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[zfs-discuss] How to properly read "zpool iostat -v" ? ;)

Subject: [zfs-discuss] How to properly read "zpool iostat -v" ? ;)
From: Jim Klimov
Date: Thu, 02 Jun 2011 00:29:49 +0400
Hello experts,

I've had a lingering question for some time: when I
use "zpool iostat -v" the values do not quite sum up.
In the example below with a raidz2 array made of 6
drives:
* the reported 33K of writes are less than two disks'
  workload at this time (at 17.9K each), overall
  disks writes are 107.4K = 325% of 33K.
* write ops sum up to 18 = 225% of 8 ops to the pool;
* read MB on dirves is 66.2M = 190% of 34.8M read
  from pool;
* read ops on drives sum up to 2014, but pool IOs
  are reported as 804 (disk work is 250% higher);

Neither difference is the same, and neither matches
the 150% (50% increase) I had expected from 2 parity
drives with 4 data drives worth of data.

Per-disk values are consistent with "iostat -Xn"
output. So I think that accounting of pool/vdev IO
is lying somewhere...

I'm curious: What's up? ;)


               capacity     operations    bandwidth
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
pool        9.53T  1.34T    804      8  34.8M  33.0K
  raidz2    9.53T  1.34T    804      8  34.8M  33.0K
    c7t0d0      -      -    296      3  11.9M  17.9K
    c7t1d0      -      -    338      3  10.7M  17.9K
    c7t2d0      -      -    367      3  10.4M  17.9K
    c7t3d0      -      -    333      3  12.0M  17.9K
    c7t4d0      -      -    340      3  10.7M  17.9K
    c7t5d0      -      -    340      3  10.5M  17.9K
cache           -      -      -      -      -      -
  c4t1d0p2   230M  15.8G      0      0      0      0
  c4t1d0p3   231M  15.5G      0      0      0      0
----------  -----  -----  -----  -----  -----  -----


Sometimes values are even more weird, i.e. here pool
reads are even less than one component drive's workload,
not to mention all six of them:

               capacity     operations    bandwidth
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
pool        9.53T  1.34T    404      0  4.41M      0
  raidz2    9.53T  1.34T    404      0  4.41M      0
    c7t0d0      -      -    115      0  4.25M      0
    c7t1d0      -      -    141      0  4.50M      0
    c7t2d0      -      -    133      0  4.41M      0
    c7t3d0      -      -    133      0  4.22M      0
    c7t4d0      -      -    138      0  4.47M      0
    c7t5d0      -      -    141      0  4.45M      0
cache           -      -      -      -      -      -
  c4t1d0p2   230M  15.8G      0      0      0      0
  c4t1d0p3   231M  15.5G      0      0      0      0
----------  -----  -----  -----  -----  -----  -----

               capacity     operations    bandwidth
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
pool        9.53T  1.34T    371      7  3.74M  29.3K
  raidz2    9.53T  1.34T    371      7  3.74M  29.3K
    c7t0d0      -      -    115      2  3.62M  16.5K
    c7t1d0      -      -    132      2  3.70M  15.3K
    c7t2d0      -      -    137      3  3.66M  15.3K
    c7t3d0      -      -    138      3  3.55M  15.3K
    c7t4d0      -      -    145      3  3.69M  16.5K
    c7t5d0      -      -    138      2  3.69M  16.5K
cache           -      -      -      -      -      -
  c4t1d0p2   230M  15.8G      0      0      0      0
  c4t1d0p3   231M  15.5G      0      0      0      0
----------  -----  -----  -----  -----  -----  -----


Thanks,
//Jim Klimov

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