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

On 2014-08-18 13:06:15 -0400, Robert Haas wrote:
> On Mon, Aug 18, 2014 at 7:19 AM, Rahila Syed <rahilasyed90@gmail.com> wrote:
> >>According to the measurement result, the amount of WAL generated in
> >>"Multiple Blocks in one run" than that in "Single Block in one run".
> >>So ISTM that compression of multiple blocks at one run can improve
> >>the compression ratio. Am I missing something?
> >
> > Sorry for using unclear terminology. WAL generated here means WAL that gets
> > generated in each run without compression.
> > So, the value WAL generated in the  above measurement is uncompressed WAL
> > generated to be specific.
> > uncompressed WAL = compressed WAL  + Bytes saved.
> >
> > Here, the measurements are done for a constant amount of time rather than
> > fixed number of transactions. Hence amount of WAL generated does not
> > correspond to compression ratios of each algo. Hence have calculated bytes
> > saved in order to get accurate idea of the amount of compression in each
> > scenario and for various algorithms.
> >
> > Compression ratio i.e Uncompressed WAL/compressed WAL in each of the above
> > scenarios are as follows:
> >
> > Compression algo       Multiple Blocks in one run    Single Block in one run
> >
> > LZ4                              1.21                                   1.27
> >
> > Snappy                        1.19                                   1.25
> >
> > pglz                             1.14                                   1.16
> >
> > This shows compression ratios of both the scenarios Multiple blocks and
> > single block  are nearly same for this benchmark.
> 
> I don't agree with that conclusion.  The difference between 1.21 and
> 1.27, or between 1.19 and 1.25, is quite significant.  Even the
> difference beyond 1.14 and 1.16 is not trivial.  We should try to get
> the larger benefit, if it is possible to do so without an unreasonable
> effort.

Agreed.

One more question: Do I see it right that multiple blocks compressed
together compress *worse* than compressing individual blocks? If so, I
have a rather hard time believing that the patch is sane.

Greetings,

Andres Freund

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