On Wed, 6 Jun 2007, Daniel Korstad wrote: > You say you have a RAID with three drive (I assume RAID5) with a read > performance of 133MB/s. There are lots of variables, file system > type, cache tuning, but that sounds very reasonable to me. I just did the math quickly - assuming each drive can sustain (give or take) 70MB/s, and I'm using a 3-drive raid5, then (block device) reads will only use 2 of those 3 drives resulting in a maximum of 140MB/s. I've measured (consistently) 133MB/s using dd (again, with iflag=direct) and a blocksize of 768K, so I'm OK with that. A question posed by a friend is this: assuming 64K blocks, if I read a single stripe from a raid (128K, right?) then two drives will be used (say, drive A and drive B). If I want the "next" 128K then which drives are most likely to be used? Drive A will now have the parity block in it's "next" block for head position, but drive B has the next data block. Nobody knows where drive C is. Does raid5 use an algorithm similar to raid1 in that it chooses the 2 drives whose heads are closest or does it utilize some other algorithm? > Here is a site with some test for RAID5 and 8 drives in the set using > high end hardware raid. > http://www.rhic.bnl.gov/hepix/talks/041019pm/schoen.pdf > 8 drives RAID 5 7200 rpm SATA drives = ~180MB/s I would have expected a peak transfer rate (sustained, block, linear access) of MAX(8*70,533) which is 533MB/s, assuming 64bit, 66MHz bus and 70MB/s drives. I recently completed a series of some 800+ tests on a 4 disk raid5 varying the I/O scheduler, readahead of the components, readahead of the raid, bitmap present or not, and filesystem and arrived at some fairly interesting results. I hope to throw them together in a more usable form in the near future. -- Jon Nelson <jnelson-linux-raid@xxxxxxxxxxx> - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
