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
