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Forum Discussion
5tb
I have two ReadyNas (Ultra 4 and Ultra Pro 4). My drives are currently full so i was planning to wait in december until WD releases the 5TB red. I was just wondering if the 5tb drives will work out...
mdgm's illustration doesn't quite work for me. Here's another way to think about the redundancy. Probably more than you want to know. :D
With single redundancy and 4 disks, the volume is divided into groups of blocks. There are 3 data blocks and one parity block in each group. Each of these blocks are on a different disk.
The parity block is computed using xor
A xor B xor C => P
You can reconstruct any missing block from the other three.
A = B xor C xor P
B = A xor C xor P
C = A xor B xor P
P = A xor B xor C
This works if (a) there is only one block missing and (b) you know which block that is. That lets you solve the proper equation. If more than 1 block is missing in the group, then you are screwed. If you don't know which block is bad, you are also screwed. When you replace a drive, the NAS reconstructs all the blocks that are now missing from the remaining drives using the appropriate equations above. That is what a resync does (and is why a resync fails if there are any bad blocks on the other drives).
RAID-6 uses two parity blocks, the second one is computed using a different formula. That creates simultaneous equations, so you can reconstruct any 2 missing blocks from the rest. Like Raid-5 you need to know which two are missing, and if more than 2 blocks are missing in the group you are screwed.
On the sparc ReadyNAS all the P blocks on the the last drive in the array. The newer NAS spread the P blocks across all the drives (using a different striping pattern). Every write requires rewriting the P block, so you get better performance if you spread them across all the drives.
BTW, one consequence of cloning is that the NAS thinks all the blocks on the cloned drive are correct. Since that is not really the case (some of the original blocks couldn't be read), it will result in some data corruption.
With single redundancy and 4 disks, the volume is divided into groups of blocks. There are 3 data blocks and one parity block in each group. Each of these blocks are on a different disk.
The parity block is computed using xor
A xor B xor C => P
You can reconstruct any missing block from the other three.
A = B xor C xor P
B = A xor C xor P
C = A xor B xor P
P = A xor B xor C
This works if (a) there is only one block missing and (b) you know which block that is. That lets you solve the proper equation. If more than 1 block is missing in the group, then you are screwed. If you don't know which block is bad, you are also screwed. When you replace a drive, the NAS reconstructs all the blocks that are now missing from the remaining drives using the appropriate equations above. That is what a resync does (and is why a resync fails if there are any bad blocks on the other drives).
RAID-6 uses two parity blocks, the second one is computed using a different formula. That creates simultaneous equations, so you can reconstruct any 2 missing blocks from the rest. Like Raid-5 you need to know which two are missing, and if more than 2 blocks are missing in the group you are screwed.
On the sparc ReadyNAS all the P blocks on the the last drive in the array. The newer NAS spread the P blocks across all the drives (using a different striping pattern). Every write requires rewriting the P block, so you get better performance if you spread them across all the drives.
BTW, one consequence of cloning is that the NAS thinks all the blocks on the cloned drive are correct. Since that is not really the case (some of the original blocks couldn't be read), it will result in some data corruption.
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