On 20/06/16 18:44, Jens-U. Mozdzen wrote:
Hi Adam,
Zitat von Adam Goryachev <adam@xxxxxxxxxxxxxxxxxxxxxx>:
Hi,
I have a RAID5 array which consists of 8 x Intel 480GB SSD, single
partition on each covering 100% of the drive.
[...]
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
[...]
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.
looking at those numbers, it might not be the (effective) utilization
that's higher, but the time the SSDs spend handling the requests.
As you already ruled out model issues for sda, further probable causes
that I'd check might be
- a different firmware level for sda
All the Series 520 drives are running identical firmware (checked with
smartctl) but I can't confirm if that is the latest firmware or not, I
can find the intel tool to upgrade the firmware, but it doesn't specify
what the current firmware version is for this model.
- disk problems (anything useful in the SMART numbers?)
No, what started all this is I did find some unusual numbers on one
disk, but that was a 160GB SSD used for the OS itself, not part of the
array, and it has now been replaced (purchased a new one, but Intel will
replace the old one eventually). All other drives SMART details look
reasonable....
- connection issues (are all disks connected to the same (type of)
controller?)
All disks are connected to the same controller....
01:00.0 Serial Attached SCSI controller: LSI Logic / Symbios Logic
SAS2308 PCI-Express Fusion-MPT SAS-2 (rev 05)
I can't comment on the RAID parameter questions, though.
I just get the feeling that specific drives are being "worked" harder
than others, and I'm not sure why.
I'm considering moving to either RAID10 or RAID50 in the future to try
to improve performance, but I'm honestly not sure that this is really
the problem anyway. By my calculations, if I double the number of
drives, and move to RAID10, then I can double the read performance and
improve write performance (I'm not exactly sure of the math here, how
does one calculate write performance on RAID5 when you need to do
read/modify/write?), alternatively, RAID50 (with 16 drives, with 4
drives in 4 RAID5 sub-arrays) should also double read performance, but
also improve write performance compared to the current, but not as much
as RAID10 would. Although RAID50 will give more storage capacity than
the RAID10....
I think my real issue is perhaps latency, and that the real "bottleneck"
is at the DRBD layer rather than raid, but I'm trying to optimise each
part that doesn't look right as I go.
Regards,
Adam
--
Adam Goryachev Website Managers www.websitemanagers.com.au
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