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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 <tls%panix.com@localhost> 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newfs(8) says -a is obsolete...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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