A multiple session restore operation allows backup-archive clients to start multiple sessions that use no-query restore operations, thus increasing the speed of restore operations. A multiple session restore operation is similar to a multiple session backup operation.
When you request a backup or archive, the client can establish more than one session with the server. The default is to use two sessions: one to query the server and one to send file data.
Parallel (concurrent) backup and restore operations that work with sequential file or tape storage pools require multiple mount points. A mount point is a tape or a file device class volume. The resourceutilization client option governs the maximum number of concurrent backup or restore sessions that the client can use. The MAXNUMMP server parameter, on the UPDATE NODE or REGISTER NODE commands, and the MOUNTLIMIT setting in the DEFINE DEVCLASS and UPDATE DEVCLASS commands, determines how many mount points a client node can use, at one time.
Configure these settings according to your requirements and available hardware. Take into account the number of mount points that all nodes might need, at any one time. For example, if you have four client nodes and only eight tape drives, if you configure all four nodes with MAXNUMMP 8, one node can seize all of the tape drives, leaving no tape drives for other nodes to use.
If all the files are on random disk, only one session is used. There is no multiple session restore for a random-access disk-only storage pool restore. However, if you are restoring files and the files are on four sequential disk volumes (or on four tape volumes) and other files are on random access disk, you can use up to five sessions during the restore.
Server settings take precedence over client settings. If the client resourceutilization option value exceeds the value of the server MAXNUMMP setting for a node, you are limited to the number of sessions that are specified by the MAXNUMMP parameter.
dsmc restore /home/*The server sends the MAXNUMMP value to the client during sign-on. During a no-query restore operation, if the client receives a notification from the server that another volume that contains data to be restored was found, the client checks the MAXNUMMP value. If another session would exceed the MAXNUMMP value, the client does not start the session.
Only one producer session per file system compares attributes for incremental backup. Incremental backup throughput does not improve for a single file system with a small amount of changed data.
Data transfer sessions do not have file system affinity; each consumer session could send files from multiple file systems, which helps balance the workload. Sending files from multiple file systems is not beneficial if you are backing up directly to a tape storage pool that is collocated by file space. Do not use multiple sessions to back up objects directly to a storage pool collocated by file space. Use multiple commands, one per file space.
The setting of the resourceutilization option and internal heuristics determine whether new consumer sessions are started.
When you backup objects directly to tape, you can prevent multiple sessions, so that data is not spread across multiple volumes, by setting the resourceutilization option to 2.
Only one session is used when files are restored from random access disk storage pools.
Only one file system can be restored at a time with the command line, but multiple sessions can be used on a single file system.
Even small clients can realize improved throughput of restore operations if the data to be restored is on multiple tapes. One session can be restoring data while another might be waiting for tapes to be mounted, or be delayed while it is reading a tape, looking for the data to restore.
Tape cartridge contention might occur, especially when files are not restored from a collocated pool. Collocating files reduces the likelihood of tape cartridge contention.