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
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