Dynamic Multi-Pathing 7.3.1 Administrator's Guide - Linux

adaptive

This policy attempts to maximize overall I/O throughput from/to the disks by dynamically scheduling I/O on the paths. It is suggested for use where I/O loads can vary over time. For example, I/O from/to a database may exhibit both long transfers (table scans) and short transfers (random look ups). The policy is also useful for a SAN environment where different paths may have different number of hops. No further configuration is possible as this policy is automatically managed by DMP.

In this example, the adaptive I/O policy is set for the enclosure enc1:

# vxdmpadm setattr enclosure enc1 \
  iopolicy=adaptive

adaptiveminq

Similar to the adaptive policy, except that I/O is scheduled according to the length of the I/O queue on each path. The path with the shortest queue is assigned the highest priority.

balanced [partitionsize=size]

This policy is designed to optimize the use of caching in disk drives and RAID controllers. The size of the cache typically ranges from 120KB to 500KB or more, depending on the characteristics of the particular hardware. During normal operation, the disks (or LUNs) are logically divided into a number of regions (or partitions), and I/O from/to a given region is sent on only one of the active paths. Should that path fail, the workload is automatically redistributed across the remaining paths.

You can use the partitionsize attribute to specify the size for the partition. The partition size in blocks is adjustable in powers of 2 from 2 up to 231. A value that is not a power of 2 is silently rounded down to the nearest acceptable value.

Specifying a partition size of 0 is equivalent to specifying the default partition size.

The default value for the partition size is 512 blocks (256k). Specifying a partition size of 0 is equivalent to the default partition size of 512 blocks (256k).

The default value can be changed by adjusting the value of the dmp_pathswitch_blks_shift tunable parameter.

For example, the suggested partition size for an Hitachi HDS 9960 A/A array is from 32,768 to 131,072 blocks (16MB to 64MB) for an I/O activity pattern that consists mostly of sequential reads or writes.

The next example sets the balanced I/O policy with a partition size of 4096 blocks (2MB) on the enclosure enc0:

# vxdmpadm setattr enclosure enc0 \
  iopolicy=balanced partitionsize=4096

minimumq

This policy sends I/O on paths that have the minimum number of outstanding I/O requests in the queue for a LUN. No further configuration is possible as DMP automatically determines the path with the shortest queue.

The following example sets the I/O policy to minimumq for a JBOD:

# vxdmpadm setattr enclosure Disk \
  iopolicy=minimumq

This is the default I/O policy for all arrays.

priority

This policy is useful when the paths in a SAN have unequal performance, and you want to enforce load balancing manually. You can assign priorities to each path based on your knowledge of the configuration and performance characteristics of the available paths, and of other aspects of your system.

In this example, the I/O policy is set to priority for all SENA arrays:

# vxdmpadm setattr arrayname SENA \
  iopolicy=priority

round-robin

This policy shares I/O equally between the paths in a round-robin sequence. For example, if there are three paths, the first I/O request would use one path, the second would use a different path, the third would be sent down the remaining path, the fourth would go down the first path, and so on. No further configuration is possible as this policy is automatically managed by DMP.

The next example sets the I/O policy to round-robin for all Active/Active arrays:

# vxdmpadm setattr arraytype A/A \
  iopolicy=round-robin

singleactive

This policy routes I/O down the single active path. This policy can be configured for A/P arrays with one active path per controller, where the other paths are used in case of failover. If configured for A/A arrays, there is no load balancing across the paths, and the alternate paths are only used to provide high availability (HA). If the current active path fails, I/O is switched to an alternate active path. No further configuration is possible as the single active path is selected by DMP.

The following example sets the I/O policy to singleactive for JBOD disks:

# vxdmpadm setattr arrayname Disk \
  iopolicy=singleactive