svn commit: r39909 - head/en_US.ISO8859-1/books/handbook/geom
Warren Block
wblock at FreeBSD.org
Fri Nov 2 04:19:21 UTC 2012
Author: wblock
Date: Fri Nov 2 04:19:20 2012
New Revision: 39909
URL: http://svn.freebsd.org/changeset/doc/39909
Log:
Rework the gmirror section to create mirrors properly. The second half
of this new section was written by Hiroki Sato.
Modified:
head/en_US.ISO8859-1/books/handbook/geom/chapter.xml
Modified: head/en_US.ISO8859-1/books/handbook/geom/chapter.xml
==============================================================================
--- head/en_US.ISO8859-1/books/handbook/geom/chapter.xml Fri Nov 2 04:13:41 2012 (r39908)
+++ head/en_US.ISO8859-1/books/handbook/geom/chapter.xml Fri Nov 2 04:19:20 2012 (r39909)
@@ -234,210 +234,598 @@ Done.</screen>
<indexterm>
<primary>Disk Mirroring</primary>
</indexterm>
+ <indexterm>
+ <primary>RAID1</primary>
+ </indexterm>
- <para>Mirroring is a technology used by many corporations and home
- users to back up data without interruption. When a mirror
- exists, it simply means that diskB replicates diskA. Or,
- perhaps diskC+D replicates diskA+B. Regardless of the disk
- configuration, the important aspect is that information on one
- disk or partition is being replicated. Later, that information
- could be more easily restored, backed up without causing service
- or access interruption, and even be physically stored in a data
- safe.</para>
-
- <para>To begin, ensure the system has two disk drives of equal
- size, these exercises assume they are direct access (&man.da.4;)
- <acronym>SCSI</acronym> disks.</para>
+ <para><acronym>RAID1</acronym>, or
+ <firstterm>mirroring</firstterm>, is the technique of writing
+ the same data to more than one disk drive. Mirrors are usually
+ used to guard against data loss due to drive failure. Each
+ drive in a mirror contains an identical copy of the data. When
+ an individual drive fails, the mirror continues to work,
+ providing data from the drives that are still functioning. The
+ computer keeps running, and the administrator has time to
+ replace the failed drive without user interruption.</para>
+
+ <para>Two common situations are illustrated in these examples.
+ The first is creating a mirror out of two new drives and using
+ it as a replacement for an existing single drive. The second
+ example creates a mirror on a single new drive, copies the old
+ drive's data to it, then inserts the old drive into the
+ mirror. While this procedure is slightly more complicated, it
+ only requires one new drive.</para>
+
+ <para>Traditionally, the two drives in a mirror are identical in
+ model and capacity, but &man.gmirror.8; does not require that.
+ Mirrors created with dissimilar drives will have a capacity
+ equal to that of the smallest drive in the mirror. Extra space
+ on larger drives will be unused. Drives inserted into the
+ mirror later must have at least as much capacity as the smallest
+ drive already in the mirror.</para>
+
+ <warning>
+ <para>The mirroring procedures shown here are non-destructive,
+ but as with any major disk operation, make a full backup
+ first.</para>
+ </warning>
+
+ <sect2 id="GEOM-mirror-metadata">
+ <title>Metadata Issues</title>
+
+ <para>Many disk systems store metadata at the end of each disk.
