Ploop/Mount helpers
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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 mount
and umount commands rather than the ploop(8) tool.
This article describes how it works and can be used.
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 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
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:
-r, -o ro
- mount read-only
-v, --verbose
- be more verbose
-f, --fake
- do everything except for the actual mount
-n, --no-mtab, -s
- these options are deliberately ignored
Unmounting
To unmount, DiskDescriptor can be specified:
umount /path/to/DiskDescriptor.xml
A mount point can be used as well:
umount /mount/point
Limitations
Note that umount can only work if:
- /etc/mtab is a separate file (not a symlink to /proc/mounts);
- mounting was done using
mount(notploop mount).
Otherwise, umount binary will not be able to find ploop as the "filesystem"
field in /etc/mtab, and will not call umount.ploop helper. As a result,
file system will be unmounted, but ploop device itself will stay mounted.
Alternative
ploop mount and ploop umount commands,
as described in ploop(8) man page.
See also
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