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

On 2014-12-12 08:27:59 -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.

And it might actually result in *higher* performance in a good number of
cases if the the WAL flushes are a significant part of the cost.

IIRC he test used a single process - that's probably not too
representative...

> 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.

Agreed.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services