If the Prefer Mounted Volumes directive is set to yes, the Storage daemon is requested to select either an Autochanger or a drive with a valid Volume already mounted in preference to a drive that is not ready. This means that all jobs will attempt to append to the same Volume (providing the Volume is appropriate – right Pool, ... for that job), unless you are using multiple pools. If no drive with a suitable Volume is available, it will select the first available drive. Note, any Volume that has
been requested to be mounted, will be considered valid as a mounted volume by another job. This if multiple jobs start at the same time and they all prefer mounted volumes, the first job will request the mount, and the other jobs will use the same volume.

If the directive is set to no, the Storage daemon will prefer finding an unused drive, otherwise, each job started will append to the same Volume (assuming the Pool is the same for all jobs). Setting Prefer Mounted Volumes to no can be useful for those sites with multiple drive autochangers that prefer to maximize backup throughput at the expense of using additional drives and Volumes. This means that the job will prefer to use an unused drive rather than use a drive that is already in use.

Despite the above, we recommend against setting this directive to no since it tends to add a lot of swapping of Volumes between the different drives and can easily lead to deadlock situations in the Storage daemon. We will accept bug reports against it, but we cannot guarantee that we will be able to fix the problem in a reasonable time.

A better alternative for using multiple drives is to use multiple pools so that Bareos will be forced to mount Volumes from those Pools on different drives.
