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2.2.3. 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). To address such requirements, the mount command implements the --bind option that provides a means for duplicating certain mounts. Its usage is as follows:
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mount --bind old_directory new_directory
Although the above 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:
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mount --rbind old_directory new_directory
Additionally, to provide as much flexibility as possible, Red Hat Enterprise Linux 5.10 implements the functionality known as shared subtrees. This feature allows you to use the following four mount types:
Shared Mount
A shared mount allows you to create an exact replica of a given mount point. When a shared mount is created, any mount within the original mount point is reflected in it, and vice versa. To create a shared mount, type the following at a shell prompt:
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mount --make-shared mount_point
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
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mount --make-rshared mount_point
See Example 2.4, “Creating a Shared Mount Point” for an example usage.

Example 2.4. 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”:
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~]# mount --bind /media /media
~]# mount --make-shared /media
Then create its duplicate in /mnt by using the following command:
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~]# mount --bind /media /mnt
You can now verify that a mount within /media also appears in /mnt. For example, if you have non-empty media in your CD-ROM drive and the /media/cdrom/ directory exists, run the following commands:
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~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
Similarly, you can verify that any file system mounted in the /mnt directory is reflected in /media. For instance, if you have a non-empty USB flash drive that uses the /dev/sdc1 device plugged in and the /mnt/flashdisk/ directory is present, type:
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~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
en-US  publican.cfg
~]# ls /mnt/flashdisk
en-US  publican.cfg
Slave Mount
A slave mount allows you to create a limited duplicate of a given mount point. When a slave mount is created, any mount within the original mount point is reflected in it, but no mount within a slave mount is reflected in its original. To create a slave mount, type the following at a shell prompt:
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mount --make-slave mount_point
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
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mount --make-rslave mount_point
See Example 2.5, “Creating a Slave Mount Point” for an example usage.

Example 2.5. Creating a Slave Mount Point

Imagine you want the content of the /media directory to appear in /mnt as well, but you do not want any mounts in the /mnt directory to be reflected in /media. To do so, as root, first mark the /media directory as “shared”:
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~]# mount --bind /media /media
~]# mount --make-shared /media
Then create its duplicate in /mnt, but mark it as “slave”:
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~]# mount --bind /media /mnt
~]# mount --make-slave /mnt
You can now verify that a mount within /media also appears in /mnt. For example, if you have non-empty media in your CD-ROM drive and the /media/cdrom/ directory exists, run the following commands:
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~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
You can also verify that file systems mounted in the /mnt directory are not reflected in /media. For instance, if you have a non-empty USB flash drive that uses the /dev/sdc1 device plugged in and the /mnt/flashdisk/ directory is present, type: :
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~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
Private Mount
A private mount allows you to create an ordinary mount. When a private mount is created, no subsequent mounts within the original mount point are reflected in it, and no mount within a private mount is reflected in its original. To create a private mount, type the following at a shell prompt:
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mount --make-private mount_point
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
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mount --make-rprivate mount_point
See Example 2.6, “Creating a Private Mount Point” for an example usage.

Example 2.6. Creating a Private Mount Point

Taking into account the scenario in Example 2.4, “Creating a Shared Mount Point”, assume that you have previously created a shared mount point by using the following commands as root:
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~]# mount --bind /media /media
~]# mount --make-shared /media
~]# mount --bind /media /mnt
To mark the /mnt directory as “private”, type:
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~]# mount --make-private /mnt
You can now verify that none of the mounts within /media appears in /mnt. For example, if you have non-empty media in your CD-ROM drive and the /media/cdrom/ directory exists, run the following commands:
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~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
~]#
You can also verify that file systems mounted in the /mnt directory are not reflected in /media. For instance, if you have a non-empty USB flash drive that uses the /dev/sdc1 device plugged in and the /mnt/flashdisk/ directory is present, type:
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~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
Unbindable Mount
An unbindable mount allows you to prevent a given mount point from being duplicated whatsoever. To create an unbindable mount, type the following at a shell prompt:
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mount --make-unbindable mount_point
Alternatively, you can change the mount type for the selected mount point and all mount points under it:
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mount --make-runbindable mount_point
See Example 2.7, “Creating an Unbindable Mount Point” for an example usage.

Example 2.7. Creating an Unbindable Mount Point

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