I've got a server with LVM over RAID1. One of the volumes has a consistently higher IO latency (as measured by the diskstats_latency munin plugin) than the other volumes from the same group.
As you can see, the dark orange /root volume has consistently high IO latency. Actually ten times the average latency of the physical devices. It also has the highest Min and Max values. My main concern are not the peaks, which occur under high load, but the constant load on (semi-)idle.
The server is running Debian Squeeze with the VServer kernel and has four VServer containers and one KVM guest.
I'm looking for ways to fix – or at least understand – this situation.
Here're some parts of the system configuration:
root@kvmhost2:~# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/system--host-root
19G 3.8G 14G 22% /
tmpfs 16G 0 16G 0% /lib/init/rw
udev 16G 224K 16G 1% /dev
tmpfs 16G 0 16G 0% /dev/shm
/dev/md0 942M 37M 858M 5% /boot
/dev/mapper/system--host-isos
28G 19G 8.1G 70% /srv/isos
/dev/mapper/system--host-vs_a
30G 23G 6.0G 79% /var/lib/vservers/a
/dev/mapper/system--host-vs_b
5.0G 594M 4.1G 13% /var/lib/vservers/b
/dev/mapper/system--host-vs_c
5.0G 555M 4.2G 12% /var/lib/vservers/c
/dev/loop0 4.4G 4.4G 0 100% /media/debian-6.0.0-amd64-DVD-1
/dev/loop1 4.4G 4.4G 0 100% /media/debian-6.0.0-i386-DVD-1
/dev/mapper/system--host-vs_d
74G 55G 16G 78% /var/lib/vservers/d
root@kvmhost2:~# cat /proc/mounts
rootfs / rootfs rw 0 0
none /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
none /proc proc rw,nosuid,nodev,noexec,relatime 0 0
none /dev devtmpfs rw,relatime,size=16500836k,nr_inodes=4125209,mode=755 0 0
none /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
/dev/mapper/system--host-root / ext3 rw,relatime,errors=remount-ro,data=ordered 0 0
tmpfs /lib/init/rw tmpfs rw,nosuid,relatime,mode=755 0 0
tmpfs /dev/shm tmpfs rw,nosuid,nodev,relatime 0 0
fusectl /sys/fs/fuse/connections fusectl rw,relatime 0 0
/dev/md0 /boot ext3 rw,sync,relatime,errors=continue,data=ordered 0 0
/dev/mapper/system--host-isos /srv/isos ext3 rw,relatime,errors=continue,data=ordered 0 0
/dev/mapper/system--host-vs_a /var/lib/vservers/a ext3 rw,relatime,errors=continue,data=ordered 0 0
/dev/mapper/system--host-vs_b /var/lib/vservers/b ext3 rw,relatime,errors=continue,data=ordered 0 0
/dev/mapper/system--host-vs_c /var/lib/vservers/c ext3 rw,relatime,errors=continue,data=ordered 0 0
/dev/loop0 /media/debian-6.0.0-amd64-DVD-1 iso9660 ro,relatime 0 0
/dev/loop1 /media/debian-6.0.0-i386-DVD-1 iso9660 ro,relatime 0 0
/dev/mapper/system--host-vs_d /var/lib/vservers/d ext3 rw,relatime,errors=continue,data=ordered 0 0
root@kvmhost2:~# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sda2[0] sdb2[1]
975779968 blocks [2/2] [UU]
md0 : active raid1 sda1[0] sdb1[1]
979840 blocks [2/2] [UU]
unused devices: <none>
root@kvmhost2:~# iostat -x
Linux 2.6.32-5-vserver-amd64 (kvmhost2) 06/28/2012 _x86_64_ (8 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
3.09 0.14 2.92 1.51 0.00 92.35
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 23.25 161.12 7.46 37.90 855.27 1596.62 54.05 0.13 2.80 1.76 8.00
sdb 22.82 161.36 7.36 37.66 850.29 1596.62 54.35 0.54 12.01 1.80 8.09
md0 0.00 0.00 0.00 0.00 0.14 0.02 38.44 0.00 0.00 0.00 0.00
md1 0.00 0.00 53.55 198.16 768.01 1585.25 9.35 0.00 0.00 0.00 0.00
dm-0 0.00 0.00 0.48 20.21 16.70 161.71 8.62 0.26 12.72 0.77 1.60
dm-1 0.00 0.00 3.62 10.03 28.94 80.21 8.00 0.19 13.68 1.59 2.17
dm-2 0.00 0.00 0.00 0.00 0.00 0.00 9.17 0.00 9.64 6.42 0.00
dm-3 0.00 0.00 6.73 0.41 53.87 3.28 8.00 0.02 3.44 0.12 0.09
dm-4 0.00 0.00 17.45 18.18 139.57 145.47 8.00 0.42 11.81 0.76 2.69
dm-5 0.00 0.00 2.50 46.38 120.50 371.07 10.06 0.69 14.20 0.46 2.26
dm-6 0.00 0.00 0.02 0.10 0.67 0.81 12.53 0.01 75.53 18.58 0.22
dm-7 0.00 0.00 0.00 0.00 0.00 0.00 7.99 0.00 11.24 9.45 0.00
dm-8 0.00 0.00 22.69 102.76 407.25 822.09 9.80 0.97 7.71 0.39 4.95
dm-9 0.00 0.00 0.06 0.08 0.50 0.62 8.00 0.07 481.23 11.72 0.16
root@kvmhost2:~# ls -l /dev/mapper/
total 0
crw------- 1 root root 10, 59 May 11 11:19 control
lrwxrwxrwx 1 root root 7 Jun 5 15:08 system--host-kvm1 -> ../dm-4
lrwxrwxrwx 1 root root 7 Jun 5 15:08 system--host-kvm2 -> ../dm-3
lrwxrwxrwx 1 root root 7 Jun 5 15:06 system--host-isos -> ../dm-2
lrwxrwxrwx 1 root root 7 May 11 11:19 system--host-root -> ../dm-0
lrwxrwxrwx 1 root root 7 Jun 5 15:06 system--host-swap -> ../dm-9
lrwxrwxrwx 1 root root 7 Jun 5 15:06 system--host-vs_d -> ../dm-8
lrwxrwxrwx 1 root root 7 Jun 5 15:06 system--host-vs_b -> ../dm-6
lrwxrwxrwx 1 root root 7 Jun 5 15:06 system--host-vs_c -> ../dm-7
lrwxrwxrwx 1 root root 7 Jun 5 15:06 system--host-vs_a -> ../dm-5
lrwxrwxrwx 1 root root 7 Jun 5 15:08 system--host-kvm3 -> ../dm-1
root@kvmhost2:~#
Best Answer
I'd be inclined to check your partition, device mapper, file-system, raid stripe alignment
Compare
blk_readandblk_wrtnagainst all the layers, they should be even if everything is aligned. Also please post the output ofdf -handcat /proc/mountsYou'll probably find that you are doing far more reads/writes on that particular logical volume moreso than any others.
Alignment is key to having healthy, fast I/O - otherwise, you are needlessly multiplying r/w between all the storage layers.
Bear in mind, if a given partition is more active than any other it will also cause what you describe. Along with the choice of file system and flags set for when that FS is mounted. Simply adding
noatimeandnodiratimecould be all it takes to reduce the I/O activity a little, and thus, the latency.