Re: Slow Soft-RAID 5 performance
- From: "J.A. Magallón" <jamagallon@xxxxxxx>
- Date: Wed, 18 Jul 2007 16:49:21 +0200
On Wed, 18 Jul 2007 10:56:11 +0100, Rui Santos <rsantos@xxxxxxxxxxxx> wrote:
Hi,...
I'm getting a strange slow performance behavior on a recently installed
Server. Here are the details:
...
I can get a write throughput of 60 MB/sec on each HD by issuing the
command 'time `dd if=/dev/zero of=test.raw bs=4k count=$(( 1024 * 1024 /
4 )); sync`'
The RAID device I'm testing on is /dev/md2. Now, by issuing the same
command 'dd if=/dev/zero of=test.raw bs=4k count=$(( 1024 * 1024 / 4 ));
sync`' on the raid device mount point, I get the following speeds:
With stripe_cache_size at default '265': 51 MB/sec
With stripe_cache_size at '8192': 73 MB/sec
I know many people consider this stupid, but can you post some hdparm -tT
data ?
The culprit can be the filesystem+pagecache, the md driver or the disk
driver, so I think trying just hdparm will show if the disk o md are
going nuts...
In my case, I have a box with 2 raids, one with SCSI disks and one with
IDE ones.
Some results:
lsscsi:
[0:0:0:0] disk IBM DDYS-T18350N S96H /dev/sda
[2:0:0:0] disk SEAGATE ST336807LW 0C01 /dev/sdb
[2:0:1:0] disk SEAGATE ST336807LW 0C01 /dev/sdc
[2:0:2:0] disk SEAGATE ST336807LW 0C01 /dev/sdd
[2:0:3:0] disk SEAGATE ST336807LW 0C01 /dev/sde
[3:0:0:0] disk ATA ST3120022A 3.06 /dev/sdf
[3:0:1:0] cd/dvd HL-DT-ST DVDRAM GSA-4040B A300 /dev/sr0
[4:0:0:0] disk ATA ST3120022A 3.76 /dev/sdg
/dev/md0:
Version : 00.90.03
Creation Time : Mon Jun 18 13:40:57 2007
Raid Level : raid5
Array Size : 107522304 (102.54 GiB 110.10 GB)
Used Dev Size : 35840768 (34.18 GiB 36.70 GB)
Raid Devices : 4
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Wed Jul 18 13:31:22 2007
State : clean
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 256K
UUID : 51ad72a7:a4d20d15:0f3ea3a1:5ccb49a0
Events : 0.2
Number Major Minor RaidDevice State
0 8 17 0 active sync /dev/sdb1
1 8 33 1 active sync /dev/sdc1
2 8 49 2 active sync /dev/sdd1
3 8 65 3 active sync /dev/sde1
This is, four scsi disks on a Adaptec U320, doing raid5:
/dev/sdb:
Timing cached reads: 904 MB in 2.00 seconds = 451.84 MB/sec
Timing buffered disk reads: 228 MB in 3.00 seconds = 75.90 MB/sec
/dev/sdc:
Timing buffered disk reads: 226 MB in 3.01 seconds = 75.01 MB/sec
/dev/sdd:
Timing buffered disk reads: 228 MB in 3.00 seconds = 75.88 MB/sec
/dev/sde:
Timing buffered disk reads: 226 MB in 3.00 seconds = 75.31 MB/sec
/dev/md0:
Timing buffered disk reads: 562 MB in 3.01 seconds = 186.88 MB/sec
Nearly 75x3 = 215 Mb/s. And this looks like a small regression, I remember
to have seen 200Mb on this setup on previous kernels.
Performance is like 186/215 = 86%.
And /dev/md1, raid0 on 2 IDE disks:
/dev/sdf:
Timing buffered disk reads: 148 MB in 3.02 seconds = 48.93 MB/sec
/dev/sdg:
Timing buffered disk reads: 124 MB in 3.00 seconds = 41.33 MB/sec
/dev/md1:
Timing buffered disk reads: 204 MB in 3.01 seconds = 67.68 MB/sec
Performance: 67 / 90 = 75%, more or less...not too good.
Now that I read the hdparm man page, perhaps would be better to repeat
the tests with hdparm --direct.
--
J.A. Magallon <jamagallon()ono!com> \ Software is like sex:
\ It's better when it's free
Mandriva Linux release 2008.0 (Cooker) for i586
Linux 2.6.21-jam12 (gcc 4.2.1 20070704 (4.2.1-3mdv2008.0)) SMP PREEMPT
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
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