Sorry if my post was not clear enough. I picked the term "cross-fs" from the implementation of vfs_copy_file_range. Below snippet, inside vfs_copy_file_range - 1578 /* this could be relaxed once a method supports cross-fs copies */ 1579 if (inode_in->i_sb != inode_out->i_sb) 1580 return -EXDEV; And I was not trying reflink for copy, rather I just used example code for copy_file_range syscall - http://man7.org/linux/man-pages/man2/copy_file_range.2.html This code creates a new file first, and later issues "copy_file_range" for copying. I was trying this sort of copy among files residing on btrfs, ext4, and xfs. Although vfs_copy_file_range internally uses clone_file_range (which would be true for btrfs and xfs). On Mon, Oct 1, 2018 at 8:18 PM Qu Wenruo <quwenruo.btrfs@xxxxxxx> 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. > > 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. > > > It seems current implement has below check, that disables such copy. > > > > 1577 /* this could be relaxed once a method supports cross-fs copies */ > > 1578 if (inode_in->i_sb != inode_out->i_sb) > > 1579 return -EXDEV; > > > > May I know what are the thoughts behind disabling cross-fs copy? > > Code has the comment "once a method supports", but that leaves me > > wondering exactly what 'method' is expected, and from whom. > > > > I disabled the check, and copy across volumes seemed to work fine. At > > least for a single file (1G size), with no data mismatch, and faster > > speed than regular copy. > > Please provide the steps or script about how you did the reflink, in > case I misunderstand your "cross-fs" definition. > > And just in case you're really doing cross filesystem reflink, please > also run "btrfs check" on both fs. > > Thanks, > Qu > > -- Joshi
