Date: Mon, 31 Mar 2003 23:10:37 +0200 From: Jens Rehsack <rehsack@liwing.de> To: Jason Andresen <jandrese@mitre.org> Cc: Lukas Ertl <l.ertl@univie.ac.at> Subject: Re: vinum performance Message-ID: <3E88AECD.10607@liwing.de> References: <20030330125138.K23911@leelou.in.tern> <3E870CC7.5000204@mac.com> <20030330175605.E23911@leelou.in.tern> <3E87204C.5060304@ludd.luth.se> <3E88524A.1060600@mitre.org>
next in thread | previous in thread | raw e-mail | index | archive | help
Jason Andresen wrote: > 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. Remember for that case that a block covered by the raid-system may be larger than 512 bytes. I use 32K for my fileserver, so to skip reading old data I had to write 32K blocks at once. Of course, the system software (either vinum or the controller software) caches a little bit, so if you write enough small data you may get a 32K block (or whatever you use), full. > I get 4565 K/sec on modern ATA/133 HDDs. > > Reading is much better at 91908 K/sec at least. > So long, Jens
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?3E88AECD.10607>