Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

124 Administering dynamic multipathing (DMP)
How DMP works
See “Changing the disk-naming scheme” on page 92 for details of how to change the
naming scheme that VxVM uses for disk devices.
See “Discovering and configuring newly added disk devices” on page 81 for a description
of how to make newly added disk hardware known to a host system.
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 thread 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 has remained healthy for a
given period of time (by default, 5 minutes). DMP can be configured with different
policies for checking the paths as described in “Configuring DMP path restoration
policies” on page 152.
The statistics-gathering thread 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 FAILFAST 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 124 and “I/O throttling” on page 125 for more
information about these features of DMP.
Path failover mechanism
The DMP feature of VxVM 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.