From: Craig A. James <cjames@modgraph-usa.com>
To: pgsql-performance@postgresql.org
Subject: Re: Bad iostat numbers
Date: Tue, 05 Dec 2006 06:46:34 -0800
Message-ID: <4575864A.8060005@modgraph-usa.com> (raw)
In-Reply-To: <33c6269f0612050457s4a8eb0eas662d7a50fb5dfa89@mail.gmail.com>
References: <PEEPKDFEHHEMKBBFPOOKKEHCDKAA.carlos.reimer@opendb.com.br>
<Pine.GSO.4.64.0612032336250.19679@westnet.com>
<33c6269f0612032317t9f0bc77n3ff791a5cd88fc80@mail.gmail.com>
<Pine.GSO.4.64.0612041946120.9323@westnet.com>
<33c6269f0612042221x5820e98bl96f8d0dc4a87bd3f@mail.gmail.com>
<20061205121507.GZ1622@mathom.us>
<33c6269f0612050457s4a8eb0eas662d7a50fb5dfa89@mail.gmail.com>
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
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To: pgsql-performance@postgresql.org
Cc: cjames@modgraph-usa.com
Subject: Re: Bad iostat numbers
In-Reply-To: <4575864A.8060005@modgraph-usa.com>
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