Sv: Re: Sv: Re: Sv: Re: Sv: Re: data-checksums - Mailing list pgsql-general
På onsdag 10. januar 2018 kl. 01:01:26, skrev Andres Freund <andres@anarazel.de>:
On 2018-01-10 00:25:08 +0100, Andreas Joseph Krogh wrote:
> På tirsdag 09. januar 2018 kl. 23:42:45, skrev Rob Sargent <
> robjsargent@gmail.com <mailto:robjsargent@gmail.com>>:
>
>
> On 01/09/2018 03:30 PM, Andreas Joseph Krogh wrote:
> På tirsdag 09. januar 2018 kl. 23:06:06, skrev Andres Freund <
> andres@anarazel.de <mailto:andres@anarazel.de>>:
> Hi,
>
> On 2018-01-09 21:47:17 +0100, Andreas Joseph Krogh wrote:
> > Does PG use HW-accellerated crc if CPU supports it[1]?
>
> Yes we do, for WAL checksums. The page checksums are a different
> algorithm though, one which has the advantage of being SIMD compatible.
>
> The checksum computations have some impact, but if there's bigger impact
> it's much more likely to be related to the fact that some hint bit
> writes to a page now needs to be WAL logged.
>
> But SIMD-instructions are also HW-accellerated by modern CPUs IIUC?
Sure. Still measurable, but even if weren't, it's irrelevant given my
primary point:
> The checksum computations have some impact, but if there's bigger impact
> it's much more likely to be related to the fact that some hint bit
> writes to a page now needs to be WAL logged.
which isn't mitigated by SIMD / hardware CRC / whatnot.
Aha, so enabling CRC causes hint-bits to be written causing extra WAL-logging, which woudn't be the case without CRC enabled?
Thanks for pointing that out.
-- 
Andreas Joseph Krogh
CTO / Partner - Visena AS
Mobile: +47 909 56 963
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