For backups I want to use an external USB3 Drive: two HDD with RAID 1 (mirror) in one 2-bay enclosure. As filesystem I'll use btrfs because of subvolumes and checksums. Performance isn't important, but data safety. For use of RAID1 I see 3 possibilities:
- built-in hardware RAID of the external enclosure
- the RAID capability from btrfs (and setup the 2 drives as separate/JBOD)
- use software RAID from linux (mdadm and JBOD for the disks)
What would you recommend?
For the built-in hardware RAID I see the disadvantages that perhaps S.M.A.R.T. and power saving mode won't go through the controller and wont't work. This will depend on the product and perhaps SMART is dispensable, but I would miss a power saving mode (not because of power consumption and HDD lifetime, but because of the noise; I'll use the backup system only 1-3 times each day for some minutes).
For the btrfs RAID I see the big advantage that because of the checksums in the filesystem, btrfs can detect which disk is corrupted, also which parts/files, and can repair it (overwrite the corrupted data with the valid). A hardware RAID only helps if a complete disk fails, but doesn't know which runnig disks have corrupted data. Yes, the btrfs checksums also helps after disk fault with hardware RAID, but it seams much easier with the btrfs RAID, e.g. if different parts of data of both disks are corrupted.
But I mistrust the combination JBOD / ext. USB. Both disks will be connected over one USB cable and one USB hub and the controller have to handle both disks over this single/shared connection. In my experience USB makes problems with even less complex configurations.
And the software RAID (mdadm) solution. I can't see any advantages over the other two.
What do you think? Which configuration would you recommend?