Date: Tue, 4 Apr 1995 14:36:04 -0700 (PDT) From: "Rodney W. Grimes" <rgrimes@gndrsh.aac.dev.com> To: dufault@hda.com (Peter Dufault) Cc: hackers@FreeBSD.org Subject: Re: large filesystems/multiple disks [RAID] Message-ID: <199504042136.OAA08422@gndrsh.aac.dev.com> In-Reply-To: <199504042048.QAA00922@hda.com> from "Peter Dufault" at Apr 4, 95 04:48:45 pm
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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. The problem is with RAID to have the reliabilty of any 1 drive going bad means you must write data to at least 2 drives for all write opertions. This means unless you greatly increase the density of your storage by going to mirrors you are going to lose performance. > > 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'll never get N times the throughput because you always have to write to 2 drives to keep the parity data up, thus your bandwidth increase is more like (N/2). I agree that the time loss for parity calculations is near zero. To achive a N factor performance increase you must go to N * 10 drives using RAID :-(, a very large cost hit. > You're paying again but not in throughput, unless you are comparing this > with a 10 way stripe. A 5 wide stripe will have better performance (N=5) than a 5 drive raid system (N=5/2=2.5). -- Rod Grimes rgrimes@gndrsh.aac.dev.com Accurate Automation Company Custom computers for FreeBSD
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