On 19/08/11 12:52, Mark Kirkwood wrote:
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).
FWIW - here's what our ServerRaid (M5015) output looks like for a RAID 1
array configured with writeback, reads not cached on the card's memory,
physical disk caches disabled:
$ MegaCli64 -LDInfo -L0 -a0
Adapter 0 -- Virtual Drive Information:
Virtual Drive: 0 (Target Id: 0)
Name :
RAID Level : Primary-1, Secondary-0, RAID Level Qualifier-0
Size : 67.054 GB
State : Optimal
Strip Size : 64 KB
Number Of Drives : 2
Span Depth : 1
Default Cache Policy: WriteBack, ReadAheadNone, Direct, No Write Cache
if Bad BBU
Current Cache Policy: WriteBack, ReadAheadNone, Direct, No Write Cache
if Bad BBU
Access Policy : Read/Write
Disk Cache Policy : Disabled
Encryption Type : None
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