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18.2.3. Sharing Mounts

Occasionally, certain system administration tasks require access to the same file system from more than one place in the directory tree (for example, when preparing a chroot environment). This is possible, and Linux allows you to mount the same file system to as many directories as necessary. Additionally, the mount command implements the --bind option that provides a means for duplicating certain mounts. Its usage is as follows: 
mount --bind old_directory new_directory
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Although this command allows a user to access the file system from both places, it does not apply on the file systems that are mounted within the original directory. To include these mounts as well, type: 
mount --rbind old_directory new_directory
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Additionally, to provide as much flexibility as possible, Red Hat Enterprise Linux 6 implements the functionality known as shared subtrees. This feature allows the use of the following four mount types:
Shared Mount
A shared mount allows the creation of an exact replica of a given mount point. When a mount point is marked as a shared mount, any mount within the original mount point is reflected in it, and vice versa. To change the type of a mount point to a shared mount, type the following at a shell prompt: 
mount --make-shared mount_point
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Alternatively, to change the mount type for the selected mount point and all mount points under it, type: 
mount --make-rshared mount_point
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See Example 18.4, “Creating a Shared Mount Point” for an example usage.

Example 18.4. Creating a Shared Mount Point

There are two places where other file systems are commonly mounted: the /media directory for removable media, and the /mnt directory for temporarily mounted file systems. By using a shared mount, you can make these two directories share the same content. To do so, as root, mark the /media directory as “shared”: 
~]# mount --bind /media /media
~]# mount --make-shared /media
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Then create its duplicate in /mnt by using the following command: 
~]# mount --bind /media /mnt
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It is now possible to verify that a mount within /media also appears in /mnt. For example, if the CD-ROM drive contains non-empty media and the /media/cdrom/ directory exists, run the following commands: 
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
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Similarly, it is possible to verify that any file system mounted in the /mnt directory is reflected in /media. For instance, if a non-empty USB flash drive that uses the /dev/sdc1 device is plugged in and the /mnt/flashdisk/ directory is present, type: 
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
en-US  publican.cfg
~]# ls /mnt/flashdisk
en-US  publican.cfg
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Slave Mount
A slave mount allows the creation of a limited duplicate of a given mount point. When a mount point is marked as a slave mount, any mount within the original mount point is reflected in it, but no mount within a slave mount is reflected in its original. To change the type of a mount point to a slave mount, type the following at a shell prompt: 
mount --make-slave mount_point
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Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it by typing: 
mount --make-rslave mount_point
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See Example 18.5, “Creating a Slave Mount Point” for an example usage.

Example 18.5. Creating a Slave Mount Point

This example shows how to get the content of the /media directory to appear in /mnt as well, but without any mounts in the /mnt directory to be reflected in /media. As root, first mark the /media directory as “shared”: 
~]# mount --bind /media /media
~]# mount --make-shared /media
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Then create its duplicate in /mnt, but mark it as “slave”: 
~]# mount --bind /media /mnt
~]# mount --make-slave /mnt
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Now verify that a mount within /media also appears in /mnt. For example, if the CD-ROM drive contains non-empty media and the /media/cdrom/ directory exists, run the following commands: 
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
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Also verify that file systems mounted in the /mnt directory are not reflected in /media. For instance, if a non-empty USB flash drive that uses the /dev/sdc1 device is plugged in and the /mnt/flashdisk/ directory is present, type: 
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
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Private Mount
A private mount is the default type of mount, and unlike a shared or slave mount, it does not receive or forward any propagation events. To explicitly mark a mount point as a private mount, type the following at a shell prompt: 
mount --make-private mount_point
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Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it: 
mount --make-rprivate mount_point
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See Example 18.6, “Creating a Private Mount Point” for an example usage.

Example 18.6. Creating a Private Mount Point

Taking into account the scenario in Example 18.4, “Creating a Shared Mount Point”, assume that a shared mount point has been previously created by using the following commands as root: 
~]# mount --bind /media /media
~]# mount --make-shared /media
~]# mount --bind /media /mnt
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To mark the /mnt directory as “private”, type: 
~]# mount --make-private /mnt
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It is now possible to verify that none of the mounts within /media appears in /mnt. For example, if the CD-ROM drives contains non-empty media and the /media/cdrom/ directory exists, run the following commands: 
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
~]#
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It is also possible to verify that file systems mounted in the /mnt directory are not reflected in /media. For instance, if a non-empty USB flash drive that uses the /dev/sdc1 device is plugged in and the /mnt/flashdisk/ directory is present, type: 
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
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Unbindable Mount
In order to prevent a given mount point from being duplicated whatsoever, an unbindable mount is used. To change the type of a mount point to an unbindable mount, type the following at a shell prompt: 
mount --make-unbindable mount_point
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Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it: 
mount --make-runbindable mount_point
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See Example 18.7, “Creating an Unbindable Mount Point” for an example usage.

Example 18.7. Creating an Unbindable Mount Point

To prevent the /media directory from being shared, as root, type the following at a shell prompt: 
~]# mount --bind /media /media
~]# mount --make-unbindable /media
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This way, any subsequent attempt to make a duplicate of this mount will fail with an error: 
~]# mount --bind /media /mnt
mount: wrong fs type, bad option, bad superblock on /media,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail  or so
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