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 bind' for more information.
Bind mounts can be used to make directories on the hardware node visible to the container.
OpenVZ uses two directories. Assuming our container is numbered 777, these directories are:
$VZROOT/private/777 $VZROOT/root/777
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Note: $VZROOT is usually /vz, on Debian systems however this is /var/lib/vz. In this document this is further referred to as $VZROOT -- substitute it with what you have.
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The $VZROOT/private directory contains root directory contents. This directory or subdirectory may be symlinked onto a different file system, for example:
$VZROOT/private -> /mnt/openvz
Requirement
On the HN we have a directory /home which we wish to make available (shared) to all containers.
You would think that you could bind mount this directory, as in: mount --bind /home $VZROOT/private/777/home but this does not work — the contents of /home cannot be seen within the container.
This is where the second directory listed above ($VZROOT/root/777) is used. If a container is not started, this directory is empty. But after starting a container, this directory contains what the container sees as its mounted file systems.
The correct command to issue on the HN is:
mount --bind /home $VZROOT/root/777/home
The container must be started 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% / tmpfs 484712 0 484712 0% /lib/init/rw tmpfs 484712 0 484712 0% /dev/shm ext3 117662052 104510764 7174408 94% /home
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 do remount (i.e. mount with -o remount,ro flags).