Mount –bind in the same filesystem move files as in the same filesystem

bind-mountmountsymlink

If I mount --bind /a in /b and if I mv /a/bigfile /b/ it will take a lot of type (if the origin file is big). This will actually copy the file and then delete from the origin instead of simple update the filesystem's file table.

I understand how and why mount --bind works. As pointed out by others, there are some good explanation on that:

Is there any way of telling mount that a mount --bind is in the same filesystem so that operations like this (move) are done by actually update the filesystem's file table?

I'm asking for solving this problem and not to understand it: maybe a kernel patch already available that I'm not aware of it or a missing parameter I didn't find already (etc).

Why: In my current setup there is a service (nextcloud) that only supports the mount --bind. I can't use symlinks. If I need to, say, have a shared folder inside each account of nextcloud it must be with bind. I may accept any solution that works with nextcloud if it is made in the filesystem/kernel level. This is because the current setup also includes access to these files in other ways, like ssh. In other words, I would like be able to move a file as in example using any command or application.

Best Answer

Apparently not, the man page rename(2) mentions this:

EXDEV oldpath and newpath are not on the same mounted filesystem. (Linux permits a filesystem to be mounted at multiple points, but rename() does not work across different mount points, even if the same filesystem is mounted on both.

If I recall correctly, a bind mount is the same as just mounting the same filesystem multiple times, i.e. there's no "source" and "target" for the bind after the fact.

There's also another answer to another question on the same subject from a couple of months ago. That one has pointers to a discussion about it with the kernel developers. So go upvote that.

Related Solutions

mount –bind Other User as Myself

You can't do that with the Linux bind mount kernel feature. But you can do it with the FUSE filesystem bindfs. Bindfs is slower than bind mounts and doesn't pass extended attributes, but on the flip side, it can be used by non-root users and on Unix variants other than Linux, and most importantly for you, allows simple transformations of ownership and permissions.

bindfs -u cqqrstoo ~bob/stuff ~cqqrstoo/bobs-stuff

In /etc/fstab, that would translate to:

bindfs#/home/bob/stuff /home/cqqrstoo/bobs-stuff fuse force-user=cqqrstoo 0 0

Linux – Why doesn’t mount respect the read only option for bind mounts

Bind mount is just... well... a bind mount. I.e. it's not a new mount. It just "links"/"exposes"/"considers" a subdirectory as a new mount point. As such it cannot alter the mount parameters. That's why you're getting complaints:

# mount /mnt/1/lala /mnt/2 -o bind,ro
mount: warning: /mnt/2 seems to be mounted read-write.

But as you said a normal bind mount works:

# mount /mnt/1/lala /mnt/2 -o bind

And then a ro remount also works:

# mount /mnt/1/lala /mnt/2 -o bind,remount,ro

However what happens is that you're changing the whole mount and not just this bind mount. If you take a look at /proc/mounts you'll see that both bind mount and the original mount change to read-only:

/dev/loop0 /mnt/1 ext2 ro,relatime,errors=continue,user_xattr,acl 0 0
/dev/loop0 /mnt/2 ext2 ro,relatime,errors=continue,user_xattr,acl 0 0

So what you're doing is like changing the initial mount to a read-only mount and then doing a bind mount which will of course be read-only.

UPDATE 2016-07-20:

The following are true for 4.5 kernels, but not true for 4.3 kernels (This is wrong. See update #2 below):

The kernel has two flags that control read-only:

The MS_READONLY: Indicating whether the mount is read-only
The MNT_READONLY: Indicating whether the "user" wants it read-only

On a 4.5 kernel, doing a mount -o bind,ro will actually do the trick. For example, this:

# mkdir /tmp/test
# mkdir /tmp/test/a /tmp/test/b
# mount -t tmpfs none /tmp/test/a
# mkdir /tmp/test/a/d
# mount -o bind,ro /tmp/test/a/d /tmp/test/b

will create a read-only bind mount of /tmp/test/a/d to /tmp/test/b, which will be visible in /proc/mounts as:

none /tmp/test/a tmpfs rw,relatime 0 0
none /tmp/test/b tmpfs ro,relatime 0 0

A more detailed view is visible in /proc/self/mountinfo, which takes into consideration the user view (namespace). The relevant lines will be these:

363 74 0:49 / /tmp/test/a rw,relatime shared:273 - tmpfs none rw
368 74 0:49 /d /tmp/test/b ro,relatime shared:273 - tmpfs none rw

Where on the second line, you can see that it says both ro (MNT_READONLY) and rw (!MS_READONLY).

The end result is this:

# echo a > /tmp/test/a/d/f
# echo a > /tmp/test/b/f
-su: /tmp/test/b/f: Read-only file system

UPDATE 2016-07-20 #2:

A bit more digging into this shows that the behavior in fact depends on the version of libmount which is part of util-linux. Support for this was added with this commit and was released with version 2.27:

commit 9ac77b8a78452eab0612523d27fee52159f5016a
Author: Karel Zak 
Date:   Mon Aug 17 11:54:26 2015 +0200

    libmount: add support for "bind,ro"

    Now it's necessary t use two mount(8) calls to create a read-only
    mount:

      mount /foo /bar -o bind
      mount /bar -o remount,ro,bind

    This patch allows to specify "bind,ro" and the remount is done
    automatically by libmount by additional mount(2) syscall. It's not
    atomic of course.

    Signed-off-by: Karel Zak

which also provides the workaround. The behavior can be seen using strace on an older and a newer mount:

Old:

mount("/tmp/test/a/d", "/tmp/test/b", 0x222e240, MS_MGC_VAL|MS_RDONLY|MS_BIND, NULL) = 0 <0.000681>

New:

mount("/tmp/test/a/d", "/tmp/test/b", 0x1a8ee90, MS_MGC_VAL|MS_RDONLY|MS_BIND, NULL) = 0 <0.011492>
mount("none", "/tmp/test/b", NULL, MS_RDONLY|MS_REMOUNT|MS_BIND, NULL) = 0 <0.006281>

Conclusion:

To achieve the desired result one needs to run two commands (as @Thomas already said):

mount SRC DST -o bind
mount DST -o remount,ro,bind

Newer versions of mount (util-linux >=2.27) do this automatically when one runs

mount SRC DST -o bind,ro