Hi,
I have a RAID5 array which consists of 8 x Intel 480GB SSD, single
partition on each covering 100% of the drive.
md1 : active raid5 sde1[7] sdc1[11] sdd1[10] sdb1[12] sdg1[9] sdh1[5]
sdf1[8] sda1[6]
3281935552 blocks super 1.2 level 5, 64k chunk, algorithm 2 [8/8]
[UUUUUUUU]
/dev/md1:
Version : 1.2
Creation Time : Wed Aug 22 00:47:03 2012
Raid Level : raid5
Array Size : 3281935552 (3129.90 GiB 3360.70 GB)
Used Dev Size : 468847936 (447.13 GiB 480.10 GB)
Raid Devices : 8
Total Devices : 8
Persistence : Superblock is persistent
Update Time : Mon Jun 20 16:02:10 2016
State : active
Active Devices : 8
Working Devices : 8
Failed Devices : 0
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 64K
Name : san1:1 (local to host san1)
UUID : 707957c0:b7195438:06da5bc4:485d301c
Events : 2092476
Number Major Minor RaidDevice State
7 8 65 0 active sync /dev/sde1
6 8 1 1 active sync /dev/sda1
8 8 81 2 active sync /dev/sdf1
5 8 113 3 active sync /dev/sdh1
9 8 97 4 active sync /dev/sdg1
12 8 17 5 active sync /dev/sdb1
10 8 49 6 active sync /dev/sdd1
11 8 33 7 active sync /dev/sdc1
I'm finding that the underlying disk utilisation is "uneven" ie, one or
two disks is used a lot more heavily than the others. This is best seen
with iostat:
iostat -x -N /dev/sd? 5
This will show 5 second averages... so we should expect the average
utilisation of all disks to be equal ( I expect, I am probably wrong).
Ignoring the first output, since that is values since the system was
booted, I've copied three sample from after that.
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz
avgqu-sz await r_await w_await svctm %util
sdf 128.00 194.00 86.80 141.20 897.70 1289.60
19.19 0.04 0.18 0.16 0.20 0.13 2.96
sdh 110.80 138.60 83.40 139.20 808.80 1063.20
16.82 0.08 0.34 0.21 0.42 0.31 6.96
sde 120.80 162.00 90.60 117.80 866.40 1073.60
18.62 0.09 0.42 0.12 0.65 0.38 7.84
sdb 141.80 184.60 110.60 130.60 1104.30 1219.20
19.27 0.04 0.15 0.14 0.16 0.11 2.64
sda 126.00 153.80 89.80 120.40 921.00 1048.00
18.73 0.13 0.61 0.14 0.96 0.57 12.08
sdg 132.20 168.40 113.00 122.80 1037.60 1116.80
18.27 0.05 0.21 0.28 0.15 0.15 3.60
sdd 122.20 180.80 99.80 135.60 958.40 1219.20
18.50 0.04 0.16 0.20 0.13 0.10 2.40
sdc 112.80 178.60 87.40 115.20 824.00 1128.80
19.28 0.17 0.85 0.43 1.17 0.75 15.20
sdf 97.00 147.80 107.40 139.80 911.30 1084.00
16.14 0.04 0.15 0.14 0.15 0.11 2.72
sdh 104.80 139.20 99.00 133.60 901.60 1024.00
16.56 0.03 0.13 0.15 0.12 0.10 2.24
sde 97.60 124.00 98.20 109.40 889.60 868.00
16.93 0.03 0.15 0.08 0.21 0.12 2.48
sdb 91.80 144.60 96.00 117.00 839.80 983.20
17.12 0.03 0.13 0.15 0.12 0.12 2.48
sda 73.80 106.40 94.80 120.00 762.20 837.60
14.90 0.12 0.58 0.10 0.95 0.55 11.76
sdg 97.00 143.80 104.80 114.60 894.50 968.80
16.99 0.06 0.29 0.11 0.45 0.28 6.16
sdd 88.40 140.80 93.00 121.00 770.90 980.00
16.36 0.09 0.41 0.16 0.61 0.40 8.56
sdc 92.60 137.00 94.40 106.20 830.70 908.00
17.33 0.21 1.07 0.48 1.59 0.90 18.00
sdf 71.60 138.60 91.60 137.40 813.80 1040.00
16.19 0.08 0.33 0.12 0.47 0.30 6.96
sdh 87.20 137.20 99.20 124.60 927.10 983.20
17.07 0.03 0.14 0.21 0.08 0.12 2.64
