On 2014-12-12 09:18:01 -0500, Bruce Momjian 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@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.
Err. Streaming?
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
