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
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