On 10/06/2009 18:17, Steven Haigh wrote:
After a week and a bit of googling, experimenting and frustration I'm
posting here and hoping I can get some clues on what could be wrong with
my 5 disk RAID5 SATA array.
The array in question is:
md1 : active raid5 sdg1[0] sdf1[1] sde1[3] sdd1[2] sdc1[4]
1172131840 blocks level 5, 1024k chunk, algorithm 2 [5/5] [UUUUU]
All 5 drives are connection to a sil_sata controller (a 3112 & a 3114)
set up as a simple SATA controller (ie no RAID here).
Once the system buffer is full, write speeds to the array are usually
under 20MB/sec.
I am currently running CentOS 5.3 (kernel 2.6.18-128.1.10.el5.centos.plus).
I have lodged a bug report against RHEL 5.3, as I believe something is
not quite right here, but haven't been able to narrow down the exact issue.
https://bugzilla.redhat.com/show_bug.cgi?id=502499
Using bonnie++ to benchmark the array, it shows sequential block reads
at 90MB/sec but writes at 11MB/sec across the RAID5 array - a difference
I really didn't expect.
Any pointers on how to try to tackle this one and figure out the root
cause of the problem would be VERY helpful!
Do your drives rattle furiously while writing? Do you have a write
intent bitmap? I found my write performance quite sucky until changing
the bitmap settings, see
http://marc.info/?l=linux-raid&m=124027469610660&w=2 for the thread. In
short, changing the bitmap from its default 2M chunk size to 32M got me
a substantial speed-up; here's my bonnie++ output on a 3-drive RAID-5:
# bonnie++ -d /mnt/store/bonnie/ -u john -n 256 -m beast
Using uid:500, gid:500.
Writing with putc()...done
Writing intelligently...done
Rewriting...done
Reading with getc()...done
Reading intelligently...done
start 'em...done...done...done...
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version 1.03 ------Sequential Output------ --Sequential Input-
--Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block--
--Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP
/sec %CP
beast 4G 72011 79 59334 10 35775 4 66455 64 174049 6
215.8 0
------Sequential Create------ --------Random
Create--------
-Create-- --Read--- -Delete-- -Create-- --Read---
-Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
/sec %CP
256 61524 81 377753 96 22143 28 53153 73 425291 99
8327 11
beast,4G,72011,79,59334,10,35775,4,66455,64,174049,6,215.8,0,256,61524,81,377753,96,22143,28,53153,73,425291,99,8327,11
Cheers,
John.
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