+ Old metadata should be erased before reusing the disk for a
+ mirror. Most problems are caused by two particular types of
+ leftover metadata: GPT partition tables, and old
+ &man.gmirror.8; metadata from a previous mirror.</para>
+
+ <para>GPT metadata can be erased with &man.gpart.8;. This
+ example erases both primary and backup GPT partition tables
+ from disk <devicename>ada8</devicename>:</para>
+
+ <screen>&prompt.root; <userinput>gpart destroy -F ada8</userinput></screen>
+
+ <para>&man.gmirror.8; can remove a disk from an active mirror
+ and erase the metadata in one step. Here, the example disk
+ <devicename>ada8</devicename> is removed from the active
+ mirror <devicename>gm4</devicename>:</para>
+
+ <screen>&prompt.root; <userinput>gmirror remove gm4 ada8</userinput></screen>
+
+ <para>If the mirror is not running but old mirror metadata is
+ still on the disk, use <command>gmirror clear</command> to
+ remove it:</para>
+
+ <screen>&prompt.root; <userinput>gmirror clear ada8</userinput></screen>
+
+ <para>&man.gmirror.8; stores one block of metadata at the end of
+ the disk. Because GPT partition schemes also store metadata
+ at the end of the disk, mirroring full GPT disks with
+ &man.gmirror.8; is not recommended. MBR partitioning is used
+ here because it only stores a partition table at the start of
+ the disk and does not conflict with &man.gmirror.8;.</para>
+ </sect2>
<sect2>
- <title>Mirroring Primary Disks</title>
-
- <para>Assuming &os; has been installed on the first,
- <devicename>da0</devicename> disk device, &man.gmirror.8;
- should be told to store its primary data there.</para>
-
- <para>Before building the mirror, enable additional debugging
- information and opening access to the device by setting the
- <varname>kern.geom.debugflags</varname> &man.sysctl.8; option
- to the following value:</para>
-
- <screen>&prompt.root; <userinput>sysctl kern.geom.debugflags=17</userinput></screen>
-
- <para>Now create the mirror. Begin the process by storing
- meta-data information on the primary disk device,
- effectively creating the
- <filename class="devicefile">/dev/mirror/gm</filename> device
- using the following command:</para>
-
- <warning>
- <para>Creating a mirror out of the boot drive may result in
- data loss if any data has been stored on the last sector of
- the disk. This risk is reduced if creating the mirror is
- done promptly after a fresh install of &os;. The following
- procedure is also incompatible with the default installation
- settings of &os; 9.<replaceable>X</replaceable> which
- use the new <acronym>GPT</acronym> partition scheme. GEOM
- will overwrite <acronym>GPT</acronym> metadata, causing data
- loss and possibly an unbootable system.</para>
- </warning>
+ <title>Creating a Mirror with Two New Disks</title>
- <screen>&prompt.root; <userinput>gmirror label -vb round-robin gm0 /dev/da0</userinput></screen>
+ <para>In this example, &os; has already been installed on a
+ single disk, <devicename>ada0</devicename>. Two new disks,
+ <devicename>ada1</devicename> and
+ <devicename>ada2</devicename>, have been connected to the
+ system. A new mirror will be created on these two disks and
+ used to replace the old single disk.</para>
+
+ <para>&man.gmirror.8; requires a kernel module,
+ <filename>geom_mirror.ko</filename>, either built into the
+ kernel or loaded at boot- or run-time. Manually load the
+ kernel module now:</para>
- <para>The system should respond with:</para>
-
- <screen>Metadata value stored on /dev/da0.
-Done.</screen>
+ <screen>&prompt.root; <userinput>gmirror load</userinput></screen>
- <para>Initialize GEOM, this will load the
- <filename>/boot/kernel/geom_mirror.ko</filename> kernel
- module:</para>
+ <para>Create the mirror with the two new drives.</para>
- <screen>&prompt.root; <userinput>gmirror load</userinput></screen>
+ <screen>&prompt.root; <userinput>gmirror label -v gm0 /dev/ada1 /dev/ada2</userinput></screen>
- <note>
- <para>When this command completes successfully, it creates the
- <devicename>gm0</devicename> device node under the
- <filename class="directory">/dev/mirror</filename>
- directory.</para>
- </note>
+ <para><devicename>gm0</devicename> is a user-chosen device name
+ assigned to the new mirror. After the mirror has been
+ started, this device name will appear in the
+ <filename>/dev/mirror/</filename> directory.</para>
+
+ <para>MBR and bsdlabel partition tables can now be created on
+ the mirror with &man.gpart.8;. Here we show a traditional
+ split-filesystem layout, with partitions for
+ <filename>/</filename>, swap, <filename>/var</filename>,
+ <filename>/tmp</filename>, and <filename>/usr</filename>. A
+ single <filename>/</filename> filesystem and a swap partition
+ will also work.</para>
+
+ <para>Partitions on the mirror do not have to be the same size
+ as those on the existing disk, but they must be large enough
+ to hold all the data already present on
+ <devicename>ada0</devicename>.</para>
+
+ <screen>&prompt.root; <userinput>gpart create -s MBR mirror/gm0</userinput>
+&prompt.root; <userinput>gpart add -t -a 4kfreebsd mirror/gm0</userinput>
+&prompt.root; <userinput>gpart show mirror/gm0</userinput>
+=> 63 156301423 mirror/gm0 MBR (74G)
+ 63 63 - free - (31k)
+ 126 156301299 1 freebsd (74G)
+ 156301425 61 - free - (30k)</screen>
+
+ <screen>&prompt.root; <userinput>gpart create -s BSD mirror/gm0s1</userinput>
+&prompt.root; <userinput>gpart add -t freebsd-ufs -a 4k -s 2g mirror/gm0s1</userinput>
+&prompt.root; <userinput>gpart add -t freebsd-swap -a 4k -s 4g mirror/gm0s1</userinput>
+&prompt.root; <userinput>gpart add -t freebsd-ufs -a 4k -s 2g mirror/gm0s1</userinput>
+&prompt.root; <userinput>gpart add -t freebsd-ufs -a 4k -s 1g mirror/gm0s1</userinput>
+&prompt.root; <userinput>gpart add -t freebsd-ufs -a 4k mirror/gm0s1</userinput>
+&prompt.root; <userinput>gpart show mirror/gm0s1</userinput>
+=> 0 156301299 mirror/gm0s1 BSD (74G)
+ 0 2 - free - (1.0k)
+ 2 4194304 1 freebsd-ufs (2.0G)
+ 4194306 8388608 2 freebsd-swap (4.0G)
+ 12582914 4194304 4 freebsd-ufs (2.0G)
+ 16777218 2097152 5 freebsd-ufs (1.0G)
+ 18874370 137426928 6 freebsd-ufs (65G)
+ 156301298 1 - free - (512B)</screen>
+
+ <para>Make the mirror bootable by installing bootcode in the MBR
+ and bsdlabel and setting the active slice:</para>
+
+ <screen>&prompt.root; <userinput>gpart bootcode -b /boot/mbr mirror/gm0</userinput>
+&prompt.root; <userinput>gpart set -a active -i 1 mirror/gm0</userinput>
+&prompt.root; <userinput>gpart bootcode -b /boot/boot mirror/gm0s1</userinput></screen>
+
+ <para>Format the filesystems on the new mirror, enabling
+ soft-updates.</para>
+
+ <screen>&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1a</userinput>
+&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1d</userinput>
+&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1e</userinput>
+&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1f</userinput></screen>
+
+ <para>Filesystems from the original disk
+ (<devicename>ada0</devicename>) can now be copied onto the
+ mirror with &man.dump.8; and &man.restore.8;.</para>
+
+ <screen>&prompt.root; <userinput>mount /dev/mirror/gm0s1a /mnt</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - / | (cd /mnt && restore -rf -)</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1d /mnt/var</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1e /mnt/tmp</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1f /mnt/usr</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - /var | (cd /mnt/var && restore -rf -)</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - /tmp | (cd /mnt/tmp && restore -rf -)</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - /usr | (cd /mnt/usr && restore -rf -)</userinput></screen>
+
+ <para><filename>/mnt/etc/fstab</filename> must be edited to
+ point to the new mirror filesystems:</para>
+
+ <programlisting># Device Mountpoint FStype Options Dump Pass#
+/dev/mirror/gm0s1a / ufs rw 1 1
+/dev/mirror/gm0s1b none swap sw 0 0
+/dev/mirror/gm0s1d /var ufs rw 2 2
+/dev/mirror/gm0s1e /tmp ufs rw 2 2
+/dev/mirror/gm0s1f /usr ufs rw 2 2</programlisting>
+
+ <para>If the &man.gmirror.8; kernel module has not been built
+ into the kernel, <filename>/mnt/boot/loader.conf</filename> is
