RAID5 on two disks

When recovering data from a client's NETGEAR ReadyNAS device we saw a rather strange layout - RAID5 of two disks, which appeared once ReclaiMe File Recovery has processed md-raid records. After looking at it for a while we realized that this is, surprisingly, possible given that we ignore the requirement of a minimum number of disks. So let's see what happens when placing data and parity blocks on two disks in a RAID5 layout where a stripe contains, say, three sectors:

Now let's see what are there in the parity blocks in such a layout? In order to get an even row of two elements it is needed that all the elements would be the same, so the content of the parity block is the same as the content of its corresponding data block. Therefore we deal with a typical RAID1 (mirror) layout. Despite all the wildness of the layout, it meets all the basic criteria of a RAID5:
  • survives a single disk failure,
  • the disk space overhead equals to the capacity of one member disk.
As far as data recovery goes, such a RAID5 layout is better than a typical one since it doesn't require the recovery of RAID configuration parameters should the md-raid setup fail. In this case it would be enough to get each disk in turn, recover data as in case of a regular hard drive recovery, compare the results, and chose the best.


  1. seems like a shady setup to be honest. Why not just run a mirror?

    1. That was a NetGear ReadyNAS with its FlexRAID or whatever. I suppose they use this trick to simplify the online expansion when the disk is added.


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