Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

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.
See “Configuring DMP path restoration policies” on page 189.
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 141.
See “I/O throttling” on page 142.
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.
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).
141Administering Dynamic Multipathing
How DMP works