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

On Wed, Dec 17, 2014 at 12:00 AM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com
> wrote:
>
> Actually, the original length of the compressed block in saved in
> PGLZ_Header, so if we are fine to not check the size of the block
> decompressed when decoding WAL we can do without the hole filled with
> zeros, and use only 1 bit to see if the block is compressed or not.
>
And.. After some more hacking, I have been able to come up with a patch
that is able to compress blocks without the page hole, and that keeps the
WAL record length the same as HEAD when compression switch is off. The
numbers are pretty good, CPU is saved in the same proportions as previous
patches when compression is enabled, and there is zero delta with HEAD when
compression switch is off.

Here are the actual numbers:
test_name | pg_size_pretty | user_diff | system_diff
-------------------------------+----------------+-----------+-------------
FPW on + 2 bytes, ffactor 50 | 582 MB | 42.391894 | 0.807444
FPW on + 2 bytes, ffactor 20 | 229 MB | 14.330304 | 0.729626
FPW on + 2 bytes, ffactor 10 | 117 MB | 7.335442 | 0.570996
FPW off + 2 bytes, ffactor 50 | 746 MB | 25.330391 | 1.248503
FPW off + 2 bytes, ffactor 20 | 293 MB | 10.537475 | 0.755448
FPW off + 2 bytes, ffactor 10 | 148 MB | 5.762775 | 0.763761
FPW on + 0 bytes, ffactor 50 | 582 MB | 42.174297 | 0.790596
FPW on + 0 bytes, ffactor 20 | 229 MB | 14.424233 | 0.770459
FPW on + 0 bytes, ffactor 10 | 117 MB | 7.057195 | 0.584806
FPW off + 0 bytes, ffactor 50 | 746 MB | 25.261998 | 1.054516
FPW off + 0 bytes, ffactor 20 | 293 MB | 10.589888 | 0.860207
FPW off + 0 bytes, ffactor 10 | 148 MB | 5.827191 | 0.874285
HEAD, ffactor 50 | 746 MB | 25.181729 | 1.133433
HEAD, ffactor 20 | 293 MB | 9.962242 | 0.765970
HEAD, ffactor 10 | 148 MB | 5.693426 | 0.775371
Record, ffactor 50 | 582 MB | 54.904374 | 0.678204
Record, ffactor 20 | 229 MB | 19.798268 | 0.807220
Record, ffactor 10 | 116 MB | 9.401877 | 0.668454
(18 rows)

The new tests of this patch are "FPW off + 0 bytes". Patches as well as
results are attached.
Regards,
--
Michael