I have an ubuntu 14.04.1 LTS server which have LVM(backed by hardware RAID5) with logical volume and a volume group named "dbstore-lv" and "dbstore-vg" which have sdb1 sdb2 sdb3 created from same sdb disk. The system as 42 cores and about 128G memory. Although i dont see CPU spikes in htop the load average output from uptime is ~43+ as well as vmstat shows constant iowait of 20-40 where the context switches is constantly around 80,000-150000 and even more at peak hours, the cpu idle time is also hovers around 70-85. Below is output of iostat -xp 1 where the %util is constantly 100%
avg-cpu: %user %nice %system %iowait %steal %idle
8.91 0.00 1.31 10.98 0.00 78.80
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sda 0.00 264.00 0.00 58.00 0.00 1428.00 49.24 0.02 0.28 0.00 0.28 0.21 1.20
sda1 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
sda2 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
sda3 0.00 264.00 0.00 58.00 0.00 1428.00 49.24 0.02 0.28 0.00 0.28 0.21 1.20
sdb 0.00 316.00 4.00 86.00 512.00 1608.00 47.11 36.02 0.27 5.00 0.05 11.11 100.00
sdb1 0.00 312.00 4.00 63.00 3512.00 4500.00 60.06 34.02 100.00 5.00 0.00 14.93 100.00
sdb2 0.00 0.00 0.00 821.00 450.00 84.00 8.00 82.00 99.19 0.00 0.19 47.62 100.00
sdb3 0.00 4.00 0.00 2.00 0.00 24.00 24.00 0.00 0.00 0.00 0.00 0.00 0.00
dm-0 0.00 0.00 0.00 6.00 0.00 24.00 8.00 0.00 0.00 0.00 0.00 0.00 0.00
dm-1 0.00 0.00 4.00 396.00 512.00 1584.00 10.48 36.02 8180.00 5.00 8180.00 2.50 100.00
dm-2 0.00 0.00 0.00 329.00 0.00 3896.00 23.68 0.85 2.58 0.00 2.58 0.05 1.60
dm-3 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
Similarly the TPS/iops is around 600-1000 most of the time(eg. iostat outptu below)
avg-cpu: %user %nice %system %iowait %steal %idle
22.24 0.35 2.56 32.08 0.00 42.77
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 527.00 3828.00 1536.00 3828 1536
sdb 576.00 8532.00 2804.00 8532 2804
sdc 42.00 280.00 156.00 280 156
dm-0 0.00 0.00 0.00 0 0
dm-1 956.00 8400.00 2804.00 8400 2804
dm-2 569.00 4108.00 1692.00 4108 1692
dm-3 0.00 0.00 0.00 0 0
Here is vmstat 1 output
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
2 22 986520 621704 440356 23588860 0 0 2560 8140 27032 132198 16 2 46 36 0
7 23 986520 672528 440204 23532752 0 0 2360 8 26659 107002 10 2 48 41 0
22 18 986520 697048 440084 23496096 0 0 3152 22520 60223 187651 25 5 46 25 0
2 18 986520 688596 439984 23501104 0 0 2436 684 50451 210261 20 5 49 26 0
13 33 986520 663680 439984 23495812 0 0 1712 149956 38549 136294 15 4 45 36 0
9 34 986520 647308 439968 23507944 0 0 1484 1832 51501 174355 19 4 38 39 0
14 18 986520 608364 439340 23531976 12 0 1828 21344 63692 134934 15 4 48 33 0
11 23 986520 588220 437636 23549852 0 0 2528 192 33461 116199 13 3 50 35 0
3 17 986520 601892 438080 23542508 0 0 3224 16376 74679 167580 20 5 40 34 0
1 16 986520 567092 438080 23574776 0 0 2272 76624 40944 136229 16 4 51 29 0
6 16 986520 584120 438380 23560932 0 0 18568 0 32038 108119 12 3 56 29 0
17 17 986520 568012 438392 23575828 0 0 2572 67248 54320 168767 19 4 51 26 0
5 23 986520 566384 438124 23575640 0 0 2656 360 60057 158031 18 5 49 28 0
1 29 986520 632216 438604 23546316 0 0 2520 28528 49198 109391 10 4 50 37 0
19 14 986508 621236 438616 23560516 0 0 2528 9368 39632 169120 19 4 44 32 0
8 31 986532 653172 440340 23548788 32 0 2460 208 29679 116036 14 4 42 40 0
28 26 986532 675568 440344 23551600 0 0 4552 3928 29385 113816 16 3 39 42 0
10 34 986532 654700 440352 23561616 0 0 2712 816 31667 155532 20 3 40 37 0
15 20 986520 630768 440356 23577388 32 0 4416 4348 35499 175319 30 3 35 32 0
Below is excerpt of lsblk which shows lvm associated to disks
sdb 8:16 0 19.7T 0 disk
├─sdb1 8:17 0 7.7T 0 part
│ └─dbstore-lv (dm-1) 252:1 0 9.4T 0 lvm /var/db/st01
├─sdb2 8:18 0 1.7T 0 part
│ └─dbstore-lv (dm-1) 252:1 0 9.4T 0 lvm /var/db/st01
└─sdb3 8:19 0 10.3T 0 part
└─archive--archivedbstore--lv (dm-0) 252:0 0 10.3T 0 lvm /opt/archive/
Queue Depth for sdb
cat /sys/block/sdb/device/queue_depth
1020
I am assuming this is due to disk seek problem as the same disk partitions are used for same LVM or may be its due to saturation of the disks(i dont have the vendor provided IOPS data of this disk yet). As initial tuning i have set vm.dirty_ratio to 5 and dirty_background_ratio to 2 + tried deadline scheduler (currently noop) but this doesnt seem to help to reduce the iowait. Any suggestions please ?
