I do not see a reason to layer any more protocols than required, if avoidable (that seems to be more popular in the Windows world where inefficiency is expected and even welcomed with great grandure).
I do have the Drobo B800i currently configured with HFS+ (which I did successfully mount under Linux, but its kind of not optimal), with an encrypted container that has ext4 on it.
However, I have a second Drobo B800i (in a different city and I need to retrieve it). Whence I pick up that 800i, I am going to try the following configuration:
a) Create no volumes upon the B800i (thus, whence Linux logs into it via iSCSI I expect to see 8 1TB individual raw volumes)
b) I will then use LUKS to encrypt each of the 8 raw volumes
c) I will then create a ZFS pool with each of the encrypted raw volumes
This would then mean that if any DASD failed, it could be removed from the ZFS pool (and the Drobo B800i for that matter), in the absence of requiring data cleansing as it was encrypted to start with, and merely replaced, encrypted again and brought back into the ZFS pool.
At such time as I have both Drobo B800i units here, I could do that with both of them and use ZFS to appoint my redundancy, volume management and LUKS for encryption.
With the Drobo B800i being a “legacy unit” now, it seems rather doubtful that Drobo will create an ext4 firmware for the unit, and using it as a pure iSCSI SAN allows me to leverage any filesystem such as BTRfs or ReFS or anything else I desire that be applied on top of iSCSI targets.
If I use RAIDZ2, then I can loose up to 2 individual DASDs before I am SOL. Moreover, ZFS, BTRfs, ReFS, etc… all have a very easy mechanism for growing filesystems, something that was not so easily done at the time Drobo started to produce such units as I own now. Now, if Drobo were to offer a firmware that had ZFS on it for the legacy units, I am quite sure they could get people to pay something for that!
When I first got the Drobos I was not sure how I would use them, and a friend that had used them in the past suggested that I go about using their internal firmware capabilities. He mostly uses Macs (and I do too), but I also wanted a filesystem that was accessible from Solaris, MacOS, Linux and potentially Linux running on System p.
As best I can understand, it seems that as long as any additional DASDs I obtain (presuming they are higher density 4K types, 4TB or 6TB) can do 512k emulation and the sum total of drives does not exceed 32TB, then I ought be fine. Having two B800i Drobos, that is 64TB of space potentially, not nearly what I even project my space needs to be in far term, save the near term.
I believe I am going to start to purchase 4TB DASDs (as they are coming down in price now, given the higher density choices coming to the market) and buying 8 of them works out well to give 32TB in single unit fully balanced across all the DASDs. Although, if I use ZFS that will be less critical as it will figure out balancing them not the Drobo firmware.