I know quite well that I/O wait has been discussed multiple times on this site, but all the other topics seem to cover constant I/O latency, while the I/O problem we need to solve on our server occurs at irregular (short) intervals, but is ever-present with massive spikes of up to 20k ms a-wait and service times of 2 seconds. The disk affected is /dev/sdb (Seagate Barracuda, for details see below).
A typical iostat -x output would at times look like this, which is an extreme sample but by no means rare:
iostat (Oct 6, 2013)
tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz await svctm %util
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
16.00 0.00 156.00 9.75 21.89 288.12 36.00 57.60
5.50 0.00 44.00 8.00 48.79 2194.18 181.82 100.00
2.00 0.00 16.00 8.00 46.49 3397.00 500.00 100.00
4.50 0.00 40.00 8.89 43.73 5581.78 222.22 100.00
14.50 0.00 148.00 10.21 13.76 5909.24 68.97 100.00
1.50 0.00 12.00 8.00 8.57 7150.67 666.67 100.00
0.50 0.00 4.00 8.00 6.31 10168.00 2000.00 100.00
2.00 0.00 16.00 8.00 5.27 11001.00 500.00 100.00
0.50 0.00 4.00 8.00 2.96 17080.00 2000.00 100.00
34.00 0.00 1324.00 9.88 1.32 137.84 4.45 59.60
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
22.00 44.00 204.00 11.27 0.01 0.27 0.27 0.60
Let me provide you with some more information regarding the hardware. It's a Dell 1950 III box with Debian as OS where uname -a reports the following:
Linux xx 2.6.32-5-amd64 #1 SMP Fri Feb 15 15:39:52 UTC 2013 x86_64 GNU/Linux
The machine is a dedicated server that hosts an online game without any databases or I/O heavy applications running. The core application consumes about 0.8 of the 8 GBytes RAM, and the average CPU load is relatively low. The game itself, however, reacts rather sensitive towards I/O latency and thus our players experience massive ingame lag, which we would like to address as soon as possible.
iostat:
avg-cpu: %user %nice %system %iowait %steal %idle
1.77 0.01 1.05 1.59 0.00 95.58
Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sdb 13.16 25.42 135.12 504701011 2682640656
sda 1.52 0.74 20.63 14644533 409684488
Uptime is:
19:26:26 up 229 days, 17:26, 4 users, load average: 0.36, 0.37, 0.32
Harddisk controller:
01:00.0 RAID bus controller: LSI Logic / Symbios Logic MegaRAID SAS 1078 (rev 04)
Harddisks:
Array 1, RAID-1, 2x Seagate Cheetah 15K.5 73 GB SAS
Array 2, RAID-1, 2x Seagate ST3500620SS Barracuda ES.2 500GB 16MB 7200RPM SAS
Partition information from df:
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sdb1 480191156 30715200 425083668 7% /home
/dev/sda2 7692908 437436 6864692 6% /
/dev/sda5 15377820 1398916 13197748 10% /usr
/dev/sda6 39159724 19158340 18012140 52% /var
Some more data samples generated with iostat -dx sdb 1 (Oct 11, 2013)
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sdb 0.00 15.00 0.00 70.00 0.00 656.00 9.37 4.50 1.83 4.80 33.60
sdb 0.00 0.00 0.00 2.00 0.00 16.00 8.00 12.00 836.00 500.00 100.00
sdb 0.00 0.00 0.00 3.00 0.00 32.00 10.67 9.96 1990.67 333.33 100.00
sdb 0.00 0.00 0.00 4.00 0.00 40.00 10.00 6.96 3075.00 250.00 100.00
sdb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4.00 0.00 0.00 100.00
sdb 0.00 0.00 0.00 2.00 0.00 16.00 8.00 2.62 4648.00 500.00 100.00
sdb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.00 0.00 0.00 100.00
sdb 0.00 0.00 0.00 1.00 0.00 16.00 16.00 1.69 7024.00 1000.00 100.00
sdb 0.00 74.00 0.00 124.00 0.00 1584.00 12.77 1.09 67.94 6.94 86.00
Characteristic charts generated with rrdtool can be found here:
iostat plot 1, 24 min interval: http://imageshack.us/photo/my-images/600/yqm3.png/
iostat plot 2, 120 min interval: http://imageshack.us/photo/my-images/407/griw.png/
As we have a rather large cache of 5.5 GBytes, we thought it might be a good idea to test if the I/O wait spikes would perhaps be caused by cache miss events. Therefore, we did a sync and then this to flush the cache and buffers:
echo 3 > /proc/sys/vm/drop_caches
and directly afterwards the I/O wait and service times virtually went through the roof, and everything on the machine felt like slow motion. During the next few hours the latency recovered and everything was as before - small to medium lags in short, unpredictable intervals.
Now my question is: does anybody have any idea what might cause this annoying behaviour? Is it the first indication of the disk array or the raid controller dying, or something that can be easily mended by rebooting? (At the moment we're very reluctant to do this, however, because we're afraid that the disks might not come back up again.)
Any help is greatly appreciated.
Thanks in advance,
Chris.
