This example describes how to configure load balancing in a SAN environment where there are multiple primary paths to an Active/Passive device through several SAN switches. As shown in the sample output from the vxdisk list command, the device has eight primary paths:

# vxdisk list c3t2d15s2

Device: c3t2d15s2
    .
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numpaths: 8
c2t0d15s2 state=enabled type=primary
c2t1d15s2 state=enabled type=primary
c3t1d15s2 state=enabled type=primary
c3t2d15s2 state=enabled type=primary
c4t2d15s2 state=enabled type=primary
c4t3d15s2 state=enabled type=primary
c5t3d15s2 state=enabled type=primary
c5t4d15s2 state=enabled type=primary

In addition, the device is in the enclosure ENC0, belongs to the disk group mydg, and contains a simple concatenated volume myvol1.

The first step is to enable the gathering of DMP statistics:

# vxdmpadm iostat start

Next, use the dd command to apply an input workload from the volume:

# dd if=/dev/vx/rdsk/mydg/myvol1 of=/dev/null &

Use the vxdmpadm iostat command to display the DMP statistics for the device. The sample output shows that all I/O is being directed to one path.

# vxdmpadm iostat show dmpnodename=c3t2d15s2 interval=5 count=2
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cpu usage = 11294us per cpu memory = 32768b
              OPERATIONS           KBYTES          AVG TIME(ms)
PATHNAME    READS   WRITES   READS     WRITES   READS      WRITES
c2t0d15s2   0       0        0         0         0.00        0.00
c2t1d15s2   0       0        0         0         0.00        0.00
c3t1d15s2   0       0        0         0         0.00        0.00
c3t2d15s2   0       0        0         0         0.00        0.00
c4t2d15s2   0       0        0         0         0.00        0.00
c4t3d15s2   0       0        0         0         0.00        0.00
c5t3d15s2   0       0        0         0         0.00        0.00
c5t4d15s2   10986   0        5493      0         0.41        0.00

Use the vxdmpadm getattr command to display the I/O policy for the enclosure that contains the device:

# vxdmpadm getattr enclosure ENC0 iopolicy

ENCLR_NAME     DEFAULT          CURRENT
============================================
ENC0           MinimumQ      Single-Active

The above output shows that the policy for the enclosure is set to singleactive, which explains why all the I/O is taking place on one path.

To balance the I/O load across the multiple primary paths, set the policy to round-robin as shown here:

# vxdmpadm setattr enclosure ENC0 iopolicy=round-robin
# vxdmpadm getattr enclosure ENC0 iopolicy
ENCLR_NAME    DEFAULT            CURRENT
============================================
ENC0          MinimumQ           Round-Robin

Reset the DMP statistics:

# vxdmpadm iostat reset

With the workload still running, you can see the effect of changing the I/O policy to balance the load across the primary paths.

# vxdmpadm iostat show dmpnodename=c3t2d15s2 interval=5 count=2
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cpu usage = 14403us per cpu memory = 32768b
             OPERATIONS            KBYTES         AVG TIME(ms)
PATHNAME   READS   WRITES    READS     WRITES   READS      WRITES
c2t0d15s2  2041    0         1021      0         0.39        0.00
c2t1d15s2  1894    0         947       0         0.39        0.00
c3t1d15s2  2008    0         1004      0         0.39        0.00
c3t2d15s2  2054    0         1027      0         0.40        0.00
c4t2d15s2  2171    0         1086      0         0.39        0.00
c4t3d15s2  2095    0         1048      0         0.39        0.00
c5t3d15s2  2073    0         1036      0         0.39        0.00
c5t4d15s2  2042    0         1021      0         0.39        0.00

Enter the following command to return the enclosure to the single active I/O policy.

# vxdmpadm setattr enclosure ENC0 iopolicy=singleactive