Finish editorial review of Memory Disks chapter.
Sponsored by: iXsystems
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Dru Lavigne
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@ -2030,6 +2030,11 @@ scsibus1:
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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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<indexterm>
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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 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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@ -2044,48 +2049,26 @@ scsibus1:
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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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<title>Creating a File- or Memory-Backed 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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<para>&os; also supports memory disks where the storage to use
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is allocated from either a hard disk or an area of memory.
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The first method is commonly referred to as a file-backed file
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system and the second method as a memory-backed file system.
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Both types can be created using
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<command>mdconfig</command>.</para>
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<example>
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<title>Creating a New File-Backed Disk with
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<command>mdconfig</command></title>
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<screen>&prompt.root; <userinput>dd if=/dev/zero of=<replaceable>newimage</replaceable> bs=1k count=<replaceable>5</replaceable>k</userinput>
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5120+0 records in
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5120+0 records out
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&prompt.root; <userinput>mdconfig -a -t vnode -f <replaceable>newimage</replaceable> -u <replaceable>0</replaceable></userinput>
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&prompt.root; <userinput>bsdlabel -w md<replaceable>0</replaceable> auto</userinput>
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&prompt.root; <userinput>newfs md<replaceable>0</replaceable>a</userinput>
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/dev/md0a: 5.0MB (10224 sectors) block size 16384, fragment size 2048
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using 4 cylinder groups of 1.25MB, 80 blks, 192 inodes.
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super-block backups (for fsck -b #) at:
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160, 2720, 5280, 7840
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&prompt.root; <userinput>mount /dev/md<replaceable>0</replaceable>a <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/md0a 4710 4 4330 0% /mnt</screen>
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</example>
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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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<para>To create a new memory-backed file system, specify a type
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of <literal>swap</literal> and the size of the memory disk to
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create. Then, format the memory disk with a file system and
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mount as usual. This example creates a 5M memory disk on unit
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<literal>1</literal>. That memory disk is then formatted with
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the <acronym>UFS</acronym> file system before it is
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mounted:</para>
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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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@ -2098,47 +2081,55 @@ super-block backups (for fsck -b #) at:
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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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comes with &man.mdmfs.8; which automatically configures a
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&man.md.4; disk using &man.mdconfig.8;, puts a UFS file system
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on it using &man.newfs.8;, and mounts it using &man.mount.8;.
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For example, to create and mount the same file system image as
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above, type the following:</para>
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<example>
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<title>Configure and Mount a File-Backed Disk with
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<command>mdmfs</command></title>
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<para>
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To create a new file-backed memory disk, first allocate an
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area of disk to use. This example creates an empty 5K file
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named <filename>newimage</filename>:</para>
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<screen>&prompt.root; <userinput>dd if=/dev/zero of=<replaceable>newimage</replaceable> bs=1k count=<replaceable>5</replaceable>k</userinput>
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5120+0 records in
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5120+0 records out
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&prompt.root; <userinput>mdmfs -F <replaceable>newimage</replaceable> -s <replaceable>5</replaceable>m md<replaceable>0</replaceable> <replaceable>/mnt</replaceable></userinput>
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5120+0 records out</screen>
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<para>Next, attach that file to a memory disk, label the memory
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disk and format it with the <acronym>UFS</acronym> file
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system, mount the memory disk, and verify the size of the
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file-backed disk:</para>
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<screen>&prompt.root; <userinput>mdconfig -f <replaceable>newimage</replaceable> -u <replaceable>0</replaceable></userinput>
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&prompt.root; <userinput>bsdlabel -w md<replaceable>0</replaceable> auto</userinput>
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&prompt.root; <userinput>newfs md<replaceable>0</replaceable>a</userinput>
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/dev/md0a: 5.0MB (10224 sectors) block size 16384, fragment size 2048
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using 4 cylinder groups of 1.25MB, 80 blks, 192 inodes.
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super-block backups (for fsck -b #) at:
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160, 2720, 5280, 7840
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&prompt.root; <userinput>mount /dev/md<replaceable>0</replaceable>a <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/md0 4718 4 4338 0% /mnt</screen>
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</example>
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/dev/md0a 4710 4 4330 0% /mnt</screen>
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<para>When <option>md</option> is used without a unit number,
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&man.mdmfs.8; uses the &man.md.4; auto-unit feature to
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automatically select an unused device. For more details
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about &man.mdmfs.8;, refer to its manual page.</para>
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<para>It takes several commands to create a file- or memory-backed file
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system using <command>mdconfig</command>. &os; also
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comes with <command>mdmfs</command> which automatically configures a
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memory disk, formats it with the <acronym>UFS</acronym> file system,
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and mounts it. For example, after creating
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<replaceable>newimage</replaceable> with
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<command>dd</command>, this one command is equivalent to
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running the <command>bsdlabel</command>,
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<command>newfs</command>, and <command>mount</command>
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commands shown above:</para>
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<indexterm>
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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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<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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<screen>&prompt.root; <userinput>mdmfs -F <replaceable>newimage</replaceable> -s <replaceable>5</replaceable>m md<replaceable>0</replaceable> <replaceable>/mnt</replaceable></userinput></screen>
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<screen>&prompt.root; <userinput>mdmfs -s <replaceable>5</replaceable>m md<replaceable>2</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/md2 4846 2 4458 0% /mnt</screen>
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</example>
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<para>To instead create a new memory-based memory disk with
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<command>mdmfs</command>, use this one command:</para>
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<screen>&prompt.root; <userinput>mdmfs -s <replaceable>5</replaceable>m md<replaceable>1</replaceable> <replaceable>/mnt</replaceable></userinput></screen>
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<para>If the unit number is not specified,
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<command>mdmfs</command> will
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automatically select an unused memory device. For more details
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about <command>mdmfs</command>, refer to &man.mdmfs.8;.</para>
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</sect2>
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</sect1>
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