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RAIDZ space efficiency #475

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RAIDZ space efficiency #475

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Update RAIDZ.rst
jameskimmel Nov 28, 2023
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13 changes: 8 additions & 5 deletions docs/Basic Concepts/RAIDZ.rst
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Expand Up @@ -51,20 +51,23 @@ we will allocate on disk:

- one 4K block of data

- one 4K padding block
- one 4K parity block

and usable space ratio will be 50%, same as with double mirror.


Another example for ``ashift=12`` and ``recordsize=128K`` for raidz1 of 3 disks:
Another example for ``ashift=12`` and ``recordsize=128K`` for RAIDZ1 of 3 disks:

- total stripe width is 3

- one stripe can have up to 2 data parts of 4K size because of 1 parity blocks
- one stripe can have up to 2 data parts and 1 parity block, each 4k in size

- we will have 128K/2 = 64 stripes with 8K of data and 4K of parity each
- we have 128k (recordsize) diveded by 8k (two 4k data parts) = 16 stripes

so usable space ratio in this case will be 66%.
- Each of these 16 stripes, has 8k of data and 4k of parity

16 stripes * 12k (data + parity) = 192k.
We write 192k to store 128k, so usable space ratio in this case will be 66%.


The more disks RAIDZ has, the wider the stripe, the greater the space
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