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

From: Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++
Date: 2011-08-18 07:07:51
Message-ID: 4E4CBA47.5070600@catalyst.net.nz (view raw or flat)
Thread:
Lists: pgsql-performance
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


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