[Linux] Filesystem mounts

[Robert Citek]

On Mon, Feb 25, 2008 at 12:17 PM, Robert Citek <robert.citek@gmail.com> wrote:
> On Mon, Feb 25, 2008 at 12:00 PM, Danny Rathjens <dannyr@wirespring.com> wrote:
>  > Robert Citek wrote:
>  >  > On Mon, Feb 25, 2008 at 9:47 AM,  <kwan@digitalhermit.com> wrote:
>  >  >>  Question is, without unmount /foo/bar, how can I view the directory
>  >  >>  contents of the mount point /foo/bar.
>  >  >
>  >  > This might help.  man mount:
>  >  >
>  >  >        Since Linux 2.5.1 it is possible to atomically move a mounted
>  >  > tree to another place. The call is
>  >  >               mount --move olddir newdir
>  >  >
>  >  I'm confused as to why you can't just unmount and recover any data coincidentally
>  >  not overwritten and mount it again.
>
>  You can't unmount a filesystem if a processes is using it, but you can move it.

This script demonstrates moving a filesystem under Ubuntu using a
loopback device:

#!/bin/bash

sudo true && (
set -x
#### setup
tmp=$(mktemp -d tmp.XXXXXX) || exit
cd $tmp
mkdir ext3-{A,B}
touch ext3-A/file-A
touch ext3-B/file-B
ls -l ext3-{A,B}

# create loopback filesystem
dd if=/dev/zero bs=1M count=100 of=ext3.img >& /dev/null
mkfs.ext3 -F ext3.img  >& /dev/null

# show the files when mounted
sudo mount -o loop ext3.img ext3-A

# notice file-A is no longer seen
ls -l ext3-{A,B}

# by unmounting, file-A is viewable again
sudo umount ext3-A
ls -l ext3-{A,B}

# however, if ext3-A is busy, it cannot be unmounted
sudo mount -o loop ext3.img ext3-A
( cd ext3-A ; sleep 3 ) &
sudo umount ext3-A

# if instead, ext3-A is moved to ext3-B, then file-A is visible but
file-B become invisible, even though ext3-A is busy
sudo mount --move ext3-A ext3-B
ls -l ext3-{A,B}

# and if ext3-B is moved to ext3-A, then file-B is visible again and
file-A is once again invisible, even though ext3-A is busy
sudo umount ext3-B
sudo mount --move ext3-B ext3-A
ls -l ext3-{A,B}

# unmount and clean up
sleep 3
sudo umount ext3-A &&
rm -rf ../$tmp
) 2>&1 | tee output.txt

Browse through output.txt to view the results.

Regards,
- Robert