Marianne
Thanks for your assistance. The NetBackup server (everything NB-related is centralized in one single physical server) has 32 GB of RAM, I understand that should be enough for deduplicating backups: the sum of all the full backups made during a weekend are about 15 TB.
All backups are made to SLP which backups first on the MSDP disk pool and then duplicates everything to tape. I reduced the concurrent jobs the disk storage unit can handle to 30 and relocated the SLP duplication schedule so backups and duplications don't coincide. We didn't observe any positive influence in the problem.
We monitored the performance and noticed that the RAM use stays at about 55% (18 GB used of the total 32 GB), CPU usage also only uses half. When the Disk Pool goes down we see the CPU usage jumps to only 30% of usage or lower, but I think that's a consequence of jobs not working. RAM usage stays the same. Attached: picture of the performance monitor, showing the CPU usage going down after the Disk Pool goes down.
I'll have to check throughly the TN, I'm not sure of the impact of changing NTFS settings on existing data. I can confirm we already checked the antivirus was excluded from the MSDP drive and filesystem indexing was also disabled. MSDP is on the Master server, I know this isn't recommended but the performance is good.