sde 85.40 126.60 84.20 102.20 830.50 850.40
18.04 0.02 0.08 0.11 0.05 0.06 1.12
sdb 90.40 153.00 94.40 117.00 907.40 1019.20
18.23 0.02 0.11 0.13 0.10 0.08 1.68
sda 77.60 134.40 84.40 121.40 813.10 958.40
17.22 0.13 0.65 0.13 1.01 0.62 12.72
sdg 101.80 140.60 109.20 112.20 1038.30 946.40
17.93 0.06 0.28 0.22 0.34 0.25 5.44
sdd 90.00 131.20 83.40 111.20 810.60 907.20
17.65 0.02 0.11 0.12 0.10 0.07 1.36
sdc 85.40 136.00 83.00 101.80 817.70 888.80
18.47 0.23 1.27 0.61 1.81 1.13 20.80
As you can see, sdc (and sda) has a much higher utilisation compared to
all the other drives, but we can see the actual reads/writes are similar
across all drives.
Trying to find/explain the differences in performance, I originally
assumed one drive was being "targeted" more heavily, perhaps due to a
bad configuration (eg, chunk size, resulting in all filesystem
read/write on the same physical disk (plus checksum)).
However, it also seems that one drive is a different model:
sda: Model Family: Intel 520 Series SSDs
sdb: Model Family: Intel 520 Series SSDs
sdc: Model Family: Intel 530 Series SSDs
sdd: Model Family: Intel 520 Series SSDs
sde: Model Family: Intel 520 Series SSDs
sdf: Model Family: Intel 520 Series SSDs
sdg: Model Family: Intel 520 Series SSDs
sdh: Model Family: Intel 520 Series SSDs
The disk sector sizes:
512 bytes logical/physical across all disks/drives.
Except we see that sda is the same model as the rest and also seems to
be affected, though not as much as sdc.
So, should I try to swap sdc with another drive of the same model (520
series)?
Is there something else I can do to better optimise the array?
Should I migrate to RAID50 with 12 drives, or RAID10 with 16 drives
(which would also add 480G capacity)?
Would moving to RAID6 help (I doubt it)?
I don't think there is a single threaded CPU issue, since using top and
watching each individual CPU, I don't see the idle reduce to zero), plus
rsync is using most CPU not the md1_raid5 thread.
Should I use a smaller chunk size to better "spread" the load across the
disks? Would a value of 4k be better (the minimum value permitted
apparently)? Or would it be better to go the other way, and increase the
chunk size even more (I think this would help with throughput, but not
so random workload like we are getting).
Some other details that might be relevant:
Linux san1 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u4
(2016-02-29) x86_64 GNU/Linux
free
total used free shared buffers cached
Mem: 7902324 3287360 4614964 1203468 196836 2440864
-/+ buffers/cache: 649660 7252664
Swap: 3939324 23436 3915888
vmstat doesn't show anything for si or so, so RAM doesn't seem to be a
problem
I'm using lvm on top of the md1 device, and each LV is used by DRBD,
which is then exported with iSCSI to another linux box, and then used as
the block device for Xen Windows machines.
Any other suggestions on where to look, or additional information I
should provide?
Regards,
Adam
--
Adam Goryachev
Website Managers
P: +61 2 8304 0000 adam@xxxxxxxxxxxxxxxxxxxxxx
F: +61 2 8304 0001 www.websitemanagers.com.au
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