From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
To: pgsql-performance@postgresql.org
Subject: Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++
Date: Thu, 18 Aug 2011 19:07:51 +1200
Message-ID: <4E4CBA47.5070600@catalyst.net.nz> (raw)
In-Reply-To: <4E4CA4BC.9050908@ringerc.id.au>
References: <959BD2FA-0312-458B-BD5F-ED358D2C47ED@darkstatic.com>
<f4e06a7cad34ee9217a3e57fd4eb0e67.squirrel@sq.gransy.com>
<D89151D3-D2E5-47E3-8B90-4A573DAB5FFA@darkstatic.com>
<75A85649-2B95-43D5-8404-4776D2E580EC@darkstatic.com>
<4E4C2FB4.4010409@2ndQuadrant.com>
<207E881B-73CD-4C31-A863-2C238838C41F@darkstatic.com>
<4E4CA4BC.9050908@ringerc.id.au>
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: mark.kirkwood@catalyst.net.nz
Subject: Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++
In-Reply-To: <4E4CBA47.5070600@catalyst.net.nz>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox