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Forum Discussion
Easiest RAID configuration to recover data if ReadyNAS fails
Hello, I have a ReadyNAS 104 and am not able to buy another quickly if mine fails someday (the NAS, not the drives), because I live in Brazil and hardware of this type here is prohibitely expensive...
There's some more detail on the data organization used in RAID-6 here: http://simple.wikipedia.org/wiki/RAID
Here's a 5 disk example.
A1, A2, A3, B1, B2, B3 are data blocks; Ap ,Aq, Bp, Bq, ... are the parity blocks. You can see that sequence is rotated, so that the parity blocks are distributed across all the drives.
Ap and Aq are computed from the data blocks in different ways. if two blocks are unknown, you end up with 2 equations with 2 unknowns, which are solved to reconstruct the two missing blocks.
The file system (in this case linux ext) is layered on top of this level. Even if the data blocks aren't used by the file system the parity blocks are still computed. That's why it takes as long to resync an empty volume as it does a full one.
Here's a 5 disk example.
A1, A2, A3, B1, B2, B3 are data blocks; Ap ,Aq, Bp, Bq, ... are the parity blocks. You can see that sequence is rotated, so that the parity blocks are distributed across all the drives.
Ap and Aq are computed from the data blocks in different ways. if two blocks are unknown, you end up with 2 equations with 2 unknowns, which are solved to reconstruct the two missing blocks.
The file system (in this case linux ext) is layered on top of this level. Even if the data blocks aren't used by the file system the parity blocks are still computed. That's why it takes as long to resync an empty volume as it does a full one.
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