Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

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.
See “Configuring the response to I/O failures” on page 189.
Subpaths Failover Group (SFG)
An 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 group, 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 IO performance. Currently the
criteria followed by DMP to form the subpath failover groups is to bundle the
paths with the same endpoints from the host to the array into one logical storage
failover group.
See “Configuring Subpaths Failover Groups (SFG)” on page 192.
Low Impact Path Probing (LIPP)
The restore daemon in DMP keeps probing the LUN paths periodically. This
behavior helps DMP to keep the path states up-to-date even though IO activity is
not there on the paths. 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
limited number of paths within an 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.
See “Configuring Low Impact Path Probing” on page 192.
I/O throttling
If I/O throttling is enabled, and the number of outstanding I/O requests builds up
on a path that has become less responsive, DMP can be configured to prevent new
I/O requests being sent on the path either when the number of outstanding I/O
requests has reached a given value, or a given time has elapsed since the last
successful I/O request on the path. While throttling is applied to a path, the new
I/O requests on that path are scheduled on other available paths. The throttling
is removed from the path if the HBA reports no error on the path, or if an
outstanding I/O request on the path succeeds.
See “Configuring the I/O throttling mechanism” on page 191.
Administering Dynamic Multi-Pathing
How DMP works
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