+ edited to load the module at boot:</para>
+
+ <programlisting>geom_mirror_load="YES"</programlisting>
+
+ <para>Reboot the system to test the new mirror and verify that
+ all data has been copied. The BIOS will see the mirror as two
+ individual drives rather than a mirror. Because the drives
+ are identical, it does not matter which is selected to
+ boot.</para>
+
+ <para>See the
+ <link linkend="gmirror-troubleshooting">Troubleshooting</link>
+ section if there are problems booting. Powering down and
+ disconnecting the original <devicename>ada0</devicename> disk
+ will allow it to be kept as an offline backup.</para>
- <para>Enable loading of the <filename>geom_mirror.ko</filename>
- kernel module during system initialization:</para>
+ <para>In use, the mirror will behave just like the original
+ single drive.</para>
+ </sect2>
- <screen>&prompt.root; <userinput>echo 'geom_mirror_load="YES"' >> /boot/loader.conf</userinput></screen>
+ <sect2>
+ <title>Creating a Mirror with an Existing Drive</title>
- <para>Edit the <filename>/etc/fstab</filename> file, replacing
- references to the old <devicename>da0</devicename> with the
- new device nodes of the <devicename>gm0</devicename> mirror
- device.</para>
+ <para>In this example, &os; has already been installed on a
+ single disk, <devicename>ada0</devicename>. A new disk,
+ <devicename>ada1</devicename>, has been connected to the
+ system. A one-disk mirror will be created on the new disk,
+ the existing system copied onto it, and then old disk will be
+ inserted into the mirror. This slightly complex procedure is
+ required because &man.gmirror.8; needs to put a 512-byte block
+ of metadata at the end of each disk, and the existing
+ <devicename>ada0</devicename> has usually had all of its space
+ already allocated.</para>
- <note>
- <para>If &man.vi.1; is your preferred editor, the following is
- an easy way to accomplish this task:</para>
+ <para>Load the &man.gmirror.8; kernel module.</para>
- <screen>&prompt.root; <userinput>vi /etc/fstab</userinput></screen>
+ <screen>&prompt.root; <userinput>gmirror load</userinput></screen>
- <para>In &man.vi.1; back up the current contents of
- <filename>fstab</filename> by typing
- <userinput>:w /etc/fstab.bak</userinput>. Then
- replace all old <devicename>da0</devicename> references
- with <devicename>gm0</devicename> by typing
- <userinput>:%s/da/mirror\/gm/g</userinput>.</para>
- </note>
+ <para>Check the media size of the original disk with
+ &man.diskinfo.8;.</para>
- <para>The resulting <filename>fstab</filename> file should look
- similar to the following. It does not matter if the disk
- drives are <acronym>SCSI</acronym> or <acronym>ATA</acronym>,
- the <acronym>RAID</acronym> device will be
- <devicename>gm</devicename> regardless.</para>
-
- <programlisting># Device Mountpoint FStype Options Dump Pass#
-/dev/mirror/gm0s1b none swap sw 0 0
-/dev/mirror/gm0s1a / ufs rw 1 1
-/dev/mirror/gm0s1d /usr ufs rw 0 0
-/dev/mirror/gm0s1f /home ufs rw 2 2
-#/dev/mirror/gm0s2d /store ufs rw 2 2
-/dev/mirror/gm0s1e /var ufs rw 2 2
-/dev/acd0 /cdrom cd9660 ro,noauto 0 0</programlisting>
-
- <para>Reboot the system:</para>
-
- <screen>&prompt.root; <userinput>shutdown -r now</userinput></screen>
-
- <para>During system initialization, the
- <devicename>gm0</devicename> should be used in place of the
- <devicename>da0</devicename> device. Once fully initialized,
- this may be checked by visually inspecting the output from
- the <command>mount</command> command:</para>
-
- <screen>&prompt.root; <userinput>mount</userinput>
-Filesystem 1K-blocks Used Avail Capacity Mounted on
-/dev/mirror/gm0s1a 1012974 224604 707334 24% /
-devfs 1 1 0 100% /dev
-/dev/mirror/gm0s1f 45970182 28596 42263972 0% /home
-/dev/mirror/gm0s1d 6090094 1348356 4254532 24% /usr
-/dev/mirror/gm0s1e 3045006 2241420 559986 80% /var
-devfs 1 1 0 100% /var/named/dev</screen>
-
- <para>The output looks good, as expected. Finally, to begin
- synchronization, insert the <devicename>da1</devicename> disk
- into the mirror using the following command:</para>
-
- <screen>&prompt.root; <userinput>gmirror insert gm0 /dev/da1</userinput></screen>
