linux-ext4 - Re: cross-fs copy support

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Date:   Mon, 1 Oct 2018 20:45:45 +0530
From:   Joshi <joshiiitr@...il.com>
To:     quwenruo.btrfs@....com
Cc:     linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: cross-fs copy support

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@....com> 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