NETGEAR is aware of a growing number of phone and online scams. To learn how to stay safe click here.
Forum Discussion
Xraid 5 Parity and Stripping how much capacity?
I've been trying to get my head round this one. Take a 4 drive nas for instance with four 500gb drives. With xraid5 the volume availanle is 1500gb. That's where I'm stumped and I know it's because of ...
mdgm likes those equations, but I also find them confusing (and I do know how it works).
There is a diagram here (http://www.thegeekstuff.com/2010/08/rai ... -tutorial/). The relevant section is Raid-5 (though it shows only three disks).
The parity block is a logical XOR of the three data blocks. Let's call a group of blocks D1, D2, D3, P. the D's are data, the P is the parity
P is computed so
D1 xor D2 xor D3 xor P = 0 (this is for every byte in the block).
If a block is missing, you can rebuild it from the other 3 by solving the equation above. Any one of the 4 blocks could be missing, the 4 equations are:
P = D1 xor D2 xor D3
D1 = P xor D2 xor D3
D2 = D1 xor P xor D3
D3 = D1 xor D2 xor P
You arrange the blocks in a known pattern so that D1, D2, D3, and P are always on different disks (this is shown in the picture I linked to). So if one disk is replaced, you can rebuild all the blocks on it using the above equations. If 2 disks fail, you don't have the information you need to rebuild anything.
RAID-6 is similar, except there are 2 parity blocks, and they are constructed in different ways. You end up with 2 simultaneous equations, so you can solve them if you have 2 unknowns (any two missing blocks can be reconstructed).
Of course if you have a Duo, there are only two disks. In that case, the two disks are simply mirrored (that is called RAID-1).
There is a diagram here (http://www.thegeekstuff.com/2010/08/rai ... -tutorial/). The relevant section is Raid-5 (though it shows only three disks).
The parity block is a logical XOR of the three data blocks. Let's call a group of blocks D1, D2, D3, P. the D's are data, the P is the parity
P is computed so
D1 xor D2 xor D3 xor P = 0 (this is for every byte in the block).
If a block is missing, you can rebuild it from the other 3 by solving the equation above. Any one of the 4 blocks could be missing, the 4 equations are:
P = D1 xor D2 xor D3
D1 = P xor D2 xor D3
D2 = D1 xor P xor D3
D3 = D1 xor D2 xor P
You arrange the blocks in a known pattern so that D1, D2, D3, and P are always on different disks (this is shown in the picture I linked to). So if one disk is replaced, you can rebuild all the blocks on it using the above equations. If 2 disks fail, you don't have the information you need to rebuild anything.
RAID-6 is similar, except there are 2 parity blocks, and they are constructed in different ways. You end up with 2 simultaneous equations, so you can solve them if you have 2 unknowns (any two missing blocks can be reconstructed).
Of course if you have a Duo, there are only two disks. In that case, the two disks are simply mirrored (that is called RAID-1).
NETGEAR Academy
Boost your skills with the Netgear Academy - Get trained, certified and stay ahead with the latest Netgear technology!
Join Us!