-
- <para>As the mirror is built the status may be checked using
- the following command:</para>
-
- <screen>&prompt.root; <userinput>gmirror status</userinput></screen>
-
- <para>Once the mirror has been built and all current data
- has been synchronized, the output from the above command
- should look like:</para>
-
- <screen> Name Status Components
-mirror/gm0 COMPLETE da0
- da1</screen>
-
- <para>If there are any issues, or the mirror is still
- completing the build process, the example will show
- <literal>DEGRADED</literal> in place of
- <literal>COMPLETE</literal>.</para>
+ <screen>&prompt.root; <userinput>diskinfo -v ada0 | head -n3</userinput>
+/dev/ada0
+ 512 # sectorsize
+ 1000204821504 # mediasize in bytes (931G)</screen>
+
+ <para>Create a mirror on the new disk. To make certain that the
+ mirror capacity is not any larger than the original drive,
+ &man.gnop.8; is used to create a fake drive of the exact same
+ size. This drive does not store any data, but is used only to
+ limit the size of the mirror. When &man.gmirror.8; creates
+ the mirror, it will restrict the capacity to the size of
+ <devicename>gzero.nop</devicename> even if the new drive
+ (<devicename>ada1</devicename>) has more space. Note that the
+ <replaceable>1000204821504</replaceable> in the second line
+ should be equal to <devicename>ada0</devicename>'s media size
+ as shown by &man.diskinfo.8; above.</para>
+
+ <screen>&prompt.root; <userinput>geom zero load</userinput>
+&prompt.root; <userinput>gnop create -s 1000204821504 gzero</userinput>
+&prompt.root; <userinput>gmirror label -v gm0 gzero.nop ada1</userinput>
+&prompt.root; <userinput>gmirror forget gm0</userinput></screen>
+
+ <para><devicename>gzero.nop</devicename> does not store any
+ data, so the mirror does not see it as connected. The mirror
+ is told to <quote>forget</quote> unconnected components,
+ removing references to <devicename>gzero.nop</devicename>.
+ The result is a mirror device containing only a single disk,
+ <devicename>ada1</devicename>.</para>
+
+ <para>After creating <devicename>gm0</devicename>, view the
+ partition table on <devicename>ada0</devicename>.</para>
+
+ <para>This output is from a 1 TB drive. If there is some
+ unallocated space at the end of the drive, the contents may be
+ copied directly from <devicename>ada0</devicename> to the new
+ mirror.</para>
+
+ <para>However, if the output shows that all of the space on the
+ disk is allocated like the following listing, there is no
+ space available for the 512-byte &man.gmirror.8; metadata at
+ the end of the disk.</para>
+
+ <screen>&prompt.root; <userinput>gpart show ada0</userinput>
+=> 63 1953525105 ada0 MBR (931G)
+ 63 1953525105 1 freebsd [active] (931G)</screen>
+
+ <para>In this case, the partition table must be edited to reduce
+ the capacity by one sector on
+ <devicename>mirror/gm0</devicename>. The procedure will
+ be explained later.</para>
+
+ <para>In either case, partition tables on the primary disk
+ should be copied first. It can be done by using &man.gpart.8;
+ <command>backup</command> and <command>restore</command>
+ subcommands.</para>
+
+ <screen>&prompt.root; <userinput>gpart backup ada0 > table.ada0</userinput>
+&prompt.root; <userinput>gpart backup ada0s1 > table.ada0s1</userinput></screen>
+
+ <para>These commands create two files,
+ <filename>table.ada0</filename> and
+ <filename>table.ada0s1</filename>. This example is from a
+ 1 TB drive:</para>
+
+ <screen>&prompt.root; <userinput>cat table.ada0</userinput>
+MBR 4
+1 freebsd 63 1953525105 [active]</screen>
+
+ <screen>&prompt.root; <userinput>cat table.ada0s1</userinput>
+BSD 8
+1 freebsd-ufs 0 4194304
+2 freebsd-swap 4194304 33554432
+4 freebsd-ufs 37748736 50331648
+5 freebsd-ufs 88080384 41943040
+6 freebsd-ufs 130023424 838860800
+7 freebsd-ufs 968884224 984640881</screen>
+
+ <para>If the whole disk was used in the output of &man.gpart.8;
+ <command>show</command>, the capacity in these partition
+ tables must be reduced by one sector. Edit the two files,
+ reducing the size of both the slice and last partition by one.
