The other day I was needing to dig into I/O performance on a Linux box and in the past iostat has served me well, so I tried the good old
but wasn’t quite sure what to look at.
Now the box I was working on was enjoying the benefits of RAID and LVM and so the output I saw looked as follows:
avg-cpu: %user %nice %system %iowait %steal %idle 0.01 0.00 0.01 0.00 0.00 99.98 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util sda 0.00 0.89 0.01 0.57 0.28 11.70 20.63 0.00 2.47 0.32 0.02 sdb 0.00 0.00 0.00 0.00 0.00 0.00 13.03 0.00 6.52 6.44 0.00 dm-0 0.00 0.00 0.01 1.46 0.28 11.70 8.12 0.02 13.90 0.13 0.02 dm-1 0.00 0.00 0.00 0.00 0.00 0.00 8.00 0.00 7.76 1.60 0.00
I thought “Ok, so what is the mount point I should focus on” and it turned out to be /dev/mapper/VolGroup00-LogVol00.
I was pretty sure that this path mapped to one of the dm-0 or dm-1 and had I been forced to guess, I would have chosen dm-0 because of the high number of 0s, but that just seemed sloopy.
So I started searching on how to map them exactly. I played with lvdisplay and pvdisplay and ls /dev but there were no clear symlinks or references to follow it clearly.
Finally after much to much time, I found /sys/block/, which actually contained (among others) directories named dm-0 and dm-1. Unfortunately, with I did not find a clear pointer back to the /dev/mapper hierarchy.
finally after much googling I found some references that suggest that the key lies in the major and minor numbers.
So, the secret is that the devices in /dev/mappper have a major and minor number (that’s not the secret). That major and minor number can be found as MAJ:MIN in one of the files under /dev/block/*/dev where the * will end up being something like dm-0. Then you can use iostat to query that device.
That seemed painful to me, but easy to execute, once I found it.
Oh and while I was digging and searching and hunting about I came across a really nice tutorial on LVM2