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

How DMP monitors I/O on paths
In older releases of VxVM, DMP had one kernel daemon (errord) that performed
error processing, and another (restored) that performed path restoration
activities.
From release 5.0, DMP maintains a pool of kernel threads that are used to perform
such tasks as error processing, path restoration, statistics collection, and SCSI
request callbacks. The vxdmpadm stat command can be used to provide
information about the threads. The names errord and restored have been retained
for backward compatibility.
One kernel thread responds to I/O failures on a path by initiating a probe of the
host bus adapter (HBA) that corresponds to the path. Another thread then takes
the appropriate action according to the response from the HBA. The action taken
can be to retry the I/O request on the path, or to fail the path and reschedule the
I/O on an alternate path.
The restore kernel task is woken periodically (typically every 5 minutes) to check
the health of the paths, and to resume I/O on paths that have been restored. As
some paths may suffer from intermittent failure, I/O is only resumed on a path
if the path has remained healthy for a given period of time (by default, 5 minutes).
DMP can be configured with different policies for checking the paths.
See “Configuring DMP path restoration policies” on page 194.
The statistics-gathering task records the start and end time of each I/O request,
and the number of I/O failures and retries on each path. DMP can be configured
to use this information to prevent the SCSI driver being flooded by I/O requests.
This feature is known as I/O throttling.
If an I/O request relates to a mirrored volume, VxVM specifies the B_NDELAY
flag. In such cases, DMP does not retry failed I/O requests on the path, and instead
marks the disks on that path as having failed.
See “Path failover mechanism” on page 145.
See “I/O throttling” on page 146.
Path failover mechanism
DMP enhances system reliability when used with multiported disk arrays. 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).
145Administering Dynamic Multi-Pathing
How DMP works