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

On Fri, Dec 12, 2014 at 03:22:24PM +0100, Andres Freund wrote:
> 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?

Well, you can already set up SSL for compression while streaming.  In
fact, I assume many are already using SSL for streaming as the majority
of SSL overhead is from connection start.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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