+ These are the last numbers in each listing.</para>
+
+ <screen>&prompt.root; <userinput>cat table.ada0</userinput>
+MBR 4
+1 freebsd 63 <emphasis>1953525104</emphasis> [active]</screen>
+
+ <screen>&prompt.root; <userinput>cat table.ada0s1</userinput>
+BSD 8
+1 freebsd-ufs 0 4194304
+2 freebsd-swap 4194304 33554432
+4 freebsd-ufs 37748736 50331648
+5 freebsd-ufs 88080384 41943040
+6 freebsd-ufs 130023424 838860800
+7 freebsd-ufs 968884224 <emphasis>984640880</emphasis></screen>
+
+ <para>If at least one sector was unallocated at the end of the
+ disk, these two files can be used without modification.</para>
+
+ <para>Now restore the partition table into
+ <devicename>mirror/gm0</devicename>.</para>
+
+ <screen>&prompt.root; <userinput>gpart restore mirror/gm0 < table.ada0</userinput>
+&prompt.root; <userinput>gpart restore mirror/gm0s1 < table.ada0s1</userinput></screen>
+
+ <para>Check the partition table with the &man.gpart.8;
+ <command>show</command>. This example has
+ <devicename>gm0s1a</devicename> for <filename>/</filename>,
+ <devicename>gm0s1d</devicename> for <filename>/var</filename>,
+ <devicename>gm0s1e</devicename> for <filename>/usr</filename>,
+ <devicename>gm0s1f</devicename> for
+ <filename>/data1</filename>, and
+ <devicename>gm0s1g</devicename> for
+ <filename>/data2</filename>.</para>
+
+ <screen>&prompt.root; <userinput>gpart show mirror/gm0</userinput>
+=> 63 1953525104 mirror/gm0 MBR (931G)
+ 63 1953525042 1 freebsd [active] (931G)
+ 1953525105 62 - free - (31k)
+
+&prompt.root; <userinput>gpart show mirror/gm0s1</userinput>
+=> 0 1953525042 mirror/gm0s1 BSD (931G)
+ 0 2097152 1 freebsd-ufs (1.0G)
+ 2097152 16777216 2 freebsd-swap (8.0G)
+ 18874368 41943040 4 freebsd-ufs (20G)
+ 60817408 20971520 5 freebsd-ufs (10G)
+ 81788928 629145600 6 freebsd-ufs (300G)
+ 710934528 1242590514 7 freebsd-ufs (592G)
+ 1953525042 63 - free - (31k)</screen>
+
+ <para>Both the slice and the last partition should have some
+ free space at the end of each disk.</para>
+
+ <para>Create filesystems on these new partitions. The
+ number of partitions will vary, matching the partitions on the
+ original disk, <devicename>ada0</devicename>.</para>
+
+ <screen>&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1a</userinput>
+&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1d</userinput>
+&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1e</userinput>
+&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1f</userinput>
+&prompt.root; <userinput>newfs -U /dev/mirror/gm0s1g</userinput></screen>
+
+ <para>Make the mirror bootable by installing bootcode in the MBR
+ and bsdlabel and setting the active slice:</para>
+
+ <screen>&prompt.root; <userinput>gpart bootcode -b /boot/mbr mirror/gm0</userinput>
+&prompt.root; <userinput>gpart set -a active -i 1 mirror/gm0</userinput>
+&prompt.root; <userinput>gpart bootcode -b /boot/boot mirror/gm0s1</userinput></screen>
+
+ <para>Adjust <filename>/etc/fstab</filename> to use the
+ new partitions on the mirror. Back up this file first by
+ copying it to <filename>/etc/fstab.orig</filename>.</para>
+
+ <screen>&prompt.root; <userinput>cp /etc/fstab /etc/fstab.orig</userinput></screen>
+
+ <para>Edit <filename>/etc/fstab</filename>, replacing
+ <devicename>/dev/ada0</devicename> with
+ <devicename>mirror/gm0</devicename>.</para>
+
+ <programlisting># Device Mountpoint FStype Options Dump Pass#
+/dev/mirror/gm0s1a / ufs rw 1 1
+/dev/mirror/gm0s1b none swap sw 0 0
+/dev/mirror/gm0s1d /var ufs rw 2 2
+/dev/mirror/gm0s1e /usr ufs rw 2 2
+/dev/mirror/gm0s1f /data1 ufs rw 2 2
+/dev/mirror/gm0s1g /data2 ufs rw 2 2</programlisting>
+
+ <para>If the &man.gmirror.8; kernel module has not been built
+ into the kernel, edit <filename>/boot/loader.conf</filename>
+ to load it:</para>
+
+ <programlisting>geom_mirror_load="YES"</programlisting>
+
+ <para>Filesystems from the original disk can now be copied onto
+ the mirror with &man.dump.8; and &man.restore.8;. Note that
+ it may take some time to create a snapshot for each filesystem
+ dumped with <command>dump -L</command>.</para>
+
