Bind mounts

Recent Linux kernels support an operation called 'bind mounting' which makes part of a mounted filesystem visible at some other mount point. See 'man mount' for more information.

Bind mounts can be used to make directories on the hardware node visible to the container.

Contents

Filesystem layout

OpenVZ uses two directories. Assuming our container is numbered 777, these directories are:

• VE_PRIVATE: $VZDIR/private/777
• VE_ROOT: $VZDIR/root/777

Note: $VZDIR is usually /vz, on Debian systems however this is /var/lib/vz. In this document this is further referred to as $VZDIR -- substitute it with what you have.

VE_PRIVATE is a place for all the container files. VE_ROOT is the mount point to which VE_PRIVATE is mounted during container start (or when you run vzctl mount

Warning: If you want to do a bind mount for container, you need to use VE_ROOT (not VE_PRIVATE!) and make sure that container is mounted (this can be checked using vzctl status).

Manual mount example

On the HN we have a directory /home which we wish to make available (shared) to container 777.

The correct command to issue on the HN is:

 mount --bind /home $VZDIR/root/777/home

The container must be started (or at least mounted) and the destination directory must exist. The container will see this directory mounted like this:

# df
Filesystem           1K-blocks      Used Available Use% Mounted on
simfs                 10485760    298728  10187032   3% /
ext3                 117662052 104510764   7174408  94% /home

During the container stop vzctl unmounts that bind mount, so you have to mount it again when you start the container for the next time. Luckily there is a way to automate it.

Make the mount persistent

Put a mount script in OpenVZ configuration directory (/etc/vz/conf/) with the name CTID.mount (where CTID is container ID, like 777). This script will be executed every time you run vzctl mount or vzctl start for a particular container. If you need to the same for all containers, use the global mount script named vps.mount.

From any mount script you can use the following environment variables:

• ${VEID} -- container ID (like 777).
• ${VE_CONFFILE} -- container configuration file (like /etc/vz/conf/777.conf)

Now, in order to get the value of VE_ROOT you need to source both the global OpenVZ configuration file, and then the container configuration file, in that particular order. This is the same way vzctl uses to determine VE_ROOT.

Mount script example

Here is an example of such a mount script (it can either be /etc/vz/conf/vps.mount or /etc/vz/conf/CTID.mount)

#!/bin/bash
source /etc/vz/vz.conf
source ${VE_CONFFILE}
mount -n --bind /mnt/disk ${VE_ROOT}/mnt/disk

After creating script please make it executable by issuing "chmod +x CTID.mount" at command line otherwise vm fails to start

Unmount script example

For unmounting a filesystem, /etc/vz/conf/vps.umount or /etc/vz/conf/CTID.umount script can be used in the same way:

#!/bin/bash
source /etc/vz/vz.conf
source ${VE_CONFFILE}
umount ${VE_ROOT}/mnt/disk

Note: CTID.umount script is not strictly required, since vzctl tries to unmount everything on CT stop. But you'd better have it anyway.

umount scripts could cause trouble and errors on VM start and might not be required if using the -n option on mount. read forum post

When mounting whitout -n option, umount script become required, but display errors, because of recursivity of umount procedure initiated before by libvzctl.

Even if no option -n was specified at mount, and no umount script was run, system file /etc/mtab could become wrong in the HN, causing trouble to commands like df.

Read-only bind mounts

Since Linux kernel 2.6.26, bind mounts can be made read-only. The trick is to first mount as usual, and then remount it read-only:

mount -n --bind /home $VZDIR/root/777/home
mount -n --bind -oremount,ro $VZDIR/root/777/home

With some kernels you need to add the sourcedirectory also: mount -n --bind -oremount,ro /home $VZDIR/root/777/home

Sometimes it is usefull to have a folder read-only mounted in a VPS, but also be able to put files in that directory. If you want that, just create an other directory and simlink the read only files into that folder:

vzctl exec2 777 "mkdir /addfileshere && ln -s /home/* /addfileshere/"

Now the /addfileshere folder is fully writable and it even feels like it is possible to delete files (but that are only the simlinks).

See also

• NFS
• FUSE
• Mounting filesystems