Accelerator could be used with a STU Group if it is configured in Failover mode. The following could be found in the Nbu 7.6 admin guide, as well as a 7.6 technote.
"Storage unit groups are supported only if the storage unit selection in the group is Failover."
This is also true for 7.5, but curiously, it isn't mentioned in the 7.5 guides.
What that line does not elaborate on, is that the Failover mode STU Group could only contain Dedup STUs.
An Accelerator policy would not check out if, for example, it uses a Failover mode STU Group that contains both Dedup STUs and Tape/BasicDisk STUs. (Makes sense, because Accelerator relies on Deduplication.)
In other words, you could only use a Failover mode STU Group if there is more than one Dedup Storage Server in the Nbu domain. If only one of the Media Servers has a Dedup Storage Server, then there will be nothing to failover to.
Currently, a MSDP Dedup Storage Server must coexist on the same server as a Media Server. Standalone Dedup Storage Servers exist in the form of the NetBackup 50xx series appliance, as well as 3rd party OST appliances.
One of the benefits of Standalone Storage Servers is that there is no single point of failure between the Media Servers using the same one. In this config, however, the Standalone Storage Server itself will be the single point of failure. It all depends on the level of resiliency you are willing to accept. (Well, at least they employ raid1 / raid6 in these appliances.)
There was a similar discussion in this post.