Edited to add: we do see one or two processes go to 'D' state in top, one of which seems to be kjournald rather frequently. If I'm not mistaken, however, this does not indicate the processes causing the latency, but rather those affected by it - correct me if I'm wrong. Does the information about uninterruptibly sleeping processes help us in any way to address the problem?
@Andy Shinn requested smartctl data, here it is:
smartctl -a -d megaraid,2 /dev/sdb yields:
smartctl 5.40 2010-07-12 r3124 [x86_64-unknown-linux-gnu] (local build)
Copyright (C) 2002-10 by Bruce Allen, http://smartmontools.sourceforge.net
Device: SEAGATE ST3500620SS Version: MS05
Serial number:
Device type: disk
Transport protocol: SAS
Local Time is: Mon Oct 14 20:37:13 2013 CEST
Device supports SMART and is Enabled
Temperature Warning Disabled or Not Supported
SMART Health Status: OK
Current Drive Temperature: 20 C
Drive Trip Temperature: 68 C
Elements in grown defect list: 0
Vendor (Seagate) cache information
Blocks sent to initiator = 1236631092
Blocks received from initiator = 1097862364
Blocks read from cache and sent to initiator = 1383620256
Number of read and write commands whose size <= segment size = 531295338
Number of read and write commands whose size > segment size = 51986460
Vendor (Seagate/Hitachi) factory information
number of hours powered up = 36556.93
number of minutes until next internal SMART test = 32
Error counter log:
Errors Corrected by Total Correction Gigabytes Total
ECC rereads/ errors algorithm processed uncorrected
fast | delayed rewrites corrected invocations [10^9 bytes] errors
read: 509271032 47 0 509271079 509271079 20981.423 0
write: 0 0 0 0 0 5022.039 0
verify: 1870931090 196 0 1870931286 1870931286 100558.708 0
Non-medium error count: 0
SMART Self-test log
Num Test Status segment LifeTime LBA_first_err [SK ASC ASQ]
Description number (hours)
# 1 Background short Completed 16 36538 - [- - -]
# 2 Background short Completed 16 36514 - [- - -]
# 3 Background short Completed 16 36490 - [- - -]
# 4 Background short Completed 16 36466 - [- - -]
# 5 Background short Completed 16 36442 - [- - -]
# 6 Background long Completed 16 36420 - [- - -]
# 7 Background short Completed 16 36394 - [- - -]
# 8 Background short Completed 16 36370 - [- - -]
# 9 Background long Completed 16 36364 - [- - -]
#10 Background short Completed 16 36361 - [- - -]
#11 Background long Completed 16 2 - [- - -]
#12 Background short Completed 16 0 - [- - -]
Long (extended) Self Test duration: 6798 seconds [113.3 minutes]
smartctl -a -d megaraid,3 /dev/sdb yields:
smartctl 5.40 2010-07-12 r3124 [x86_64-unknown-linux-gnu] (local build)
Copyright (C) 2002-10 by Bruce Allen, http://smartmontools.sourceforge.net
Device: SEAGATE ST3500620SS Version: MS05
Serial number:
Device type: disk
Transport protocol: SAS
Local Time is: Mon Oct 14 20:37:26 2013 CEST
Device supports SMART and is Enabled
Temperature Warning Disabled or Not Supported
SMART Health Status: OK
Current Drive Temperature: 19 C
Drive Trip Temperature: 68 C
Elements in grown defect list: 0
Vendor (Seagate) cache information
Blocks sent to initiator = 288745640
Blocks received from initiator = 1097848399
Blocks read from cache and sent to initiator = 1304149705
Number of read and write commands whose size <= segment size = 527414694
Number of read and write commands whose size > segment size = 51986460
Vendor (Seagate/Hitachi) factory information
number of hours powered up = 36596.83
number of minutes until next internal SMART test = 28
Error counter log:
Errors Corrected by Total Correction Gigabytes Total
ECC rereads/ errors algorithm processed uncorrected
fast | delayed rewrites corrected invocations [10^9 bytes] errors
read: 610862490 44 0 610862534 610862534 20470.133 0
write: 0 0 0 0 0 5022.480 0
verify: 2861227413 203 0 2861227616 2861227616 100872.443 0
Non-medium error count: 1
SMART Self-test log
Num Test Status segment LifeTime LBA_first_err [SK ASC ASQ]
Description number (hours)
# 1 Background short Completed 16 36580 - [- - -]
# 2 Background short Completed 16 36556 - [- - -]
# 3 Background short Completed 16 36532 - [- - -]
# 4 Background short Completed 16 36508 - [- - -]
# 5 Background short Completed 16 36484 - [- - -]
# 6 Background long Completed 16 36462 - [- - -]
# 7 Background short Completed 16 36436 - [- - -]
# 8 Background short Completed 16 36412 - [- - -]
# 9 Background long Completed 16 36404 - [- - -]
#10 Background short Completed 16 36401 - [- - -]
#11 Background long Completed 16 2 - [- - -]
#12 Background short Completed 16 0 - [- - -]
Long (extended) Self Test duration: 6798 seconds [113.3 minutes]
