LinuxQuestions.org - iostat output

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We have complaints from the users that there are read write issues in our VPS server. This is the sample output of the iostat command.

Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
sda 0.14 6.86 0.54 2.48 0.01 0.04 32.02 0.63 206.76 6.24 1.89
sdb 56.84 41.72 73.37 133.98 1.56 1.52 30.42 3.25 15.66 1.65 34.19
dm-0 0.00 0.00 0.00 0.00 0.00 0.00 9.05 0.00 25.89 24.28 0.00

where dm-0 indicated an LVM partition in the server. Each VPS server is created as a separate LVM. There is a LVM partition which always shows more than 90% disk usage.

dm-21 0.00 0.00 416.00 0.00 8.25 0.00 40.60 15.53 36.72 2.35 97.60

Is there any way to find out the details of this dm-21? I want to know which VPS is causing the problem. The lvdisplay command does not show id dm-21. So I am unable to track the exact VPS causing the problem. Any ideas?

I don't really know if this'll work, but try "cat /sys/class/block/dm-21/dev", then "dmsetup ls" to see what LV it is.

An example

Code:

[root@athlonz devices]# cd /sys/class/block/



[root@athlonz block]# ls

dm-0  dm-2  dm-4  fd0  md1  ram1  ram11  ram13  ram15  ram3  ram5  ram7  ram9  sda1  sda3  sdb  sdb2  sdb5

dm-1  dm-3  dm-5  md0  ram0  ram10  ram12  ram14  ram2  ram4  ram6  ram8  sda  sda2  sda5  sdb1  sdb3  sr0



[root@athlonz block]# cat /sys/class/block/dm-0/dev 

253:0



[root@athlonz block]# dmsetup ls

vgz01-t (253, 5)

vgz01-hfsplus  (253, 3)

vgz00-lvz01    (253, 0)

vgz00-lvz00    (253, 1)

vgz01-maclv    (253, 4)

vgz01-lvz00    (253, 2)

Thank you for the details.. it really helped. Can you please let me know what does (253, 3) indicate? I checked the man pages and could not find any helping details.

(253,3) are the major and minor number of the fourth LVM Logical Volume that you created.

The explanation that I've seen is that the major number is associated with a software driver for a certain type of device and the minor numbers represent the actual devices under it.

(If you Google "linux major device numbers" you can see quite a few articles trying to explain it. It's a simple concept, but few of these articles give a simple explanation, and I can't either...)

If you cat /proc/devices you can see under the block devices (disks are block devices) that '8' is 'sd' (scsi device) and '253' is device-mapper (which LVM uses -- see the Wikipedia article on "device mapper").

Your real /dev/sda device is probably 8,0; /dev/sda1 is probably 8,1; etc. While your LVM logical volumes are 253,nn.

If you had a hardware RAID array controller, you'd have a different major number and different names for the devices.)

Here is a display from a simple home x86 machine with IDE drives, software RAID (MD devices), and LVM2. You can see the major and minor numbers with an 'ls -l'.

Code:

[root@athlonz ~]# ls -l /dev/sd*

brw-rw---- 1 root disk 8,  0 2009-07-28 12:40 /dev/sda

brw-rw---- 1 root disk 8,  1 2009-07-28 12:41 /dev/sda1

brw-rw---- 1 root disk 8,  2 2009-07-28 12:40 /dev/sda2

brw-rw---- 1 root disk 8,  3 2009-07-28 12:40 /dev/sda3

brw-rw---- 1 root disk 8,  5 2009-07-28 12:40 /dev/sda5

brw-rw---- 1 root disk 8, 16 2009-07-28 12:40 /dev/sdb

brw-rw---- 1 root disk 8, 17 2009-07-28 12:41 /dev/sdb1

brw-rw---- 1 root disk 8, 18 2009-07-28 12:40 /dev/sdb2

brw-rw---- 1 root disk 8, 19 2009-07-28 12:40 /dev/sdb3

brw-rw---- 1 root disk 8, 21 2009-07-28 12:40 /dev/sdb5



[root@athlonz ~]# ls -l /dev/md*

brw-rw---- 1 root disk 9, 0 2009-07-28 12:40 /dev/md0

brw-rw---- 1 root disk 9, 1 2009-07-28 12:41 /dev/md1



[root@athlonz ~]# ls -l /dev/mapper/*

crw-rw---- 1 root root  10, 60 2009-07-28 12:40 /dev/mapper/control

brw-rw---- 1 root disk 253,  1 2009-07-28 12:40 /dev/mapper/vgz00-lvz00

brw-rw---- 1 root disk 253,  0 2009-07-28 12:41 /dev/mapper/vgz00-lvz01

brw-rw---- 1 root disk 253,  3 2009-07-28 12:41 /dev/mapper/vgz01-hfsplus

brw-rw---- 1 root disk 253,  2 2009-07-28 12:41 /dev/mapper/vgz01-lvz00

brw-rw---- 1 root disk 253,  4 2009-07-28 12:41 /dev/mapper/vgz01-maclv

brw-rw---- 1 root disk 253,  5 2009-07-28 12:41 /dev/mapper/vgz01-t

And to make it more interesting, more than one device name can have the same major/minor numbers and therefore represent the same device.

Code:

[root@athlonz ~]# ls -l /dev/root 

brw------- 1 root root 253, 0 2009-07-28 12:40 /dev/root



[root@athlonz ~]# ls -l /dev/dm-0

brw-rw---- 1 root disk 253, 0 2009-07-28 12:40 /dev/dm-0



[root@athlonz ~]# ls -l /dev/mapper/vgz00-lvz01

brw-rw---- 1 root disk 253, 0 2009-07-28 12:41 /dev/mapper/vgz00-lvz01

Hope that helps.

Hello tommylovell,

Thanx for the explanation and the tips to get more details. I managed to use the minor number details and find out the exact VPS :-). thanx a lot again :-)