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Re: Help with low raid5 performance

On Tue, 11 Jan 2011, Greg Oster wrote:
On Tue, 11 Jan 2011 08:43:12 -0500
Thor Lancelot Simon <> wrote:
I'm using a 4-disk RAID 5 with the partition starting at 45416448
(which is very well aligned (on a 65536 boundary!).

This should not be aligned on a 64k boundary with 3 data disks.  It
should be aligned on a 48K boundary!

Actually 45416448 is both on a 48k boundary and a 64k boundary.

Greg, with RAIDframe sector 0 should always be at the beginning of a
stripe, so 48K alignment with no remainder is correct, yes?

Yes, and yes. (in this case)

Which it is (by luck in this case!).

Changing to 32 1 1 5 gave:
      -------Sequential Output-------- ---Sequential Input--
--Random-- -Per Char- --Block--- -Rewrite-- -Per Char- --Block---
--Seeks--- MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec
%CPU  /sec %CPU 2048  9137  9.6  7440  3.3  3045  1.3 61801 82.3
149558 37.1 228.2  2.4

These were formatted with: newfs -O 2 -f 4096 -b 32768

This newfs command line is wrong.  32 doesn't go evenly into 48.
Remember that the RAIDframe sectors-per-stripe is in half-kilobyte

You probably want newfs -f 2048 -b 16384 -a 3.

I'm curious to see the results of the above.

newfs(8) says -a is obsolete...

I've run the tests through and it doesn't help much. RAID5 on 3 disks _is_ significantly quicker than on 4 (4x quicker for character writes and 7x for block writes). But contig 3 vs contig 4 and using -f 2048 -b 16384 make little difference (a handful of % at best).

I've ensured all partitions are 48k aligned.


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