Message-ID: <4575864A.8060005@modgraph-usa.com>
Date: Tue, 05 Dec 2006 06:46:34 -0800
From: "Craig A. James" <cjames@modgraph-usa.com>
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To: pgsql-performance@postgresql.org
Subject: Re: Bad iostat numbers
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Alex Turner wrote:
> The problem I see with software raid is the issue of a battery backed 
> unit: If the computer loses power, then the 'cache' which is held in 
> system memory, goes away, and fubars your RAID.

I'm not sure I see the difference.  If data are cached, they're not written whether it is software or hardware RAID.  I guess if you're writing RAID 1, the N disks could be out of sync, but the system can synchronize them once the array is restored, so that's no different than a single disk or a hardware RAID.  If you're writing RAID 5, then the blocks are inherently error detecting/correcting, so you're still OK if a partial write occurs, right?

I'm not familiar with the inner details of software RAID, but the only circumstance I can see where things would get corrupted is if the RAID driver writes a LOT of blocks to one disk of the array before synchronizing the others, but my guess (and it's just a guess) is that the writes to the N disks are tightly coupled.

If I'm wrong about this, I'd like to know, because I'm using software RAID 1 and 1+0, and I'm pretty happy with it.

Craig