Date: Mon, 02 May 2005 10:48:44 -0400 From: asym <bsdlists@rfnj.org> To: Arne =?iso-8859-1?Q?=22W=F6rner=22?= <arne_woerner@yahoo.com> Cc: freebsd-performance@freebsd.org Subject: Re: Very low disk performance on 5.x Message-ID: <6.2.1.2.2.20050502104554.03703c10@mail.rfnj.org> In-Reply-To: <20050502143819.49818.qmail@web41207.mail.yahoo.com> References: <20050502143819.49818.qmail@web41207.mail.yahoo.com>
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At 10:38 5/2/2005, Arne "W=F6rner" wrote: >--- Allen <bsdlists@rfnj.org> wrote: > > Scenario B, verified read enabled: > > 1. RAID card reads up ALL blocks in the stripe (5 reads). > > 2. RAID card pretends the block requested is on a "degraded" > > drive, and > > calculates it from the other 3 + the XOR stripe. > > 3. RAID card reports the value back, or tosses some kind of > > error. > > > > You can see, the cache just doesn't play a part in what I was > > describing, > > which is basically the array performing as though it is degraded > > when in > > fact it is not, to catch failures that would otherwise be > > missed. > > >That would be a funny implementation. Step one could be done >mostly from cache in case of sequential reads from a device (like >/dev/ad0s1f or so). In this thread we always looked at sequential >reads, as far as I recall... It's not "funny" it's just something some people do to try to catch more=20 errors. RAID5 will not catch transient write errors by default. If there was say=20 noise on the bus, and so the wrong value was written to the disk after the= =20 XOR was performed, but the right XOR data was written to the disk. RAID5 would normally never catch such an error, unless you run it in a=20 verified mode where it always behaves as though it is degraded, which is=20 what I was describing. Why you would, I'm not sure. It would catch the errors I suppose, but=20 there's nothing it can do about them -- it can't know if the XOR data or=20 the actual data is corrupt. Detection without correction is better than=20 nothing though, and if performance isn't your real goal, you can turn on=20 such features in some cards.
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