Question: In a 4-drive RAID-10 setup, how many drives can I “safely” lose before the data becomes unrecoverable? It’s a bit unclear to me whether the setup provides one drive or two drives worth of redundancy.
Is the redundancy of RAID-01 different or the same as RAID-10?
Answer: RAID-10 consists of a stripe of mirrors. As long as one disk in a mirror is functional, you’re fine.
If in the below example you lost Disk 0 and 1, you would lose data. If you lost Disk 0 and 2, your data would still be accessible from the mirrored stripe.
Since RAID-01 is a mirror of stripes, it offers the same redundancy. The disks are just logically ordered in a different way.
In practice, this means that you should already be warned when one disk fails, because the second disk failing may have a one in three chance of being the one that makes you lose data.