Hello Heitor and Dan,
When a Job is despooling (disk->tape), the file daemon will wait. It will just begin spooling again (if necessary, i.e., amount of space that can be used by the job in the spool area is less then the amount of data that will be backed up for this client). The others file daemons will be spooling to disk.
So IMHO the more large the spool area you could have for a job, the minimum interleaving you will have.
It will depend also if you have jobs with very diversified amounts of backup data (total backup size per job). I would choose the average value (of the total backup size for each job) for the Maximum Spool Size, if I did not had enough space in disk to choose the highest value (the highest total backup size that a job could have), to minimize data interleaving.
In any way, you will speed up your backups since the network delay for the data travels from client to the storage is greater than the transfer speeds from disk to tape (supposing your spool area data is not traveling through network).
