DPM Overview

DPM improves the overall reliability of failover and switchover by monitoring the secondary disk-path availability. Use the scdpm command to verify availability of the disk path that is used by a resource before the resource is switched. Options that are provided with the scdpm command enable you to monitor disk paths to a single node or to all nodes in the cluster. See the scdpm(1M) man page for more information about command-line options.

The DPM components are installed from the SUNWscu package. The SUNWscu package is installed by the standard Sun Cluster installation procedure. See the scinstall(1M) man page for installation interface details. The following table describes the default location for installation of DPM components.

A multithreaded DPM daemon runs on each node. The DPM daemon (scdpmd) is started by an rc.d script when a node boots. If a problem occurs, the daemon is managed by pmfd and restarts automatically. The following list describes how the scdpmd works on initial startup.

At startup, the status for each disk path is initialized to UNKNOWN.

1. The DPM daemon gathers disk-path and node name information from the previous status file or from the CCR database. Refer to Cluster Configuration Repository (CCR) for more information about the CCR. After a DPM daemon is started, you can force the daemon to read the list of monitored disks from a specified file name.

2. The DPM daemon initializes the communication interface to answer requests from components that are external to the daemon, such as the command-line interface.

3. The DPM daemon pings each disk path in the monitored list every 10 minutes by using scsi_inquiry commands. Each entry is locked to prevent the communication interface access to the content of an entry that is being modified.

4. The DPM daemon notifies the Sun Cluster Event Framework and logs the new status of the path through the UNIX syslogd(1M) mechanism.

All errors that are related to the daemon are reported by pmfd (1M). All the functions from the API return 0 on success and -1 for any failure.

The DPM Daemon monitors the availability of the logical path that is visible through multipath drivers such as Sun StorEdge Traffic Manager, HDLM, and PowerPath. The individual physical paths that are managed by these drivers are not monitored, because the multipath driver masks individual failures from the DPM daemon.