Solstice Backup 5.1 Administration Guide

Logical Volume Backup

A logical volume is a type of primary (disk) storage on a client machine that can span several physical disk volumes. The logical volume has its own device address, and it is treated similarly to a disk partition by the filesystem. When Backup backs up data from clients, it has to determine how many save sessions to allocate to each client for best performance. To avoid contention, there should not be more than one backup operation running per physical disk. Backup attempts to allocate different sessions across different physical disks to avoid contention.

To determine how many save sessions to allocate, the Backup server probes (queries) the clients in a backup group (using the savefs -p command) to find out what data to back up and where the data is physically located. Backup tries to determine whether there are logical volumes. It stores this information in two variables, disk-number and maximum-sessions, according to the following rules:

When the group of volumes or disks that contain logical volumes is not part of the device path, all logical volumes on the client machine are assigned to the same disk-number, and maximum-sessions is set to the number of logical volumes on the client machine.
When the group of volumes or disks that contain logical volumes is part of the device path, all logical volumes within the volume group are assigned to the same disk-number, and maximum-sessions is set to the number of logical volumes within the volume group.

The server uses the output from the savefs probe to allocate its save sessions (up to the maximum server parallelism) across the clients in the backup group:

First, the server allocates one save session per client in the backup group.
Then, if there are still save sessions available, it allocates one save session per physical disk on each client.
If, after that, there are still save sessions available, it allocates save sessions to each disk-number value, up to the limits in maximum-sessions for each client and client parallelism.