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The reason that ZFS has such trouble with this notion is that a lot of its internal architecture depends on "block pointers" (offsets of data into a given vdev) being immutable - this is, among other things, why CoW snapshots are so cheap. And attempting to rewrite all of these without having any gaps in consistent on-disk state is...challenging, to say the least.

Sun apparently had internally done most(?) of the work on BP rewrite (as it's referred to), but performance sucked rather badly, and the work has not been shipped by Sun/Oracle, let alone anything that came out before the F/OSS code sharing ceased. [1]

(Performance sucked, in particular, for similar reasons to why dedup performance has such a high penalty on ZFS - they end up storing a lookup table that gets hit for every block that gets written after you turn on dedup, and so if that table stops fitting in RAM, it's a bad time - let alone the issue of making sure the disk backing the DDT storage is sufficiently performant...)

[1] - http://www.listbox.com/member/archive/182180/2012/01/sort/th...




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