Iostat shows higher await time for LVM device compared to underlying dm-multipath device on Red Hat Enterprise Linux

Solution Verified - Updated -

Issue

  • The LVM device dm-3 is showing a high response time when I compare them to the actual devices that make up the lvm, why such behavior?
Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util
sdc               0.00     0.00  145.00  103.00     8.64     0.62    76.48     0.55    2.21   0.84  20.90
sdd               0.00     0.00  140.00  108.00     8.15     2.66    89.29     1.20    4.82   0.94  23.40
sde               0.00     0.00  141.00  107.00     7.76     1.75    78.58     0.88    3.56   0.83  20.70
sdf               0.00     0.00  143.00  106.00     8.36     1.11    77.91     0.44    1.76   0.83  20.60
dm-2              0.00   974.00  569.00  424.00    32.91     6.15    80.56     3.09    3.11   0.50  49.50  <-- multipath await = 3.11ms
dm-3              0.00     0.00  569.00 1380.00    32.91     6.15    41.05    34.22   17.56   0.25  49.30  <-- LVM await = 17.56ms

Environment

  • Red Hat Enterprise Linux (RHEL) 6, 7
  • Storage stack consists of:
    • Block device (such as /dev/sda) --> dm-multipath device (such as mpatha) --> LVM pv --> LVM logical volume
  • Higher "Await" times are reported by iostat for the LVM device than for the underlying dm-multipath and individual paths when load is generated on the logical volume.

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