If the size of a volume is too small to host all of the repositories, the only option is to move to a new volume. There are times when Bitbucket Server serves many repositories that can grow in size past allowable volume size limits. This could be NAS or local disks.
If you were able to define multiple repository homes and have the UI element where you are able to choose which home to use for each repository you could split the repositories across several different volumes. A default home could still be applied for new repositories.
This would allow Bitbucket Server to grow by attaching new local disk or NAS storage and creating new homes for larger repositories. This would also improve disk IO since there would be more writable disks. This would also allow admin to start using the new home for new repositories as the default.
If the Bitbucket Server was pointed to a new home where the git repository was not, it should default to a new git repository. We should assume that the repository was migrated to the new home directory at startup.
The current workaround would be to use symlinks to move the repository to the new partition. This can cause problems with Bitbucket Server locking out other Bitbucket Server from using the repository.