Can anyone explain why a software raid5 array of N disks has
significantly lower read performance than a raid0 array of N-1 disks?
I'm only considering the case where there are no drive failures.
I'm using 8 Seagate ST317242A drives in UDMA-66 mode, with 4 Promise
Ultra-66 cards, one drive per channel. This is with linux-2.2.10
+ 2.2.10.uniform-ide-6.20.eridanus.patch from http://www.kernel.dk/ide,
+ raid0145-19990724-2.2.10.bz2 and raidtools-19990724-0.90.tar.bz2.
The only problem is that, for some reason, the system hangs in SMP mode,
but it works fine in uniprocessor mode. I don't know when I'll get
time to try fixing that problem, but for now uniprocessor is ok for
my testing.
On a Pentium II/400 I get ~60MB/s reading a file with raid0 on 6 drives,
but <40MB/s with raid5 on 8 drives.
Thanks,
Jan Edler
NEC Research Institute
