I have an ubuntu 14.04.1 LTS server which have LVM 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
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/
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 ? Is this high IOwait and high load average(40+) possibly due to multiple PVs(made from same sdb disk partitions) in same volume group +same LV ?