From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
To: Ogden <lists@darkstatic.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++
Date: Fri, 19 Aug 2011 12:52:20 +1200
Message-ID: <4E4DB3C4.5020307@catalyst.net.nz> (raw)
In-Reply-To: <CB8B0873-30DF-4B37-947C-093714543F62@darkstatic.com>
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<75A85649-2B95-43D5-8404-4776D2E580EC@darkstatic.com>
<4E4C2FB4.4010409@2ndQuadrant.com>
<207E881B-73CD-4C31-A863-2C238838C41F@darkstatic.com>
<4E4CA4BC.9050908@ringerc.id.au>
<4E4CBA47.5070600@catalyst.net.nz>
<CB8B0873-30DF-4B37-947C-093714543F62@darkstatic.com>
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
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To: pgsql-performance@postgresql.org
Cc: mark.kirkwood@catalyst.net.nz, lists@darkstatic.com
Subject: Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++
In-Reply-To: <4E4DB3C4.5020307@catalyst.net.nz>
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