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

On Wed, Dec 17, 2014 at 1:34 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
>
>
> 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.

I think that neither pg_control nor xl_parameter_change need to have the info
about WAL compression because each backup block has that entry.

Will review the remaining part later.

Regards,

--
Fujii Masao