multipath round-robin 0 using only one path for I/O when when added as ASM disk?

Solution Verified - Updated -

Issue

  • Device Mapper Multipath by defaults use 'round-robin 0' path selector, but use just one path. Why so?
# multipath -ll /dev/mapper/mpath1
mpath1 (3600c0ff0001187d839e9c15101000000) dm-18 IBM,2145
size=4.5T features='1 queue_if_no_path' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=50 status=enabled
| |- 4:0:1:31  sdci 69:96   active ready running
| |- 4:0:0:31  sdbr 68:80   active ready running
| |- 3:0:1:31  sds  65:32   active ready running
| `- 3:0:0:31  sdb  8:16    active ready running
`-+- policy='round-robin 0' prio=10 status=enabled
  |- 4:0:3:31  sddq 71:128  active ready running
  |- 4:0:2:31  sdcz 70:112  active ready running
  |- 3:0:3:31  sdba 67:64   active ready running
  `- 3:0:2:31  sdaj 66:48   active ready running
# iostat -x | egrep 'sdci|sdbr|sds\ |sdb\ '
sdb              18.17    11.12 472.13 717.08 74251.90 39372.26    95.55     0.18    0.14   0.05   5.80
sds               0.00     0.00  0.00  0.00     0.40     0.00   278.13     0.00    1.70   0.49   0.00
sdbr              0.00     0.00  0.00  0.00     0.39     0.00   281.46     0.00    1.59   0.55   0.00
sdbr1             0.00     0.00  0.00  0.00     0.00     0.00    29.27     0.00    1.19   0.70   0.00
sdbr2             0.00     0.00  0.00  0.00     0.00     0.00    53.47     0.00    1.22   1.22   0.00
sdbr3             0.00     0.00  0.00  0.00     0.00     0.00    42.92     0.00    0.53   0.46   0.00
sdbr4             0.00     0.00  0.00  0.00     0.00     0.00    56.79     0.00    0.38   0.38   0.00
sdci              0.00     0.00  0.00  0.00     0.33     0.00   252.07     0.00    1.59   0.57   0.00
sdci1             0.00     0.00  0.00  0.00     0.00     0.00    33.32     0.00    1.71   1.33   0.00
sdci2             0.00     0.00  0.00  0.00     0.00     0.00    78.55     0.00    2.86   2.86   0.00
sdci3             0.00     0.00  0.00  0.00     0.00     0.00    55.66     0.00    1.91   1.61   0.00
sdci4             0.00     0.00  0.00  0.00     0.00     0.00    73.53     0.00    1.91   1.91   0.00
# iostat -x | egrep 'sdci|sdbr|sds\ |sdb\ '
sdb              18.17    11.12 472.13 717.08 74251.89 39372.25    95.55     0.18    0.14   0.05   5.80
sds               0.00     0.00  0.00  0.00     0.40     0.00   278.13     0.00    1.70   0.49   0.00
sdbr              0.00     0.00  0.00  0.00     0.39     0.00   281.46     0.00    1.59   0.55   0.00
sdbr1             0.00     0.00  0.00  0.00     0.00     0.00    29.27     0.00    1.19   0.70   0.00
sdbr2             0.00     0.00  0.00  0.00     0.00     0.00    53.47     0.00    1.22   1.22   0.00
sdbr3             0.00     0.00  0.00  0.00     0.00     0.00    42.92     0.00    0.53   0.46   0.00
sdbr4             0.00     0.00  0.00  0.00     0.00     0.00    56.79     0.00    0.38   0.38   0.00
sdci              0.00     0.00  0.00  0.00     0.33     0.00   252.07     0.00    1.59   0.57   0.00
sdci1             0.00     0.00  0.00  0.00     0.00     0.00    33.32     0.00    1.71   1.33   0.00
sdci2             0.00     0.00  0.00  0.00     0.00     0.00    78.55     0.00    2.86   2.86   0.00
sdci3             0.00     0.00  0.00  0.00     0.00     0.00    55.66     0.00    1.91   1.61   0.00
sdci4             0.00     0.00  0.00  0.00     0.00     0.00    73.53     0.00    1.91   1.91   0.00
  • Multipath devices are not using preferred paths, We have been advised by our SAN support company that all our Linux servers (RHEL4 & RHEL5) are using non-optimal paths. They say that this is a OS issue as it is the OS that determines which paths to take. How to make sure all paths for multipath device being used?

Environment

  • Red Hat Enterprise Linux 5
  • Device Mapper Multipath(DM Multipath)

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