RAID strategy - 8 1TB drives

Solution 1:

Fore pure cold backup I'd say RAID5 is fine. RAID5 will take a considerable amount of time to restore on 8x1TB SATA drives, but given the chances of something breaking at low IOPS that's probably not critical. If you want the added bit of safety, go for RAID6 which will give you the ability to survive two simultaneous disk failures.

Solution 2:

Here is a good article about: Why RAID 5 stops working in 2009

Here's a quick of summary:

• Disk Failure: With 7 disks you have ~20% chance of seeing a disk failure in one year. It's quite likely you'll see a failure over the operational lifetime of the storage.
• Read Failures: "SATA drives are commonly specified with an unrecoverable read error rate (URE) of 10^14." This means that for a hypothetical 12TBs of storage you're likely to see read failures. This is just a function of the increasing amount of storage that is commonly being used. More storage, more read failures.

It works like this according to the author: More disks (increased probability of disk failure) + more disk space (increased probability of a read failure) = Increased probability of both events happening at the same time. What will kill your RAID array is when a disk fails and while your array is rebuilding you have a read failure.

The author's solution: RAID-6

Here's a comment that seems to sum it rather well:

The key point that seems to be missed in many of the comments is that when a disk fails in a RAID 5 array and it has to rebuild there is a significant chance of a non-recoverable read error during the rebuild (BER / UER). As there is no longer any redundancy the RAID array cannot rebuild, this is not dependent on whether you are running Windows or Linux, hardware or software RAID 5, it is simple mathematics. An honest RAID controller will log this and generally abort, allowing you to restore undamaged data from backup onto a fresh array.

Editor's Notes: You could just use quality SCSI or SAS drives...