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

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?

Thank you

Ogden