Multi-session backup does not reuse sessions that end

Resolving The Problem

In the case where multiple file systems are backed up or archived at the same time, it is possible that one file system may use only 1 session to backup its file, even though this file system contains a lot of files to be backed and resourceutilization = 10.

The following information is explained in the performance tuning guide,

The default resourceutilization level allows up to two sessions with the server, one for querying the server and one for sending file data. Resourceutilization=5 permits up to four sessions (two for queries and two for sending data), while resourceutilization=10 permits up to eight sessions (four for queries and four for sending data) with the server. Producer sessions scan the client system for eligible files. The remaining sessions are consumer sessions and are used for data transfer.

So assuming that a schedule is used to backup 4 file systems and that resourceutilization=10 is used, 4 producer and 4 consumer sessions are started. The client will start 1 producer and 1 consumer session for each file specification, totalling the 8 sessions.

For example, assuming the following 4 file systems are backed up as part of a schedule, /fs/small1, /fs/small2, /fs/small3, /fs/big, and assuming that files need to be backed up in each, then 1 producer and 1 consumer would be started for each file system.

When the backup is done for "/fs/small1", its producer session and its consumer session terminate. This leaves 6 sessions running. Those last 2 sessions that terminated are not reused by the client.
Similarly, when the backup is done for "/fs/small2", its producer session and its consumer session terminate. This leaves 4 sessions running. Those last 2 sessions that terminated are again not reused.
Finally, when the backup is done for "/fs/small2", its producer session and its consumer session terminate. This leaves 2 sessions running. Those last 2 sessions that terminated are also not reused.
In the end, you are left with 2 running sessions, 1 producer and 1 consumer for the
"/fs/big" file system. Eventually, the producer will also terminate, which leaves only one consumer session to backup the /fs/big file system, no matter how much more data is left to backup on that file system.

This is the design of the multi-session backup. Once the maximum number of sessions (8 in this case) is reached, no more sessions are started even if some of the previous sessions terminated. To make sure that a particularly big file system is backed up with the maximum number of sessions in a multi-session backup, it might be necessary to backup this file system by itself. In the case of schedule backup, 2 separate schedules may be used, i.e, one schedule for/fs/small1, /fs/small2, /fs/small3, and one schedule for /fs/big only.