On Fri, Oct 14, 2011 at 09:45:05AM +0100, Hugo Mills wrote: > Updating and checking the reference count is the thing > that takes time, and can't really be short-circuited.
I'm curious if you know how ZFS does this. My boss has used a set of ZFS systems for backup, and indicates that deleting a snapshot is instant, and free space is updated immediately with no apparent background I/O. I know ZFS only has read-only snapshots, but I would think it would have some of the same reference counting challenges. I see reading one of the ZFS presentations that it uses a "birth time" in each block pointer. This lets it optimizes the deletion by walking only a partial set of the nodes. Still, for any non-trivial deletion it would have to walk enough nodes to keep the delete from being as quick as is claimed. -- Bruce Guenter <br...@untroubled.org> http://untroubled.org/
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