What are the PowerPath policies and how are they used?

policy

ad

(Adaptive)

invista, ess, hitachi, hpxp, hphsx

I/O requests are assigned to paths based on an algorithm that takes into account path load and logical device priority. With a valid PowerPath license, this policy is the default for the storage classes listed in the Available storage classes column.

co

(CLARiiON

optimization)

Caution: The basic failover policy is supported only in configurations with one HBA connected to a storage system through a switch and one HBA port zoned to each SP on a CLARiiON system, or to a port on two separate FAs on a Symmetrix system. Using this policy in other configurations is not supported and may result in loss of access to data in the event of path failures. Load balancing is not in effect. I/O routing on failure is limited to one HBA and one port on each storage system interface. When a host boots, it designates one path (through one interface) for all I/O. If an I/O is issued to a logical device that cannot be reached via that path (that is, the I/O cannot reach that logical device through the device's assigned interface), the logical device is assigned to the other interface. This policy protects against CLARiiON SP failures, Symmetrix FA port failures, and back-end failures, and it allows non-disruptive upgrades to work when running PowerPath without a license key. It does not protect against HBA failures. This policy is valid only and is the default policy without a PowerPath license for the storage classes listed in the Available storage classes column. HP-UX Note: HP-UX hosts designate the first two paths to a volume for use as basic.

Load balance is based on the number of blocks in pending I/Os. I/O requests are routed to the path with the fewest queued blocks, regardless of the number of requests involved.

nr (No redirect)

symm, invista, ess, hitachi, hpxp, hphsx

Note: Do not use this policy in production environments. Use only for diagnostic purposes. Neither load balancing nor path failover is in effect. If nr is set on a pseudo device and the I/O path fails, data errors can occur. If nr is set on a native device and the I/O path fails, data errors will occur. This is the default policy for the storage classes listed in the Available storage classes except symm and hphsx, on platforms without a valid PowerPath license

All (PowerPath license), clariion only (PowerPath Base license)

For native devices, this policy uses the path that would have been used if PowerPath were not installed. For pseudo devices, it uses one arbitrary path for all I/O. For all devices, path failover is in effect, but load balancing is not. This is the default policy for CLARiiON storage systems on platforms with a valid PowerPath Base license.

rr (Round robin)

si (StreamIO)

For each possible path for an I/O to a particular volume, this policy selects the same path as was selected for the previous I/O to the volume, unless the pending I/O count since the last path change exceeds the volume's threshold value. When the threshold is exceeded, the policy selects a new path based on the adaptive policy algorithm. This policy is valid only for PowerPath 5.1 SP2 and later for Linux.

so(Symmetrix optimization)