Re: Compression of full-page-writes

On Fri, Jan 2, 2015 at 2:11 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> , I now see the compression patch as something that has negatives, so
>> has to be set by the user, and only wins in certain cases. I am
>> disappointed, and am trying to figure out how this became such a
>> marginal win for 9.5. :-(
>
> I find the notion that a multi digit space reduction is a "marginal win"
> pretty ridiculous and way too narrow focused. Our WAL volume is a
> *significant* problem in the field. And it mostly consists out of FPWs
> spacewise.

One thing I'd like to point out, is that in cases where WAL I/O is an
issue (ie: WAL archiving), usually people already compress the
segments during archiving. I know I do, and I know it's recommended on
the web, and by some consultants.

So, I wouldn't want this FPW compression, which is desirable in
replication scenarios if you can spare the CPU cycles (because of
streaming), adversely affecting WAL compression during archiving.

Has anyone tested the compressability of WAL segments with FPW compression on?

AFAIK, both pglz and lz4 output should still be compressible with
deflate, but I've never tried.