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Raid Write > Read?

H

hokatichenci

Gawd
Joined
Oct 28, 2004
Messages
722
This is sort of weird, last night I noticed on my test array that I was getting 210mb/s writes and 170mb/s reads when I set it up with raid0. Now I have it setup as a rebuilt raid5 (8 hour rebuild weaksauce) and I'm getting about 130mb/s writes, and 110-115mb/s reads. The CPU is absolutely peaked out and might even be a real bottleneck here, but still - the write speeds have been significantly faster in each test with 8 disks. When I had only 4 disks, the read speeds were faster in each scenario. Does this seem weird to anybody else?

Code: 
llama raid # cat /proc/mdstat
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4]
md0 : active raid5 sdh1[7] sdg1[6] sdf1[5] sde1[4] sdd1[3] sdc1[2] sdb1[1] sda1[0]
      2173866688 blocks level 5, 64k chunk, algorithm 2 [8/8] [UUUUUUUU]

unused devices: <none>
llama raid # ls
llama raid # dd if=/dev/zero of=1gbfile bs=1024k count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 7.86339 s, 137 MB/s
llama raid # dd if=1gbfile of=/dev/null
2097152+0 records in
2097152+0 records out
1073741824 bytes (1.1 GB) copied, 9.18942 s, 117 MB/s
llama raid # dd if=1gbfile of=/dev/null
2097152+0 records in
2097152+0 records out
1073741824 bytes (1.1 GB) copied, 9.37025 s, 115 MB/s
llama raid # dd if=/dev/zero of=10gbfile bs=1024k count=10240
10240+0 records in
10240+0 records out
10737418240 bytes (11 GB) copied, 82.4836 s, 130 MB/s
llama raid # dd if=10gbfile of=/dev/null
20971520+0 records in
20971520+0 records out
10737418240 bytes (11 GB) copied, 93.3376 s, 115 MB/s
llama raid # dd if=/dev/zero of=10gbfile bs=1024k count=10240
10240+0 records in
10240+0 records out
10737418240 bytes (11 GB) copied, 83.7587 s, 128 MB/s
 
The read speeds with raid 5 is faster than the write because when it is writing the data it has to calculate the parity bits and write them as well.
 
underdone said:
The read speeds with raid 5 is faster than the write because when it is writing the data it has to calculate the parity bits and write them as well.
Click to expand...

Thank you for not reading the numbers - or my post whatsoever. The write speeds in this test are clearly faster than the read speeds. I did find the problem though - I am not using the same block size, and the default block size is apparently so small that it kills performance. The lowest I tested was a 256k block size for dd and that jumped performance up. So overall numbers are about 150mb/s reads, 130mb/s writes - which does have some sanity to the numbers.
 
hokatichenci said:
Thank you for not reading the numbers - or my post whatsoever. The write speeds in this test are clearly faster than the read speeds. I did find the problem though - I am not using the same block size, and the default block size is apparently so small that it kills performance. The lowest I tested was a 256k block size for dd and that jumped performance up. So overall numbers are about 150mb/s reads, 130mb/s writes - which does have some sanity to the numbers.
Click to expand...
nice catch. Good to know for future reference. Thanks for posting your solution.
 
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