Re: Compression of full-page-writes

On Thu, Jan 1, 2015 at 2:39 AM, Bruce Momjian <bruce@momjian.us> wrote:
>
> On Tue, Dec 30, 2014 at 01:27:44PM +0100, Andres Freund wrote:
> > On 2014-12-30 21:23:38 +0900, Michael Paquier wrote:
> > > On Tue, Dec 30, 2014 at 6:21 PM, Jeff Davis <pgsql@j-davis.com> wrote:
> > > > On Fri, 2013-08-30 at 09:57 +0300, Heikki Linnakangas wrote:
> > > >> Speeding up the CRC calculation obviously won't help with the WAL volume
> > > >> per se, ie. you still generate the same amount of WAL that needs to be
> > > >> shipped in replication. But then again, if all you want to do is to
> > > >> reduce the volume, you could just compress the whole WAL stream.
> > > >
> > > > Was this point addressed?
> > > Compressing the whole record is interesting for multi-insert records,
> > > but as we need to keep the compressed data in a pre-allocated buffer
> > > until WAL is written, we can only compress things within a given size
> > > range. The point is, even if we define a  lower bound, compression is
> > > going to perform badly with an application that generates for example
> > > many small records that are just higher than the lower bound...
> > > Unsurprisingly for small records this was bad:
> >
> > So why are you bringing it up? That's not an argument for anything,
> > except not doing it in such a simplistic way.
>
> I still don't understand the value of adding WAL compression, given the
> high CPU usage and minimal performance improvement.  The only big
> advantage is WAL storage, but again, why not just compress the WAL file
> when archiving.
>
> I thought we used to see huge performance benefits from WAL compression,
> but not any more?