Re: [REVIEW] Re: Compression of full-page-writes

Hello,

>Well, the larger question is why wouldn't we just have the user compress
>the entire WAL file before archiving --- why have each backend do it?
>Is it the write volume we are saving?

IIUC, the idea here is to not only save the on disk size of WAL but to
reduce the overhead of flushing WAL records to disk in servers with heavy
write operations. So yes improving the performance by saving write volume
is a part of the requirement.

Thank you,
Rahila Syed

On Fri, Dec 12, 2014 at 7:48 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> On Fri, Dec 12, 2014 at 08:27:59AM -0500, Robert Haas wrote:
> > On Thu, Dec 11, 2014 at 11:34 AM, Bruce Momjian <bruce(at)momjian(dot)us>
> wrote:
> > >> compression = 'on' : 1838 secs
> > >> = 'off' : 1701 secs
> > >>
> > >> Different is around 140 secs.
> > >
> > > OK, so the compression took 2x the cpu and was 8% slower. The only
> > > benefit is WAL files are 35% smaller?
> >
> > Compression didn't take 2x the CPU. It increased user CPU from 354.20
> > s to 562.67 s over the course of the run, so it took about 60% more
> > CPU.
> >
> > But I wouldn't be too discouraged by that. At least AIUI, there are
> > quite a number of users for whom WAL volume is a serious challenge,
> > and they might be willing to pay that price to have less of it. Also,
> > we have talked a number of times before about incorporating Snappy or
> > LZ4, which I'm guessing would save a fair amount of CPU -- but the
> > decision was made to leave that out of the first version, and just use
> > pg_lz, to keep the initial patch simple. I think that was a good
> > decision.
>
> Well, the larger question is why wouldn't we just have the user compress
> the entire WAL file before archiving --- why have each backend do it?
> Is it the write volume we are saving? I though this WAL compression
> gave better performance in some cases.
>
> --
> Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
> EnterpriseDB http://enterprisedb.com
>
> + Everyone has their own god. +
>