On Aug 18, 2011, at 2:07 AM, Mark Kirkwood wrote: > On 18/08/11 17:35, Craig Ringer wrote: >> On 18/08/2011 11:48 AM, Ogden wrote: >>> Isn't this very dangerous? I have the Dell PERC H700 card - I see that it has 512Mb Cache. Is this the same thing and good enough to switch to nobarrier? Just worried if a sudden power shut down, then data can be lost on this option. >>> >>> >> Yeah, I'm confused by that too. Shouldn't a write barrier flush data to persistent storage - in this case, the RAID card's battery backed cache? Why would it force a RAID controller cache flush to disk, too? >> >> > > If the card's cache has a battery, then the cache is preserved in the advent of crash/power loss etc - provided it has enough charge, so setting 'writeback' property on arrays is safe. The PERC/SERVERRAID cards I'm familiar (LSI Megaraid rebranded models) all switch to write-though mode if they detect the battery is dangerously discharged so this is not normally a problem (but commit/fsync performance will fall off a cliff when this happens)! > > Cheers > > Mark So a setting such as this: Device Name : /dev/sdb Type : SAS Read Policy : No Read Ahead Write Policy : Write Back Cache Policy : Not Applicable Stripe Element Size : 64 KB Disk Cache Policy : Enabled Is sufficient to enable nobarrier then with these settings? Thank you Ogden -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
