Re: [REVIEW] Re: Compression of full-page-writes - Mailing list pgsql-hackers

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@momjian.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@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +