As others have commented, the RAID5 will function as its own layer below Vail's DE storage system. Theoretically, if Vail develops a fault, it will not affect the RAID and vice versa.
So to your question, overall efficiency is the product of two factors: the efficiency of your RAID5 and the efficiency of DE itself. RAID5 efficiency depends on the number of spindles, 66%, 75% and 80% for 3, 4 and 5 disks respectively. DE efficiency
is 88% without duplication and 44% with. Multiply the appropriate numbers and that will be your answer.
For your other question, I don't really know how to answer that. I am not very sure if such a configuration actually makes sense in the first place. Remember that Vail's primary advantage is that it provides a way to transparently upgrade storage
by adding disks of any size in any order, and taking care of the redundancy for you. RAID5 of course has no such functionality, so if you wanted to add a disk, you would have to add it using DE. It is certainly possible to configure a new multi-disk
RAID5 array, presented as a single disk and use DE to add that to the pool, if your controller can support that many disks and if you are willing to spend that much on an upgrade.
The cheaper and easier way would be to add a single disk without any RAID configuration to the pool, which is what Vail was designed to do. However, if you did that, you would then have a mix of redundant and non-redundant storage in your pool.
Running that type of pool without duplication would carry a risk of data loss from the failure of the non-redundant disk.
If all that sounds very complicated to you, it's complicated to me too. I don't think I've fully explored this yet but it already seems like something I wouldn't want to do. The simple answer to your question, is it wise to turn off duplication
in this scenario, NO.
However, the long answer, I think you should not do this at all. Go with a standard configuration, so that at least when you run into problems, there will be answers for you on this forum.
