Editorial review of Attaching and Detaching Existing Images.
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Dru Lavigne
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</info>
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<para>In addition to physical disks, &os; also supports
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the creation and use of memory disks.</para>
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the creation and use of memory disks. One possible use for a
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memory disk is to access the contents of an
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<acronym>ISO</acronym> file system without the overhead of first
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burning it to a <acronym>CD</acronym> or <acronym>DVD</acronym>,
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then mounting the <acronym>CD/DVD</acronym> media.</para>
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<para>In &os;, the &man.md.4; driver is used to provide support
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for memory disks. The <filename>GENERIC</filename> kernel
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includes this driver. When using a custom kernel
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configuration file, ensure it includes this line:</para>
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<indexterm>
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<primary>disks</primary>
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<secondary>memory</secondary>
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</indexterm>
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<programlisting>device md</programlisting>
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<sect2 xml:id="disks-mdconfig">
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<title>Attaching and Detaching Existing Images</title>
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<indexterm>
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<primary>disks</primary>
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<secondary>file-backed</secondary>
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<secondary>memory</secondary>
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</indexterm>
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<para>&man.mdconfig.8; is used to configure and enable memory
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disks, &man.md.4;, under &os;. To use &man.mdconfig.8;,
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&man.md.4; must be first loaded. When using a custom kernel
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configuration file, ensure it includes this line:</para>
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<programlisting>device md</programlisting>
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<para>&man.mdconfig.8; supports several types of memory backed
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virtual disks: memory disks allocated with &man.malloc.9; and
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memory disks using a file or swap space as backing. One
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possible use is the mounting of <acronym>CD</acronym>
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images.</para>
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<para>To mount an existing file system image:</para>
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<example>
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<title>Using <command>mdconfig</command> to Mount an Existing
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File System Image</title>
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<para>To mount an existing file system image, use
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<command>mdconfig</command> to specify the name of the
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<acronym>ISO</acronym> file and a free unit number. Then,
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refer to that unit number to mount it on an existing mount
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point. Once mounted, the files in the <acronym>ISO</acronym>
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will appear in the mount point. This example attaches
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<replaceable>diskimage.iso</replaceable> to the memory device
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<filename>/dev/md0</filename> then mounts that memory device
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on <filename>/mnt</filename>:</para>
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<screen>&prompt.root; <userinput>mdconfig -f <replaceable>diskimage.iso</replaceable> -u <replaceable>0</replaceable></userinput>
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&prompt.root; <userinput>mount /dev/md<replaceable>0</replaceable> <replaceable>/mnt</replaceable></userinput></screen>
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</example>
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<para>If a unit number is not specified with
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<option>-u</option>, <command>mdconfig</command> will
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automatically allocate an unused memory device and output
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the name of the allocated unit, such
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as <filename>md4</filename>. Refer to &man.mdconfig.8;
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for more details about this command and its options.</para>
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<para>When a memory disk is no
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longer in use, its resources should be released back to
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the system. First, unmount the file system, then use
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<command>mdconfig</command> to detach the disk from the system and
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release its resources. To continue this example:</para>
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<screen>&prompt.root; <userinput>umount /mnt</userinput>
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&prompt.root; <userinput>mdconfig -d -u <replaceable>0</replaceable></userinput></screen>
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<para>To determine if any memory disks are still attached to the
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system, type <command>mdconfig -l</command>.</para>
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</sect2>
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<sect2 xml:id="disks-md-freebsd5">
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<title>Creating a Memory Disk</title>
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<indexterm>
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<primary>disks</primary>
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<secondary>memory file system</secondary>
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</indexterm>
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<para>To create a new file system image with
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&man.mdconfig.8;:</para>
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@ -2050,12 +2073,32 @@ Filesystem 1K-blocks Used Avail Capacity Mounted on
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/dev/md0a 4710 4 4330 0% /mnt</screen>
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</example>
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<para>If unit number is not specified with
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<option>-u</option>, &man.mdconfig.8; uses the
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&man.md.4; automatic allocation to select an unused device.
