On Mon, Jan 12, 2009 at 01:59:10PM -0500, Moore, Joe wrote:
> Nicolas Williams wrote:
> > It'd be awesome to have a native directory->dataset conversion feature
> > in ZFS. And, relatedly, fast moves of files across datasets
> > in the same
> > volume. These two RFEs have been discussed to death in the list; see
> > the archives.
>
> This would be a nice feature to have. The most compelling technical
> problem I've seen in the idea of "reparenting" a directory to be a
> top-level dataset is that when a zfs filesystem is used, open files on
> that filesystem have a particular devid. In order to split off the
> directory onto a new zfs filesystem, you'd have to atomically change
> the devid inside all the processes that have open files under that
> directory. Finding those open files is practically impossible.
Hmmm, no, that's not impossible as far as kernel-land references --
rollback already has to do this, for example.
If a process has stat()ed a file and recorded the {st_dev, st_ino}
somewhere, then it will be confused later. But this is no different
than what would happen if you did "cp -r A B && rm -rf A && mv B A", so
it's hardly a serious problem. Similarly w.r.t. NFS file handles.
The real problem is that ZFS needs to manage what blocks are and are not
shared between the two datasets. The snapshot/clone approach does that,
but inefficiently. There's an obvious way to modify the snapshot/clone
approach so that you get the right amount of block sharing, but just
because it's obvious does not mean that it can be implemented in one
day.
Nico
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