2 drives, slow software RAID1 (md)
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by bart613
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Published on 2010-02-09T14:22:19Z
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2010/04/05
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Hello,
I've got a server from hetzner.de (EQ4) with 2* SAMSUNG HD753LJ drives (750G 32MB cache).
OS is CentOS 5 (x86_64). Drives are combined together into two RAID1 partitions:
- /dev/md0 which is 512MB big and has only /boot partitions
- /dev/md1 which is over 700GB big and is one big LVM which hosts other partitions
Now, I've been running some benchmarks and it seems like even though exactly the same drives, speed differs a bit on each of them.
# hdparm -tT /dev/sda
/dev/sda: Timing cached reads: 25612 MB in 1.99 seconds = 12860.70 MB/sec Timing buffered disk reads: 352 MB in 3.01 seconds = 116.80 MB/sec
# hdparm -tT /dev/sdb
/dev/sdb: Timing cached reads: 25524 MB in 1.99 seconds = 12815.99 MB/sec Timing buffered disk reads: 342 MB in 3.01 seconds = 113.64 MB/sec
Also, when I run eg. pgbench which is stressing IO quite heavily, I can see following from iostat output:
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 231.40 0.00 298.00 0.00 9683.20 32.49 0.17 0.58 0.34 10.24
sda1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sda2 0.00 231.40 0.00 298.00 0.00 9683.20 32.49 0.17 0.58 0.34 10.24
sdb 0.00 231.40 0.00 301.80 0.00 9740.80 32.28 14.19 51.17 3.10 93.68
sdb1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdb2 0.00 231.40 0.00 301.80 0.00 9740.80 32.28 14.19 51.17 3.10 93.68
md1 0.00 0.00 0.00 529.60 0.00 9692.80 18.30 0.00 0.00 0.00 0.00
md0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
dm-0 0.00 0.00 0.00 0.60 0.00 4.80 8.00 0.00 0.00 0.00 0.00
dm-1 0.00 0.00 0.00 529.00 0.00 9688.00 18.31 24.51 49.91 1.81 95.92
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 152.40 0.00 330.60 0.00 5176.00 15.66 0.19 0.57 0.19 6.24
sda1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sda2 0.00 152.40 0.00 330.60 0.00 5176.00 15.66 0.19 0.57 0.19 6.24
sdb 0.00 152.40 0.00 326.20 0.00 5118.40 15.69 19.96 55.36 3.01 98.16
sdb1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdb2 0.00 152.40 0.00 326.20 0.00 5118.40 15.69 19.96 55.36 3.01 98.16
md1 0.00 0.00 0.00 482.80 0.00 5166.40 10.70 0.00 0.00 0.00 0.00
md0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
dm-0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
dm-1 0.00 0.00 0.00 482.80 0.00 5166.40 10.70 30.19 56.92 2.05 99.04
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 181.64 0.00 324.55 0.00 5445.11 16.78 0.15 0.45 0.21 6.87
sda1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sda2 0.00 181.64 0.00 324.55 0.00 5445.11 16.78 0.15 0.45 0.21 6.87
sdb 0.00 181.84 0.00 328.54 0.00 5493.01 16.72 18.34 61.57 3.01 99.00
sdb1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sdb2 0.00 181.84 0.00 328.54 0.00 5493.01 16.72 18.34 61.57 3.01 99.00
md1 0.00 0.00 0.00 506.39 0.00 5477.05 10.82 0.00 0.00 0.00 0.00
md0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
dm-0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
dm-1 0.00 0.00 0.00 506.39 0.00 5477.05 10.82 28.77 62.15 1.96 99.00
And this is completely getting me confused. How come two exactly the same specced drives have such a difference in write speed (see util%)? I haven't really paid attention to those speeds before, so perhaps that something normal -- if someone could confirm I would be really grateful.
Otherwise, if someone have seen such behavior again or knows what is causing such behavior I would really appreciate answer.
I'll also add that both "smartctl -a" and "hdparm -I" output are exactly the same and are not indicating any hardware problems. The slower drive was changed already two times (to new ones). Also I asked to change the drives with places, and then sda were slower and sdb quicker (so the slow one was the same drive). SATA cables were changed two times already.
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