Date: Mon, 31 Mar 2003 09:35:54 -0500 From: Jason Andresen <jandrese@mitre.org> To: Mattias Pantzare <pantzer@ludd.luth.se> Cc: Lukas Ertl <l.ertl@univie.ac.at> Subject: Re: vinum performance Message-ID: <3E88524A.1060600@mitre.org> In-Reply-To: <3E87204C.5060304@ludd.luth.se> References: <20030330125138.K23911@leelou.in.tern> <3E870CC7.5000204@mac.com> <20030330175605.E23911@leelou.in.tern> <3E87204C.5060304@ludd.luth.se>
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Mattias Pantzare wrote: > Lukas Ertl wrote: > >> Ok. But I still don't understand why RAID 5 write performance is _so_ >> bad. >> The CPU is not the bottle neck, it's rather bored. And I don't understand >> why RAID 0 doesn't give a big boost at all. Is the ahc driver known to be >> slow? > > > To do a RAID 5 write you do this: > 1. Read the old data on the blocks that you will write to. > 2. Read the coresponding parity data. > 3. Write the new data. > 4. Write the new parity. Hmm, how about the case where you're writing new data? You shouldn't have to do steps 1 & 2, and yet the RAID5 write performance is still abysmial. I get 4565 K/sec on modern ATA/133 HDDs. Reading is much better at 91908 K/sec at least. -- \ |_ _|__ __|_ \ __| Jason Andresen jandrese@mitre.org |\/ | | | / _| Network and Distributed Systems Engineer _| _|___| _| _|_\___| Office: 703-883-7755
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