Configuring the zimbraBackupMinFreeSpace attribute helps you manage running backup session by notifying you
Set the value for attribute zimbraBackupMinFreeSpace to the amount of free space required on the backup target disk before a backup session is run. If the disk has less space than the value set in the attribute, the backup session will not run and an email notification is sent to the administrator.
The value for this attribute can be specified as a percentage of the total disk space, for example 25%, or as number of bytes, for example 300MB, 50GB, etc. The default value is 0, meaning the check is disabled and backup is always allowed to start.
Backup sessions run if the free disk space is at least the value you set. If your backup file is larger than the value, the backup session fails. You should monitor the size of the backup files and adjust the attribute value if the backup require more space than the configured value.