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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 ...
There are a lot more blocks, typically the block size is 2 kilobytes.
Also, your picture shows 12 data blocks and 4 parity blocks, and 16 blocks all together. So 75% of the space is data, and 25% is parity.
75% x 2000 = 1500 GB
25% x 2000 = 500 GB of parity overhead.
The parity block always needs to be current (and written when the disks are all operational). Otherwise you can't recover anything when a disk is replaced.
So whenever a data block is changed, the parity block is recomputed and saved.
In other words, whenever you write to this raid array, you are reading the 3 data blocks, updating one of them, recomputing the parity, and writing the changed data block and the changed parity block back to the disk.
There is a faster way to do it: (a) read the data block you are going to change, read the parity block, (b) xor the parity block with the data block, (c) update the data block, xor the parity block with the new data block, and (d) write both blocks back.
Also, your picture shows 12 data blocks and 4 parity blocks, and 16 blocks all together. So 75% of the space is data, and 25% is parity.
75% x 2000 = 1500 GB
25% x 2000 = 500 GB of parity overhead.
The parity block always needs to be current (and written when the disks are all operational). Otherwise you can't recover anything when a disk is replaced.
So whenever a data block is changed, the parity block is recomputed and saved.
In other words, whenever you write to this raid array, you are reading the 3 data blocks, updating one of them, recomputing the parity, and writing the changed data block and the changed parity block back to the disk.
There is a faster way to do it: (a) read the data block you are going to change, read the parity block, (b) xor the parity block with the data block, (c) update the data block, xor the parity block with the new data block, and (d) write both blocks back.
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