From: Eric Sandeen <sandeen@sandeen.net>
To: Qu Wenruo <quwenruo.btrfs@gmx.com>, Joshi <joshiiitr@gmail.com>,
linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org
Subject: Re: cross-fs copy support
Date: Mon, 1 Oct 2018 10:49:19 -0500 [thread overview]
Message-ID: <df92a795-3f2e-751a-8504-030015fba6d9@sandeen.net> (raw)
In-Reply-To: <2a92fff4-d005-835a-3bd7-a328b008857f@gmx.com>
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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
-Eric
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next prev parent reply other threads:[~2018-10-01 22:27 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
2018-10-01 15:49 ` Eric Sandeen [this message]
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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