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DMP enhances system availability when used with disk arrays having multiple paths. In the event of the loss of a path to a disk array, DMP automatically selects the next available path for I/O requests without intervention from the administrator.
DMP is also informed when a connection is repaired or restored, and when you add or remove devices after the system has been fully booted (provided that the operating system recognizes the devices correctly).
If required, the response of DMP to I/O failure on a path can be tuned for the paths to individual arrays. DMP can be configured to time out an I/O request either after a given period of time has elapsed without the request succeeding, or after a given number of retries on a path have failed.
A subpaths failover group (SFG) represents a group of paths which could fail and restore together. When an I/O error is encountered on a path in an SFG, DMP does proactive path probing on the other paths of that SFG as well. This behavior adds greatly to the performance of path failover thus improving I/O performance. Currently the criteria followed by DMP to form the subpaths failover groups is to bundle the paths with the same endpoints from the host to the array into one logical storage failover group.
The restore daemon in DMP keeps probing the LUN paths periodically. This behavior helps DMP to keep the path states up-to-date even when no I/O occurs on a path. Low Impact Path Probing adds logic to the restore daemon to optimize the number of the probes performed while the path status is being updated by the restore daemon. This optimization is achieved with the help of the logical subpaths failover groups. With LIPP logic in place, DMP probes only a limited number of paths within a subpaths failover group (SFG), instead of probing all the paths in an SFG. Based on these probe results, DMP determines the states of all the paths in that SFG.
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