Dynamic Multi-Pathing 7.3.1 Administrator's Guide - Linux

Example of applying load balancing in a SAN

This example describes how to use Dynamic Multi-Pathing (DMP) 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 this sample output from the vxdisk list command, the device sdm has eight primary paths:

# vxdisk list sdq

Device: sdq
    .
    .
    .
numpaths: 8
sdj state=enabled type=primary
sdk state=enabled type=primary
sdl state=enabled type=primary
sdm state=enabled type=primary
sdn state=enabled type=primary
sdo state=enabled type=primary
sdp state=enabled type=primary
sdq 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 &

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, sdq:

# vxdmpadm iostat show dmpnodename=sdq 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
sdj        0       0        0        0         0.00        0.00
sdk        0       0        0        0         0.00        0.00
sdl        0       0        0        0         0.00        0.00
sdm        0       0        0        0         0.00        0.00
sdn        0       0        0        0         0.00        0.00
sdo        0       0        0        0         0.00        0.00
sdp        0       0        0        0         0.00        0.00
sdq        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           MinimumQ      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          MinimumQ           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=sdq 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
sdj        2041    0         1021      0         0.39        0.00
sdk        1894    0         947       0         0.39        0.00
sdl        2008    0         1004      0         0.39        0.00
sdm        2054    0         1027      0         0.40        0.00
sdn        2171    0         1086      0         0.39        0.00
sdo        2095    0         1048      0         0.39        0.00
sdp        2073    0         1036      0         0.39        0.00
sdq        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