Original source:http://unix-essentials.blogspot.com/search/label/SAN
VxVM - DMP Load Balancing
How to configure load balancing across multiple primary paths in an Active/Passive DMP environment using VERITAS Volume Manager (tm) 4.x
Details:
In an Active/Passive Dynamic Multipathing (DMP) environment, it is possible that there may be multiple primary paths to a device due to a mesh of SAN switches. Volume Manager 4.0 can be configured to allow load balancing across multiple primary paths. This TechNote explains how to do this.
Note: This feature was not available in releases prior to 4.0
In the following example, device c3t2d15 belongs to a disk group named tonydg and contains a simple concatenated volume named tony1. The device has eight primary paths:
# vxdisk list c3t2d15s2
Device: c3t2d15s2
<..snip..>
numpaths: 8
c2t0d15s2 state=enabled type=primary
c2t1d15s2 state=enabled type=primary
c3t1d15s2 state=enabled type=primary
c3t2d15s2 state=enabled type=primary
c4t3d15s2 state=enabled type=primary
c4t4d15s2 state=enabled type=primary
c5t3d15s2 state=enabled type=primary
c5t5d15s2 state=enabled type=primary
DMP statistics are enabled and a read based workload is applied to the volume:
# vxdmpadm iostat start
# dd if=/dev/vx/rdsk/tonydg/tony1 of=/dev/null bs=512k count=6144
By displaying the DMP statistics for device c3t2d15, it can be seen that all I/Os are being directed to one subpath, c5t5d15:
# vxdmpadm iostat show dmpnodename=c3t2d15s2
cpu usage = 8023us per cpu memory = 32768b
OPERATIONS MBYTES AVG TIME(ms)
PATHNAME READS WRITES READS WRITES READS WRITES
c2t0d15s2 0 0 0 0 0.000000 0.000000
c2t1d15s2 0 0 0 0 0.000000 0.000000
c3t1d15s2 0 0 0 0 0.000000 0.000000
c3t2d15s2 0 0 0 0 0.000000 0.000000
c4t3d15s2 0 0 0 0 0.000000 0.000000
c4t4d15s2 0 0 0 0 0.000000 0.000000
c5t3d15s2 0 0 0 0 0.000000 0.000000
c5t5d15s2 6144 0 3145728 0 0.011081 0.000000
Using vxdmpadm, it is clear that the default iopolicy for this enclosure is Single Active (which explains the above behavior):
# vxdmpadm getattr enclosure HDS92000 iopolicy
ENCLR_NAME DEFAULT CURRENT
============================================
HDS92000 Single-Active Single-Active
To load balance across multiple primaries, the iopolicy should be set to round-robin:
# vxdmpadm setattr enclosure HDS92000 iopolicy=round-robin
# vxdmpadm getattr enclosure HDS92000 iopolicy
ENCLR_NAME DEFAULT CURRENT
============================================
HDS92000 Single-Active Round-Robin
The DMP statistics are reset and the same workload is applied to the volume, load balancing across the primary paths can be seen.
# vxdmpadm iostat reset
# dd if=/dev/vx/rdsk/tonydg/tony1 of=/dev/null bs=512k count=6144
# vxdmpadm iostat show dmpnodename=c3t2d15s2
cpu usage = 7223us per cpu memory = 32768b
OPERATIONS MBYTES AVG TIME(ms)
PATHNAME READS WRITES READS WRITES READS WRITES
c2t0d15s2 786 0 402432 0 0.011013 0.000000
c2t1d15s2 778 0 398336 0 0.011267 0.000000
c3t1d15s2 772 0 395264 0 0.011205 0.000000
c3t2d15s2 744 0 380928 0 0.011094 0.000000
c4t3d15s2 750 0 384000 0 0.011029 0.000000
c4t4d15s2 768 0 393216 0 0.011169 0.000000
c5t3d15s2 733 0 375296 0 0.010943 0.000000
c5t5d15s2 813 0 416256 0 0.011068 0.000000
The enclosure can be returned to its default policy of single active with the following:
# vxdmpadm setattr enclosure HDS92000 iopolicy=singleactive
Note: This setting is not persistent across reboots, I/O policies will revert to their default. Persistence can be configured by adding the relevant vxdmpadm command to a boot script.