+ <screen>&prompt.root; <userinput>mount /dev/mirror/gm0s1a /mnt</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - / | (cd /mnt && restore -rf -)</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1d /mnt/var</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1e /mnt/usr</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1f /mnt/data1</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1g /mnt/data2</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - /usr | (cd /mnt/usr && restore -rf -)</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - /var | (cd /mnt/var && restore -rf -)</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - /data1 | (cd /mnt/data1 && restore -rf -)</userinput>
+&prompt.root; <userinput>dump -C16 -b64 -0aL -f - /data2 | (cd /mnt/data2 && restore -rf -)</userinput></screen>
+
+ <para>Restart the system, booting from
+ <devicename>ada1</devicename>. If everything is working, the
+ system will boot from <devicename>mirror/gm0</devicename>,
+ which now contains the same data as
+ <devicename>ada0</devicename> had previously. See the
+ <link linkend="gmirror-troubleshooting">Troubleshooting</link>
+ section if there are problems booting.</para>
+
+ <para>At this point, the mirror still consists of only the
+ single <devicename>ada1</devicename> disk.</para>
+
+ <para>After booting from <devicename>mirror/gm0</devicename>
+ successfully, the final step is inserting
+ <devicename>ada0</devicename> into the mirror.</para>
+
+ <important>
+ <para>When <devicename>ada0</devicename> is inserted into the
+ mirror, its former contents will be overwritten by data on
+ the mirror. Make certain that
+ <devicename>mirror/gm0</devicename> has the same contents as
+ <devicename>ada0</devicename> before adding
+ <devicename>ada0</devicename> to the mirror. If there is
+ something wrong with the contents copied by &man.dump.8; and
+ &man.restore.8;, revert <filename>/etc/fstab</filename> to
+ mount the filesystems on <devicename>ada0</devicename>,
+ reboot, and try the whole procedure again.</para>
+ </important>
+
+ <screen>&prompt.root; <userinput>gmirror insert gm0 ada0</userinput>
+GEOM_MIRROR: Device gm0: rebuilding provider ada0</screen>
+
+ <para>Synchronization between the two disks will start
+ immediately. &man.gmirror.8; <command>status</command>
+ shows the progress.</para>
+
+ <screen>&prompt.root; <userinput>gmirror status</userinput>
+ Name Status Components
+mirror/gm0 DEGRADED ada1 (ACTIVE)
+ ada0 (SYNCHRONIZING, 64%)</screen>
+
+ <para>After a while, synchronization will finish.</para>
+
+ <screen>GEOM_MIRROR: Device gm0: rebuilding provider ada0 finished.
+&prompt.root; <userinput>gmirror status</userinput>
+ Name Status Components
+mirror/gm0 COMPLETE ada1 (ACTIVE)
+ ada0 (ACTIVE)</screen>
+
+ <para><devicename>mirror/gm0</devicename> now consists of
+ the two disks <devicename>ada0</devicename> and
+ <devicename>ada1</devicename>, and the contents are
+ automatically synchronized with each other. In use,
+ <devicename>mirror/gm0</devicename> will behave just like the
+ original single drive.</para>
</sect2>
- <sect2>
+ <sect2 id="gmirror-troubleshooting">
<title>Troubleshooting</title>
<sect3>
- <title>System Refuses to Boot</title>
+ <title>Problems with Booting</title>
- <para>If the system boots up to a prompt similar to:</para>
+ <sect4>
+ <title>BIOS Settings</title>
- <programlisting>ffs_mountroot: can't find rootvp
-Root mount failed: 6
-mountroot></programlisting>
-
- <para>Reboot the machine using the power or reset button. At
- the boot menu, select option six (6). This will drop the
- system to a &man.loader.8; prompt. Load the kernel module
- manually:</para>
-
- <screen>OK? <userinput>load geom_mirror</userinput>
-OK? <userinput>boot</userinput></screen>
-
- <para>If this works then for whatever reason the module was
- not being loaded properly. Check whether the relevant entry
- in <filename>/boot/loader.conf</filename> is correct. If
- the problem persists, place:</para>
-
- <programlisting>options GEOM_MIRROR</programlisting>
-
- <para>in the kernel configuration file, rebuild and reinstall.
