Four Drive RAID-5 on RAIDFrame Considered Harmful...

I'm building up a new server to replace my Cobalt RaQ2+ running NetBSD. 
  I got a Mini-ITX board a 1U case and hung 4 drives off it (actually 5 
while I bring it up, but the 5th will go away once the raidset is 
properly setup, and there's not much IO to it)

The drives are all 400GB Seagate drives, and because of the I/O on the 
Via EN-15000G, 2 are SATA and 2 are IDE (ide drives on their own channel).

Setting it up initially, I raided the 4 drives together with two 
partitions on each of the components: a small one for RAID-1 to load the 
kernel, and a large one for RAID-5.  Unfortunately, the RAID-5 had 
horrible performance: 2-3MB/sec sometimes and never higher than about 
12MB/sec.

I tracked it down to the problem that 2 and 3 are relatively prime :-)
Given 4 drives in a RAID-5 setup, you get 3 data blocks per raid stripe, 
but the filesystem block size Must be a power of 2, so you always have 
to write at least one partial stripe.

I'm still doing testing, but at one point I saw differences in 
performance between 3 (+1 spare) and 4 drive RAID-5 sets of 20:1.  That 
is, adding the 4th drive into the RAID set caused performance to drop by 
a factor of 20.

So far, it looks like the best overall performance I'm getting is with 3 
drives, 32 SectsPerSU (for 32K Stripes) with a filesystem block size 
also of 32K.

I'm sort of disappointed at losing 25% of my storage, but the 
performance loss just isn't worth it.  Would a hardware RAID card have 
these issues, or do they do tricks with buffering or something to get 
around it?

Once I finish testing, I'll post my results.

Thanks,

Robert
-- 
Robert Thille                7575 Meadowlark Dr.; Sebastopol, CA 95472
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