Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++

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
andgood 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'sbattery 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
hasenough charge, so setting 'writeback' property on arrays is safe. The PERC/SERVERRAID cards I'm familiar (LSI
Megaraidrebranded models) all switch to write-though mode if they detect the battery is dangerously discharged so this
isnot 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