Re: Sv: Re: Sv: Re: Sv: Re: Sv: Re: data-checksums

On 2018-01-09 20:51:17 -0500, Stephen Frost wrote:
> Greetings,
>
> * Andreas Joseph Krogh (andreas(at)visena(dot)com) wrote:
> > 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.
>
> Yes, having checksums enabled forces logging of hint bits. You can
> enable wal_log_hints independently too, without having checksums, to see
> what kind of an impact it'll have on your environment.
>
> A useful documentation update might be:
>
> ---
> With checksums enabled, wal_log_hints <link to the GUC's documentation>
> will be enabled and each page read or write will involve calculating the
> checksum for the page.
> ---
>
> I'd probably just replace the "Enabling checksums may incur a noticeable
> performance penalty" with the above, as it should be clear that doing
> more work implies an impact on performance and that avoids the whole
> question of trying to characterize in a general way something that can't
> be generalized (as it's workload dependent).

-1. I think this is underplaying the cost.

Greetings,

Andres Freund