Date: Mon, 01 Nov 2004 10:58:18 -0500 From: Richard Coleman <rcoleman@criticalmagic.com> To: Brad Knowles <brad@stop.mail-abuse.org> Cc: current@freebsd.org Subject: Re: Gvinum RAID5 performance Message-ID: <41865D1A.7080002@criticalmagic.com> In-Reply-To: <p06002006bdabc1160a6a@[10.0.1.3]> References: <002401c4bf9c$c4fee8e0$0201000a@riker> <p06002002bdab24905ad8@[10.0.1.3]> <1099286568.4185c82881654@picard.newmillennium.net.au> <p06002006bdabc1160a6a@[10.0.1.3]>
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Brad Knowles wrote: > Keep in mind that if you've got a five disk RAID-5 array, then for > any given block, four of those disks are data and would have to be > accessed on every read operation anyway, and only one disk would be > parity. The more disks you have in your RAID array, the lower the > parity to data ratio, and the less benefit you would get from > checking parity in background. My understanding is that this is true only when the system is running in a degraded mode. When you blow a disk, instead of hitting that disk, you access all the other disks and calculate the desired block using the parity. If this system is healthy, a single disk access should be sufficient. If a system with a 9 disk raid5 volume (pretty common) had to hit each disk on a block access, it would majorly suck. I think detecting and handling bad physical blocks is a whole other matter. Richard Coleman rcoleman@criticalmagic.com
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