Although I'm familiar with the principles behind ZFS and RAID systems in general (having used various hardware and software RAID systems in the past), I've never actually used ZFS for anything until now, so I'd appreciate some advice on my intended setup.
Basically what I have are two SATA to eSATA driverless JBOD adaptors*, each with a set of four disks (two 3tb, two 1.5tb). My plan is to combine these using ZFS such that I will have two RAID-Zs, which I would then mirror, hopefully giving me a single ~6tb volume which can cope with at least one failed drive or controller, and potentially more than one disk (depending upon where the failures actually occur).
Anyway, I'm wondering what the exact steps would be involved in this? Will I need to combine each pair of smaller disks first, then create the two RAID-Z's then create a mirror and finally the volume itself? Could someone give me some example commands for this? I'm also interested in any caveats; how difficult will it be to add capacity, bearing in mind that each controller can handle five disks and I already have four on each.
To try and clarify, my proposed setup would look something like this:
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ZFS Mirror (~6.0tb)
|_ RAID-Z (Controller 1)
| |_ 3.0tb Disk
| |_ 3.0tb Disk
| |_ ZFS Stripe or Concatenated Set
| |_ 1.5tb Disk
| |_ 1.5tb Disk
|_ RAID-Z (Controller 2)
|_ 3.0tb Disk
|_ 3.0tb Disk
|_ ZFS Stripe or Concatenated Set
|_ 1.5tb Disk
|_ 1.5tb Disk
Or put another way; each controller will have two 1.5tb disks combined into a single 3.0tb disk, added to the two other 3.0tb disks to give a total of three, which will be combined as a RAID-Z of ~6.0tb. The two resulting RAID-Z's will then be mirrored for a single ~6.0tb ZFS volume. If there are some other possible setups I might consider please let me know, though this is all I can think of, except for a RAID-10 style setup (3x striped arrays of 6.0tb each) but I don't think it offers any real advantage, while two RAID-Zs might potentially have more redundancy depending upon what fails.
I realise that such a mix of capabilities isn't ideal, though the smaller disks are actually much faster than the larger ones, so I'm hoping that when combined they should be a good match. Of course running the whole setup through only two connections isn't ideal either, but as I say, I'm not too bothered about performance, especially since it will basically be functioning as a NAS for periodic backup (probably nightly or weekly) anyway, so speeds in excess of 20mb/sec should be more than enough.
*For those that are interested, the controllers are Lycom UB-208RMs, and actually they do do hardware RAID but it leaves a lot to be desired including (but not limited to) actually rebuilding an array with a failed disk more than 10% of the time (if I didn't have other backups I'd have lost all of my data several times over by now). They are however perfectly capable JBOD adaptors, I just wouldn't recommend trusting their RAID capabilities.