Editing Ploop/Mount helpers

Despite the fact that ploop is not a file system, a trick exist
to make it look and feel like so -- i.e. to use usual <code>mount</code>
and <code>umount</code> commands rather than the {{Man|ploop|8}} tool.
This article describes why and how, and lists caveats and limitation.

== Introduction ==

ploop is not a filesystem per se, but a kernel [[w:loop device]] driver, providing a way
to represent a ploop image (or a set of stacked images) as a
[[w:Device file system#Block_devices|block device]].
On top of that block device provided by ploop, an ext4 file system
is created and used, for example, for storing files of a specific
container.

Therefore a complete ploop mount consists of two steps:
# "mount" ploop image(s) to create a ploop device (/dev/ploopNNNNN)
# mount a filesystem residing on this ploop device to a mount point

For simplicity, these two are usually wrapped to be done together in one step (say when you use ploop mount with -m option).
In reality, step 1 is kernel assembling a device out of image(s), and step 2 is the real mount.

== Mounting ==

Normally, ploop is mounted by <code>ploop mount</code> command.

You can use the following syntax to mount a ploop device and the filesystem inside it:

 mount -t ploop [option ...] /path/to/DiskDescriptor.xml /mount/point

The following options are supported:

<code>-r</code>, <code>-o ro</code>
: mount read-only

<code>-v</code>, <code>--verbose</code>
: be more verbose

<code>-f</code>, <code>--fake</code>
: do everything except for the actual mount

<code>-n</code>, <code>--no-mtab</code>, <code>-s</code>
: these options are deliberately ignored

== Unmounting ==

Normally, ploop is unmounted by <code>ploop umount</code> command.

But if the following is true, usual <code>umount</code> can be used:
# /etc/mtab is a separate file (not a symlink to /proc/mounts);
# mounting should have been done using <code>mount</code> (not <code>ploop mount</code>).

Any of the following can be used:

 umount /path/to/DiskDescriptor.xml
 umount /mount/point

[[Category: ploop]]
@@ Line 1: / Line 1: @@
 Despite the fact that ploop is not a file system, a trick exist
-to make it look and feel like so — i.e. to use usual <code>mount</code>
+to make it look and feel like so -- i.e. to use usual <code>mount</code>
 and <code>umount</code> commands rather than the {{Man|ploop|8}} tool.
-This article describes how it works and can be used.
+This article describes why and how, and lists caveats and limitation.
 == Introduction ==
@@ Line 21: / Line 21: @@
 == Mounting ==
+Normally, ploop is mounted by <code>ploop mount</code> command.
 You can use the following syntax to mount a ploop device and the filesystem inside it:
@@ Line 42: / Line 44: @@
 == Unmounting ==
-To unmount, DiskDescriptor can be specified:
+Normally, ploop is unmounted by <code>ploop umount</code> command.
- umount /path/to/DiskDescriptor.xml
+But if the following is true, usual <code>umount</code> can be used:
+# /etc/mtab is a separate file (not a symlink to /proc/mounts);
+# mounting should have been done using <code>mount</code> (not <code>ploop mount</code>).
-A mount point can be used as well:
+Any of the following can be used:
+ umount /path/to/DiskDescriptor.xml
   umount /mount/point
-== Limitations ==
+[[Category: ploop]]
-Note that umount can only work if:
-# /etc/mtab is a separate file (not a symlink to /proc/mounts);
-# mounting was done using <code>mount</code> (not <code>ploop mount</code>).
-Otherwise, umount binary will not be able to find <code>ploop</code> as the "filesystem"
-field in /etc/mtab, and will not call <code>umount.ploop</code> helper. As a result,
-file system will be unmounted, but ploop device itself will stay mounted.
-== Alternative ==
-<code>ploop mount</code> and <code>ploop umount</code> commands,
-as described in {{Man|ploop|8}} man page.
-== See also ==
-* [[Ploop]]
-* {{Bug|2817}}
-[[Category: Storage]]