On 19/08/11 02:09, Ogden wrote:
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?
Hmm - that output looks different from the cards I'm familiar with. I'd
want to see the manual entries for "Cache Policy=Not Applicable" and
"Disk Cache Policy=Enabled" to understand what the settings actually
mean. Assuming "Disk Cache Policy=Enabled" means what I think it does
(i.e writes are cached in the physical drives cache), this setting seems
wrong if your card has on board cache + battery, you would want to only
cache 'em in the *card's* cache (too many caches to keep straight in
one's head, lol).
Cheers
Mark
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