linux-ext4 - Re: cross-fs copy support

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Message-ID: <2a92fff4-d005-835a-3bd7-a328b008857f@gmx.com>
Date:   Mon, 1 Oct 2018 22:48:15 +0800
From:   Qu Wenruo <quwenruo.btrfs@....com>
To:     Joshi <joshiiitr@...il.com>, 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

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

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