128Administering dynamic multipathing (DMP)

How DMP works

See “Changing the disk-naming scheme” on page 91 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 82 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 160.

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 128 and “I/O throttling” on page 129 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.