Re: Raid 10 chunksize

On Wed, 1 Apr 2009, Mark Kirkwood wrote:

> Scott Carey wrote:
>>
>> A little extra info here >> md, LVM, and some other tools do not allow the
>> file system to use write barriers properly.... So those are on the bad list
>> for data integrity with SAS or SATA write caches without battery back-up.
>> However, this is NOT an issue on the postgres data partition. Data fsync
>> still works fine, its the file system journal that might have out-of-order
>> writes. For xlogs, write barriers are not important, only fsync() not
>> lying.
>>
>> As an additional note, ext4 uses checksums per block in the journal, so it
>> is resistant to out of order writes causing trouble. The test compared to
>> here was on ext4, and most likely the speed increase is partly due to that.
>>
>>
>
> [Looks at Stef's config - 2x 7200 rpm SATA RAID 0] I'm still highly
> suspicious of such a system being capable of outperforming one with the same
> number of (effective) - much faster - disks *plus* a dedicated WAL disk
> pair... unless it is being a little loose about fsync! I'm happy to believe
> ext4 is better than ext3 - but not that much!

given how _horrible_ ext3 is with fsync, I can belive it more easily with
fsync turned on than with it off.

David Lang

> However, its great to have so many different results to compare against!
>
> Cheers
>
> Mark
>
>
>