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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?

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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.

Picture from Wikipedia.

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.

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  • So, I guess you could say, "RAID-10 provides 1.66 drives worth of redundancy"? ;-)
    – IQAndreas
    Feb 9, 2016 at 15:26
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    You know, if I was in marketing, I'd say, "Up to 2 drives" :)
    – slhck
    Feb 9, 2016 at 16:15
  • That's the exact phrase I keep hearing thrown around (and causing me confusion), which is the reason I opened this question.
    – IQAndreas
    Feb 9, 2016 at 16:19
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    "it offers the same redundancy" only with 4 disks. For any Array with more than 4 disks, see this post for some "maths" on why to use 10 not 01: serverfault.com/questions/145319/…
    – dognose
    Aug 23, 2017 at 19:40
  • Yes, a 10-disk array supports the loss of "upto 5 disks" in any configuration. (One whole Group in case of Raid-01, or one disk of each group in case of Raid-10) But the Propability to actually LOOSE a critical disk is way higher for 01: After the first disk is lost, 5/9 disks have the chance to kill your Raid 01 (Any Disk of the second Raid-0-Group). In a Raid 10, only 1/9 disks has the capability to do this. (The second disk in the very same Raid-1-Group)
    – dognose
    Aug 23, 2017 at 20:05

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