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From: Joshi <>
Subject: Re: cross-fs copy support
Date: Mon, 1 Oct 2018 20:45:45 +0530	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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 -
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 <> 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


  reply	other threads:[~2018-10-01 21:54 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-01 14:32 cross-fs copy support Joshi
2018-10-01 14:48 ` Qu Wenruo
2018-10-01 15:15   ` Joshi [this message]
2018-10-01 15:49   ` Eric Sandeen
2018-10-01 19:51     ` Andreas Dilger
2018-10-02  8:15       ` David Sterba
2018-10-02 15:19         ` Darrick J. Wong
2018-10-02 18:28       ` J. Bruce Fields

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