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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 171.

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 140.

See "I/O throttling" on page 141.

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).

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 167.

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 outstanding 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 168.