A while back, a friend of mine who's a filmmaker, asked if I had any ideas on how to do fairly big storage without a huge initial cost. Performance is not a big issue, but reasonable redundancy is.
So I started thinking about using Linux software raid to build a RAID 60 of 2.5" disks over USB3. USB3 hubs are cheap, USB3 SATA controllers are cheap and 2.5" SATA disks are cheap.
Is there any reason why I couldn't add a hundred USB disks, perhaps in groups of five in md RAID6 and then striping them to get a very big RAID60?
71 Answer
While technically doable, I don't think its a very good idea - You will have difficulties tracking which disks are failing, the system will be cumbersome and prone to issues. 2.5" disks are not good value (per gig) compared with 3.5" disks. You will also likely have issues with stability of the RAID - both because of vibration and IO limits.
If I were tackling the problem I'd consider looking at a second hand NAS (with new drives), or Backblaze pods are not excessively priced and probably do what you want without a heavy premium.
IMPORTANT ADDENDUM
DO NOT USE RAID6 (or RAID5). These not scale well - the statistical chance of additional disks failing during a rebuild is surprisingly high with large disks (not to mention performance difficulties). Go for RAID10
Alternatively, I'd look for cheap computers with good 80+ PSU's, lots of hard drive bays and then use a distributed file system for redundancy and scaling - something like moosefs or gluster.
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