Date: Tue, 4 Apr 1995 16:48:45 -0400 (EDT) From: Peter Dufault <dufault@hda.com> To: rgrimes@gndrsh.aac.dev.com (Rodney W. Grimes) Cc: hackers@FreeBSD.org Subject: Re: large filesystems/multiple disks [RAID] Message-ID: <199504042048.QAA00922@hda.com> In-Reply-To: <199504041720.KAA07847@gndrsh.aac.dev.com> from "Rodney W. Grimes" at Apr 4, 95 10:20:52 am
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Rodney W. Grimes writes: > > > > > > RAID does have the negative effect of of having to write 20% more data, > > > thus cutting effective bandwidth by 20%. It is actually worse than > > > this in that all writes must write to at least 2 drives no matter how > > > small they are. The removes some of the benifits of stripping. > > > > And that is why some RAID systems use (battery backed up please ;-) RAM > > caches. This works quite nicely. > > And you find these caches will fill up and some point in a sustained > write test and you end up right back at the 20% performance loss I > was talking about. > > Pure stripping of drives always outperforms RAID, you always pay some > price for reliability, and it is usually performance or $$$. I'm not sure what you mean here. You don't always need to suffer the performance loss if you're willing to suffer with the data density loss. With a fast channel to the array and dedicated hardware driving the disks and calculating the parity you should be able to get close to N times the throughput while suffering while losing 1/(N+2) of the potential storage, where N is something like 8 and I'm assuming a parity drive and hot standby. You're paying again but not in throughput, unless you are comparing this with a 10 way stripe. -- Peter Dufault Real Time Machine Control and Simulation HD Associates, Inc. Voice: 508 433 6936 dufault@hda.com Fax: 508 433 5267
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