On 11/29/2014 07:15 AM, Shriramana Sharma wrote:
On Sat, Nov 29, 2014 at 7:58 PM, Hugo Mills <hugo@xxxxxxxxxxxxx> wrote:The latest version of mv should be able to use CoW copies to make it more efficient. It has a --reflink option, the same as cp. Note that you can't make reflinks crossing a mount boundary, but you can do so crossing a subvolume boundary (as you're doing here).
One thing to keep in mind is that mv, when crossing any of these boundaries degenerates to a copy-and-remove operation and _none_ of the source files will be removed until _all_ of the files have been copied. If any of the copy operations fail the removes will not take place at all. It would only take a couple large NOCOW files to put you over a limit somewhere.
So if you get to an out-of-space condition mid-move you are going to have to disentangle your stuff by hand anyway.
If you are consolidating sub-volumes (as per the original question) on a "nearly full" drive you may want to do it all long-hand with a script moving various chunks or something instead of just trying a move/copy of "cp --reflinks /vol1/* /vol2/" (same for mv when you get that --reflinks revision).
ASIDE: Also be aware that such a moment would be the perfect time to consider compression and so on. A regular copy (non reflinks) will apply the currently selected compress= regime (and reconsider sparsity etc) in a way that the move will not. e.g. once you decide to do intrusive maintenance you might be well served by taking the extra time to restructure your storage. 8-)
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