ZFS delayed reuse of recently freed blocks to enable rollbacks
It was proposed and seems implemented in Solaris 11 but no other distro, to delay actual reuse of those blocks which were recently freed but can be still referenced from the uberblock ring (recent 32-128 TXG numbers). In effect this better guarantees the possibility of rollback to a consistent version of the pool, as well as aid recovery of recently erroneously deleted datasets (and files).
In one of the discussions it was suggested that such a feature can be exposed as snapshots; I think that (partially - to differ from Solaris 11) it can instead be implemented as a rolling series of pool-wide snapshots.
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