wrote:
> I think your concern is that 20 min + 30 min does not equal 3 hours.
>
> It might be natural to think the contention would, at max, be 50 min x 2
> (1 hr 40 min).
>
> So what's going on?
>
> It seems disk I/O  is a primary suspect since you hint for an iostat
> replacement inside of Postgres.
>
> If it is due to disk I/O the resolution will be to add RAID 0 SSDs at best.
>
> Consider looking at io stats on the container's persistent volumes.
>
> What is the pipe connecting the database server to the disks?  If it's
> NAS, well that would explain it.
>
> HTH,
> Adam
>
>
>
> On Thu, Aug 10, 2023 at 2:37 PM Marc Millas <marc(dot)millas(at)mokadb(dot)com> wrote:
>
> Hi,
>
> I have a 15 TB db on postgres 14 (soon 15).
> shared buffers is 32 GB.
>
> It's a db with max 15 users and often less, and currently 1 or 2.
> the biggest table have 133 partitions of 150M to 200M+ rows each.
> lots of request access explicitly one of those.
>
> When I, alone, run a query "reading" 15M buffers, it takes 20 minutes
> (+-5minutes). inside the query there are 2 group by on a 200M rows
> partition, with all the rows in each group by.
> When a colleague run the same kind of request (not the same request,
> but something reading roughly the same volume ) , on a different set
> of data,   his request is completed in less than half an hour.
> If we run our requests simultaneously... my request take hours. around
> 3 hours.
>
> I am making a supposition that its some kind of "pumping" effect in
> the cache.
>
> I cannot have access to the underlying OS. I can, for sure, do some
> copy xx from program 'some command',  but its a container with very
> limited possibilities, not even 'ps'.
> So I would like to monitor from inside the db (so without iostat and
> the same)  the volumes of read that postgres do to the OS.
> I did activate track_io_timing, but the volumes I get in the explain
> analyze buffer are roughly the same alone or not alone. (the 15M
> buffers told )
> to my understanding, the volumes that are shown in pg_stat_database
> are the useful ones ie. even if the db as to read it from disk more
> than once. true ? or false ?
>
> So.. either my supposition is not correct, and I will read with a lot
> of interest other ideas
> either its correct and I would like to know how to monitor this (in
> the current context, installing a dedicated extension is not
> impossible, but is a very boring process)
>
> Thanks for your help :-)
>
> regards,
>
> PS: I know that providing the complete data model and the exact
> requests can be considered mandatory, but when I change the request I
> get the very same behaviour...
>
>
>
> Marc MILLAS
> Senior Architect
> +33607850334
> www.mokadb.com <http://www.mokadb.com>
>

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Born in Arizona, moved to Babylonia.