Re: [REVIEW] Re: Compression of full-page-writes - Mailing list pgsql-hackers

On Wed, Dec 17, 2014 at 1:34 AM, Michael Paquier
<michael.paquier@gmail.com> wrote:
>
>
> On Wed, Dec 17, 2014 at 12:00 AM, Michael Paquier
> <michael.paquier@gmail.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