Message-ID: <4E4DB3C4.5020307@catalyst.net.nz>
Date: Fri, 19 Aug 2011 12:52:20 +1200
From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
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To: Ogden <lists@darkstatic.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++
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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