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The name of the allocated unit will be output to stdout, such
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as <filename>md4</filename>. Refer to &man.mdconfig.8;
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for more details about.</para>
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<para>For a memory-based file system, <quote>swap
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backing</quote> should normally be used. This does not mean
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that the memory disk will be swapped out to disk by default,
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but rather that the memory disk will be allocated from a
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memory pool which can be swapped out to disk if needed. It is
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also possible to create memory-based disks which are
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&man.malloc.9; backed, but using large malloc backed memory
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disks can result in a system panic if the kernel runs out of
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memory.</para>
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<example>
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<title>Creating a New Memory-Based Disk with
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<command>mdconfig</command></title>
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<screen>&prompt.root; <userinput>mdconfig -a -t swap -s <replaceable>5</replaceable>m -u <replaceable>1</replaceable></userinput>
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&prompt.root; <userinput>newfs -U md<replaceable>1</replaceable></userinput>
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/dev/md1: 5.0MB (10240 sectors) block size 16384, fragment size 2048
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using 4 cylinder groups of 1.27MB, 81 blks, 192 inodes.
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with soft updates
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super-block backups (for fsck -b #) at:
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160, 2752, 5344, 7936
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&prompt.root; <userinput>mount /dev/md<replaceable>1</replaceable> <replaceable>/mnt</replaceable></userinput>
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&prompt.root; <userinput>df <replaceable>/mnt</replaceable></userinput>
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Filesystem 1K-blocks Used Avail Capacity Mounted on
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/dev/md1 4718 4 4338 0% /mnt</screen>
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</example>
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<para>While &man.mdconfig.8; is useful, it takes several
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command lines to create a file-backed file system. &os; also
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@ -2087,58 +2130,6 @@ Filesystem 1K-blocks Used Avail Capacity Mounted on
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<primary>disks</primary>
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<secondary>detaching a memory disk</secondary>
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</indexterm>
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<para>When a memory-based or file-based file system is no
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longer in use, its resources should be released back to
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the system. First, unmount the file system, then use
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&man.mdconfig.8; to detach the disk from the system and
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release the resources.</para>
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<para>For example, to detach and free all resources used by
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<filename>/dev/md4</filename>:</para>
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<screen>&prompt.root; <userinput>mdconfig -d -u <replaceable>4</replaceable></userinput></screen>
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<para>It is possible to list information about configured
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&man.md.4; devices by running
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<command>mdconfig -l</command>.</para>
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</sect2>
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<sect2 xml:id="disks-md-freebsd5">
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<title>Creating a Memory Disk</title>
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<indexterm>
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<primary>disks</primary>
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<secondary>memory file system</secondary>
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</indexterm>
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<para>For a memory-based file system, <quote>swap
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backing</quote> should normally be used. This does not mean
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that the memory disk will be swapped out to disk by default,
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but rather that the memory disk will be allocated from a
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memory pool which can be swapped out to disk if needed. It is
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also possible to create memory-based disks which are
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&man.malloc.9; backed, but using large malloc backed memory
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disks can result in a system panic if the kernel runs out of
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memory.</para>
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<example>
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<title>Creating a New Memory-Based Disk with
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<command>mdconfig</command></title>
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<screen>&prompt.root; <userinput>mdconfig -a -t swap -s <replaceable>5</replaceable>m -u <replaceable>1</replaceable></userinput>
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&prompt.root; <userinput>newfs -U md<replaceable>1</replaceable></userinput>
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/dev/md1: 5.0MB (10240 sectors) block size 16384, fragment size 2048
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using 4 cylinder groups of 1.27MB, 81 blks, 192 inodes.
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with soft updates
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super-block backups (for fsck -b #) at:
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160, 2752, 5344, 7936
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&prompt.root; <userinput>mount /dev/md<replaceable>1</replaceable> <replaceable>/mnt</replaceable></userinput>
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&prompt.root; <userinput>df <replaceable>/mnt</replaceable></userinput>
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Filesystem 1K-blocks Used Avail Capacity Mounted on
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/dev/md1 4718 4 4338 0% /mnt</screen>
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</example>
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<example>
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<title>Creating a New Memory-Based Disk with
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<command>mdmfs</command></title>
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