Re: ZFS and Postgresql - WASRe: Best OS for Postgres 8.2

On Wed, 9 May 2007, Jignesh Shah wrote:

> But we still pay the penalty on WAL while writing them in the first place I
> guess .. Is there an option to disable it.. I can test how much is the impact
> I guess couple of %s but good to verify :-) )

on modern CPU's where the CPU is significantly faster then RAM,
calculating a checksum is free if the CPU has to touch the data anyway
(cycles where it would be waiting for a cache miss are spent doing the
calculations)

if you don't believe me, hack the source to remove the checksum and see if
you can measure any difference.

David Lang

  >
> Regards,
> Jignesh
>
>
> Alvaro Herrera wrote:
>>  Jignesh Shah escribió:
>>
>>
>> >  Now comes the thing that I am still exploring
>> >  * Do we do checksum in WAL ? I guess we do .. Which means that we are
>> >  now doing double checksumming on the data. One in ZFS and one in
>> >  postgresql. ZFS does allow checksumming to be turned off (but on new
>> >  blocks allocated). But of course the philosophy is where should it be
>> >  done (ZFS or PostgreSQL).
>> >
>>
>>  Checksums on WAL are not optional in Postgres, because AFAIR they are
>>  used to determine when it should stop recovering.
>>
>>
>
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