Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

144 Administering dynamic multipathing (DMP)
Administering DMP using vxdmpadm
Scheduling I/O on the paths of an Asymmetric Active/Active array
You can specify the use_all_paths attribute in conjunction with the adaptive,
balanced, minimumq, priority and round-robin I/O policies to specify whether I/O
requests are to be scheduled on the secondary paths in addition to the primary paths of an
Asymmetric Active/Active (A/A-A) array. Depending on the characteristics of the array,
the consequent improved load balancing can increase the total I/O throughput. However,
this feature should only be enabled if recommended by the array vendor. It has no effect
for array types other than A/A-A.
For example, the following command sets the balanced I/O policy with a partition size
of 2048 blocks (2MB) on the enclosure enc0, and allows scheduling of I/O requests on
the secondary paths:
# vxdmpadm setattr enclosure enc0 iopolicy=balanced \
partitionsize=2048 use_all_paths=no
The default setting for this attribute is use_all_paths=no.
Example of applying load balancing in a SAN
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 c3t2d15 has eight primary paths:
# vxdisk list c3t2d15
Device: c3t2d15
...
numpaths: 8
c2t0d15 state=enabled type=primary
c2t1d15 state=enabled type=primary
c3t1d15 state=enabled type=primary
c3t2d15 state=enabled type=primary
c4t2d15 state=enabled type=primary
c4t3d15 state=enabled type=primary
c5t3d15 state=enabled type=primary
c5t4d15 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, c5t4d15:
# vxdmpadm iostat show dmpnodename=c3t2d15 interval=5 count=2