Hi,
I am involved in doing a ZFS implementation for a research OS developed at DTU,
Denmark. In the process, a question has come up:
Does ZFS really provide a 128 bit storage system?
Jeff Bonwick seems to think so, as he writes on his blog: "A fully-populated
128-bit [zfs?] storage pool would contain 2128 blocks = 2137bytes = 2140 bits;"
I just can't make out the math for this. The structure used for addressing data
in a ZFS pool, the DVA uses 32 bits to address the vdev and 63 bits to address
the sector offset on this vdev where a given block resides. The DVA is 128
bits, but 8 of these bits is used for Raid-Z info, 24 bits is used to hold the
size of the block, and 1 bit is used as a gang flag, and these does not
contribute to the amount of addressable blocks. When people say that ZFS is a
128 bit storage system, they refer to the number of addressable blocks in a
storage pool, right? It seems to me that one is only able to address 2^32 *
2^63 = 2^95 blocks in each pool.
If anybody can put some light on this matter, it will be greatly appreciated.
Regards,
Andreas Hindborg
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