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 can be seen in this sample output from the vxdisk list command, the device c3t2d15s2 has eight primary paths:

# vxdisk list c3t2d15s2

Device: c3t2d15s2
    .
    .
    .
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 the dd command is used to apply an input workload from the volume:

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

By running the vxdmpadm iostat command to display the DMP statistics for the device, it can be seen that all I/O is being directed to one path, c5t4d15s2:

# vxdmpadm iostat show dmpnodename=c3t2d15s2 interval=5 count=2
    .
    .
    .
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

The vxdmpadm command is used to display the I/O policy for the enclosure that contains the device:

# vxdmpadm getattr enclosure ENC0 iopolicy

ENCLR_NAME       DEFAULT         CURRENT
============================================
ENC0             Round-Robin     Single-Active

This 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, the policy is set to round-robin as shown here:

# vxdmpadm setattr enclosure ENC0 iopolicy=round-robin
# vxdmpadm getattr enclosure ENC0 iopolicy

ENCLR_NAME    DEFAULT         CURRENT
============================================
ENC0          Round-Robin         Round-Robin

The DMP statistics are now reset:

# vxdmpadm iostat reset

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

# vxdmpadm iostat show dmpnodename=c3t2d15s2 interval=5 count=2
    .
    .
    .
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

The enclosure can be returned to the single active I/O policy by entering the following command:

# vxdmpadm setattr enclosure ENC0 iopolicy=singleactive