On Oct 1, 2018, at 9:49 AM, Eric Sandeen <sandeen@xxxxxxxxxxx> wrote: > > > On 10/1/18 9:48 AM, Qu Wenruo wrote: >> >> >> On 2018/10/1 下午10:32, Joshi wrote: >>> I was wondering about the cross-fs copy through copy_file_range. >> >> The term "cross-fs" looks pretty confusing. >> >> If you mean "cross-subvolume", then it should work without problem in btrfs. >> >> If you mean reflink across two different file systems (not matter the >> same fs type or not). >> Then it's impossible to work. > > I believe Joshi is talking about vfs_copy_file_range() not > vfs_clone_file range(), although _copy_ does call _clone_ if it can. > >> Reflink (clone_file_range) works by inserting data pointers into the >> filesystem other than really copying the data. >> Thus if the source is outside of the fs, it's really impossible to work, >> as the source pointer/data is completely out of control of the dest fs. > > Yes, I would expect there to be problems with his modified kernel > for a filesystem that supports clone_file_range, because > vfs_copy_file_range() will clone if possible, and this should fail across > filesystems. > > In general, though, I don't know for sure why we don't fall back to > do_splice_direct() across filesystems, although the filesystems that > implement their own ->copy_file_range ops may have their own, > further restrictions within their implementations. > > This call /is/ documented in the manpage as only being valid for > files on the same filesystem, though: > http://man7.org/linux/man-pages/man2/copy_file_range.2.html There was a patch to allow cross-mount copy for NFS, but it hasn't landed yet. Cheers, Andreas
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