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A file system must be mounted on to a directory in another file system. When mounting file system <literal>B</literal> on to the directory <literal>A1</literal>, the root directory of <literal>B</literal> replaces <literal>A1</literal>, and the directories in <literal>B</literal> appear accordingly:
_ external ref='basics/example-dir2' md5='__failed__'
/
|
+--- A1
| |
| +--- B1
| |
| `--- B2
|
`--- A2
Any files that are in the <literal>B1</literal> or <literal>B2</literal> directories can be reached with the path <filename>/A1/B1</filename> or <filename>/A1/B2</filename> as necessary. Any files that were in <filename>/A1</filename> have been temporarily hidden. They will reappear if <literal>B</literal> is <firstterm>unmounted</firstterm> from <literal>A</literal>.
If <literal>B</literal> had been mounted on <literal>A2</literal> then the diagram would look like this:
_ external ref='basics/example-dir3' md5='__failed__'
/
|
+--- A1
|
`--- A2
|
+--- B1
|
`--- B2
and the paths would be <filename>/A2/B1</filename> and <filename>/A2/B2</filename> respectively.
File systems can be mounted on top of one another. Continuing the last example, the <literal>C</literal> file system could be mounted on top of the <literal>B1</literal> directory in the <literal>B</literal> file system, leading to this arrangement:
_ external ref='basics/example-dir4' md5='__failed__'
/
|
+--- A1
|
`--- A2
|
+--- B1
| |
| +--- C1
| |
| `--- C2
|
`--- B2
Or <literal>C</literal> could be mounted directly on to the <literal>A</literal> file system, under the <literal>A1</literal> directory:
_ external ref='basics/example-dir5' md5='__failed__'
/
|
+--- A1
| |
| +--- C1
| |
| `--- C2
|
`--- A2
|
+--- B1
|
`--- B2
It is entirely possible to have one large root file system, and not need to create any others. There are some drawbacks to this approach, and one advantage.
Benefits of Multiple File Systems
Different file systems can have different <firstterm>mount options</firstterm>. For example, the root file system can be mounted read-only, making it impossible for users to inadvertently delete or edit a critical file. Separating user-writable file systems, such as <filename>/home</filename>, from other file systems allows them to be mounted <firstterm>nosuid</firstterm>. This option prevents the <firstterm>suid</firstterm>/<firstterm>guid</firstterm> bits on executables stored on the file system from taking effect, possibly improving security.
FreeBSD automatically optimizes the layout of files on a file system, depending on how the file system is being used. So a file system that contains many small files that are written frequently will have a different optimization to one that contains fewer, larger files. By having one big file system this optimization breaks down.
FreeBSD's file systems are robust if power is lost. However, a power loss at a critical point could still damage the structure of the file system. By splitting data over multiple file systems it is more likely that the system will still come up, making it easier to restore from backup as necessary.
Benefit of a Single File System
File systems are a fixed size. If you create a file system when you install FreeBSD and give it a specific size, you may later discover that you need to make the partition bigger. This is not easily accomplished without backing up, recreating the file system with the new size, and then restoring the backed up data.
FreeBSD features the <citerefentry><refentrytitle>growfs</refentrytitle><manvolnum>8</manvolnum></citerefentry> command, which makes it possible to increase the size of file system on the fly, removing this limitation.
File systems are contained in partitions. This does not have the same meaning as the common usage of the term partition (for example, <trademark class="registered">MS-DOS</trademark> partition), because of FreeBSD's <trademark class="registered">UNIX</trademark> heritage. Each partition is identified by a letter from <literal>a</literal> through to <literal>h</literal>. Each partition can contain only one file system, which means that file systems are often described by either their typical mount point in the file system hierarchy, or the letter of the partition they are contained in.
FreeBSD also uses disk space for <firstterm>swap space</firstterm> to provide <firstterm>virtual memory</firstterm>. This allows your computer to behave as though it has much more memory than it actually does. When FreeBSD runs out of memory, it moves some of the data that is not currently being used to the swap space, and moves it back in (moving something else out) when it needs it.
Some partitions have certain conventions associated with them.
Partition
Convention
<literal>a</literal>
Normally contains the root file system.
<literal>b</literal>
Normally contains swap space.

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(itstool) path: itemizedlist/title

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book.translate.xml:7777
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a year ago
Source string age
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books/handbook.pot, string 1252