Ah, I forgot to mention - ufsdump|ufsrestore was at some time also
a recommended way of such transition ;)
I think it should be aware of all intimacies of the FS, including
sparse files which reportedly may puzzle some other archivers.
Although with any sort of ZFS compression (including lightweight
zle) zero-filled blocks should translate into zero IOs. (Maybe
some metadata would appear, to address the "holes", however).
With proper handling of sparse files you don't write any of that
voidness into the FS and you don't process anything on reads.
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