Re: Compression of full-page-writes

On Thu, Jan 1, 2015 at 12:03 PM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
wrote:
> On Thu, Jan 1, 2015 at 2:10 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > On Thu, Jan 1, 2015 at 2:39 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> >> > 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.
> When doing some tests with pgbench for a fixed number of transactions,
> I also noticed a reduction in replay time as well, see here for
> example some results here:
>
http://www.postgresql.org/message-id/CAB7nPqRv6RaSx7hTnp=g3dYqOu++FeL0UioYqPLLBdbhAyB_jQ@mail.gmail.com
>
> >> I thought we used to see huge performance benefits from WAL
compression,
> >> but not any more?
> >
> > I think there can be performance benefit for the cases when the data
> > is compressible, but it would be loss otherwise. The main thing is
> > that the current compression algorithm (pg_lz) used is not so
> > favorable for non-compresible data.
> Yes definitely. Switching to a different algorithm would be the next
> step forward. We have been discussing mainly about lz4 that has a
> friendly license, I think that it would be worth studying other things
> as well once we have all the infrastructure in place.
>
> >>Has the UPDATE WAL compression removed that benefit?
> >
> > Good question, I think there might be some impact due to that, but in
> > general for page level compression still there will be much more to
> > compress.
> That may be a good thing to put a number on. We could try to patch a
> build with a revert of a3115f0d and measure a bit that the difference
> in WAL size that it creates. Thoughts?
>

You can do that, but what inference you want to deduce from it?
I think there can be some improvement in performance as well as
compression depending on the tests (if your tests involves lot of
Updates, then you might see some better results), however the
results will be more or less on similar lines.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com