I have change stripe cache size from   4096 stripe cache to  8192, the test result show the performance improve <5%, maybe The effect is not very obvious。


------------------------------------------------------------------
发件人：Stan Hoeppner [off-list ref]
发送时间：2013年11月28日(星期四) 12:41
收件人：lilofile [off-list ref]; Linux RAID [off-list ref]
主　题：Re: 答复：md raid5 performace 6x SSD RAID5

On 11/27/2013 7:51 AM, lilofile wrote:

additional: CPU: Intel(R) Xeon(R) CPU           X5650  @ 2.67GHz
                memory:32GB

...

when I create raid5 which use six SSD(sTEC s840),
when the stripe_cache_size is set 4096. 
root@host1:/sys/block/md126/md# cat /proc/mdstat 
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md126 : active raid5 sdg[6] sdf[4] sde[3] sdd[2] sdc[1] sdb[0]
      3906404480 blocks super 1.2 level 5, 128k chunk, algorithm 2 [6/6] [UUUUUU]

the single ssd read/write performance :

root@host1:~# dd if=/dev/sdb of=/dev/zero count=100000 bs=1M
^C76120+0 records in
76119+0 records out
79816556544 bytes (80 GB) copied, 208.278 s, 383 MB/s

root@host1:~# dd of=/dev/sdb if=/dev/zero count=100000 bs=1M
100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 232.943 s, 450 MB/s

the raid read and write performance is  approx 1.8GB/s read and 1.1GB/s write performance
root@sc0:/sys/block/md126/md# dd if=/dev/zero of=/dev/md126 count=100000 bs=1M
100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 94.2039 s, 1.1 GB/s


root@sc0:/sys/block/md126/md# dd of=/dev/zero if=/dev/md126 count=100000 bs=1M
100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 59.5551 s, 1.8 GB/s

why the performance is so bad?  especially the write performace.

There are 3 things that could be, or are, limiting performance here.

1.  The RAID5 write thread peaks one CPU core as it is single threaded
2.  A 4KB stripe cache is too small for 6 SSDs, try 8KB
3.  dd issues IOs serially and will thus never saturate the hardware

#1 will eventually be addressed with a multi-thread patch to the various
RAID drivers including RAID5.  There is no workaround at this time.

To address #3 use FIO or a similar testing tool that can issue IOs in
parallel.  With SSD based storage you will never reach maximum
throughput with a serial data stream.

-- 
Stan
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