System Requirements

Bacula will require some space for temporary files and logs.

By default, the directory /opt/bacula/working is used to store the following things:

Messages spooled by the Director
BWeb log files
spooled SD attributes
DIR, FD, SD trace and dump files
Plugin log files and other plugin-specific data

It is best practice to have the following partitioning to avoid filling your root space.

It is recommended you have:

at least 10GB+ for /opt, we recommend this value as /opt/bacula/working is heavily used for caching purpose, plugin logs and single file restore mount points and logs.
a separated partition for your catalog (typically, at least 150 bytes and up to a multiple of that are used for each object and file kept in the catalog).
a separate file system backed by dedicated storage devices (striped NVMe or SSDs) for data spooling (if you use spooling to tapes). The spooling configuration (Directive SpoolSize) should be adjusted to allow for 80 to 95 percent usage of the storage hardware. Remember that spooled attributes will go by default to /opt/bacula/working.

Note

Partitioning your system will guarantee an efficient load on each partition and avoid filling of the root one.

Regarding the space allowed for disk based backup, please configure it to be extensible by using LVM/ZFS or any other mechanism, even by assigning jobs to a dedicated partition and use copy and migration jobs afterwards to keep useful jobs.