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I am trying to figure out how sysstat is reporting the utilization of the device mapper logical volume, lower than the individual disks that make up the logical volume. I keep trying to do the math but I cant make sense of it.

This is the description of that column in the man page:

%util
   Percentage of elapsed time during which I/O requests were issued to the device 
   (bandwidth utilization for the  device).  Device  saturation occurs when this 
   value is close to 100%.

I have a logical volume (dm-2) which contains xvdb, xvdc, xvdd.
if the %util is the percentage of time during I/O requests were issued, id expect that the combination of the disks that make up dm-2 would be equivalent to the value reported by dm-2.

If we add it up:
xvdb - 32.36
xvdc - 51.31
xvdd - 48.30
the value is 131.97
but dm-2 is reporting a util of 91.02.

It does not make sense to me how the logical volume in LVM can report a different utilization than the physical volumes/disks that make it.

When I add up wr_sec/s, or even tps, the results are much closer to what I would expect. but the Util I am stumped on the difference.

12:00:01 AM       DEV       tps  rd_sec/s  wr_sec/s  avgrq-sz  avgqu-sz     await     svctm     %util
...
07:56:01 AM      xvda      3.01      0.00     26.40      8.76      0.01      1.85      0.58      0.17
07:56:01 AM      xvdb    329.24      0.00   4680.19     14.22      3.71     11.26      0.98     32.36
07:56:01 AM      xvdc    370.90      0.00   5396.45     14.55      7.92     21.39      1.38     51.31
07:56:01 AM      xvdd    329.44      0.14   4392.93     13.33      7.41     22.49      1.47     48.30
07:56:01 AM      dm-0      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
07:56:01 AM      dm-1      0.20      0.00      1.62      8.00      0.00      9.83      2.00      0.04
07:56:01 AM      dm-2    953.43      0.14  14461.30     15.17     19.46     20.43      0.95     91.02
07:56:01 AM      dm-3      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
07:56:01 AM      dm-4      1.10      0.00      8.93      8.12      0.00      2.57      1.05      0.12
07:56:01 AM      dm-5      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00
07:56:01 AM      dm-6      1.81      0.00     15.84      8.75      0.00      0.54      0.10      0.02
07:56:01 AM      dm-7      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00

Anyone have the explanation of this difference?
This is not just one by the way, there are MANY examples of this.

  • have you tried to check if you have one lvm stripped volume? – c4f4t0r Jul 11 at 23:14
  • I have they are not striped. However it's three disks on OVM with a ZFS NFS Backend storage so maybe the controller is treating it as such. But if it was striped how would that explain it I don't understand. – cdiehl Jul 12 at 16:48

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