Map sar device names

"sar -d 5" is my favorite way to get disk latency numbers, but the device names it reports are just major, minor numbers. This isn't how I think about filesystems, so I need to map to a human-readable filesystem name to understand the latency numbers that are reported.

the output of sar -d 5 looks like this for me

10:58:34 AM       DEV       tps  rd_sec/s  wr_sec/s  avgrq-sz  avgqu-sz     await     svctm     %util
10:58:39 AM    dev8-0      1.60      0.00    363.20    227.00      0.02     15.00      9.50      1.52
10:58:39 AM   dev8-16      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
10:58:39 AM  dev252-0      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
10:58:39 AM  dev252-1      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
10:58:39 AM  dev252-2     45.40      0.00    363.20      8.00      0.66     14.63      0.33      1.52
10:58:39 AM   dev8-32      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
10:58:39 AM  dev252-3      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
10:58:39 AM  dev252-4      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
10:58:39 AM   dev8-64      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
10:58:39 AM   dev8-80      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
^C

I get major/minor device numbers with "ls -lL /dev/" and read through looking for a match

I happen to already know the general range of devices that have traffic, so I've narrowed down my list, but the first time I did this, I went through all of /dev. In this example, I'm looking for the dev8-0 and dev252-2 since they are busy writing at the moment.

root@weasel:~# ls -lL /dev/sda*
brw-rw---- 1 root disk 8, 0 Mar 23 08:06 /dev/sda
brw-rw---- 1 root disk 8, 1 Mar 23 08:06 /dev/sda1
brw-rw---- 1 root disk 8, 2 Mar 23 08:06 /dev/sda2
brw-rw---- 1 root disk 8, 5 Mar 23 08:06 /dev/sda5
root@weasel:~# ls -lL /dev/dm*
brw-rw---- 1 root disk 252, 0 Mar 23 08:06 /dev/dm-0
brw-rw---- 1 root disk 252, 1 Mar 23 08:06 /dev/dm-1
brw-rw---- 1 root disk 252, 2 Mar 23 08:06 /dev/dm-2
brw-rw---- 1 root disk 252, 3 Mar 23 08:06 /dev/dm-3
brw-rw---- 1 root disk 252, 4 Mar 23 08:06 /dev/dm-4

Here I see the device names of interest are /dev/sda and /dev/dm-2. Looking in /dev/mapper, I can see the friendly name for dm-2 is Ubuntu-root.

root@weasel:~# ls -l /dev/mapper
total 0
crw------- 1 root root 10, 236 Mar 23 08:06 control
lrwxrwxrwx 1 root root       7 Mar 23 08:06 netstore-netstore -> ../dm-0
lrwxrwxrwx 1 root root       7 Mar 23 08:06 netstore-store -> ../dm-1
lrwxrwxrwx 1 root root       7 Mar 23 08:06 rootback-rootback -> ../dm-4
lrwxrwxrwx 1 root root       7 Mar 23 08:06 Ubuntu-root -> ../dm-2
lrwxrwxrwx 1 root root       7 Mar 23 08:06 Ubuntu-swap_1 -> ../dm-3

dmsetup ls

Another way to map major/minor numbers and names.

root@weasel:~# dmsetup ls
Ubuntu-swap_1	(252, 3)
netstore-netstore	(252, 0)
Ubuntu-root	(252, 2)
netstore-store	(252, 1)
rootback-rootback	(252, 4)

references

• http://en.wikipedia.org/wiki/Device_mapper