- That should remedy this issue.</para>
+ <para>BIOS settings may have to be changed to boot from one
+ of the new mirrored drives. Either mirror drive can be
+ used for booting. As components of a mirror, they contain
+ identical data.</para>
+ </sect4>
+
+ <sect4>
+ <title>Boot Problems</title>
+
+ <para>If the boot stopped with this message, something is
+ wrong with the mirror device:</para>
+
+ <screen>Mounting from ufs:/dev/mirror/gm0s1a failed with error 19.
+
+Loader variables:
+ vfs.root.mountfrom=ufs:/dev/mirror/gm0s1a
+ vfs.root.mountfrom.options=rw
+
+Manual root filesystem specification:
+ <fstype>:<device> [options]
+ Mount <device> using filesystem <fstype>
+ and with the specified (optional) option list.
+
+ eg. ufs:/dev/da0s1a
+ zfs:tank
+ cd9660:/dev/acd0 ro
+ (which is equivalent to: mount -t cd9660 -o ro /dev/acd0 /)
+
+ ? List valid disk boot devices
+ . Yield 1 second (for background tasks)
+ <empty line> Abort manual input
+
+mountroot></screen>
+
+ <para>Forgetting to load the
+ <filename>geom_mirror</filename> module in
+ <filename>/boot/loader.conf</filename> can cause this
+ problem. To fix it, boot from a &os;-9 or later CD or USB
+ stick and choose <literal>Shell</literal> at the first
+ prompt. Then load the mirror module and mount the mirror
+ device:</para>
+
+ <screen>&prompt.root; <userinput>gmirror load</userinput>
+&prompt.root; <userinput>mount /dev/mirror/gm0s1a /mnt</userinput></screen>
+
+ <para>Edit <filename>/mnt/boot/loader.conf</filename>,
+ adding a line to load the mirror module:</para>
+
+ <programlisting>geom_mirror_load="YES"</programlisting>
+
+ <para>Save the file and reboot.</para>
+
+ <para>Other problems that cause <literal>error 19</literal>
+ require more effort to fix. Enter
+ <literal>ufs:/dev/ada0s1a</literal> at the prompt.
+ Although the system should boot from
+ <devicename>ada0</devicename>, another prompt to select a
+ shell appears because <filename>/etc/fstab</filename> is
+ incorrect. Press the Enter key at the prompt. Undo the
+ modifications so far by reverting
+ <filename>/etc/fstab</filename>, mounting filesystems from
+ the original disk (<devicename>ada0</devicename>) instead
+ of the mirror. Reboot the system and try the procedure
+ again.</para>
+
+ <screen>Enter full pathname of shell or RETURN for /bin/sh:
+&prompt.root; <userinput>cp /etc/fstab.orig /etc/fstab</userinput>
+&prompt.root; <userinput>reboot</userinput></screen>
+ </sect4>
</sect3>
</sect2>
<sect2>
<title>Recovering from Disk Failure</title>
- <para>The wonderful part about disk mirroring is that when a
- disk fails, it may be replaced, presumably, without losing
+ <para>The wonderful part about disk mirroring is that an
+ individual disk can fail without causing the mirror to lose
any data.</para>
- <para>Considering the previous <acronym>RAID</acronym>1
- configuration, assume that <devicename>da1</devicename>
- has failed and now needs to be replaced. To replace it,
- determine which disk has failed and power down the system.
- At this point, the disk may be swapped with a new one and
- the system brought back up. After the system has restarted,
- the following commands may be used to replace the disk:</para>
+ <para><devicename>ada0</devicename> is one of two drives making
+ up the mirror in the previous example. If
+ <devicename>ada0</devicename> fails, the mirror will continue
+ to work, providing data from the remaining working drive,
+ <devicename>ada1</devicename>.</para>
+
+ <para>To replace the failed drive, the computer is shut down and
+ the failed drive is physically replaced with a new drive of
+ equal or greater capacity. Manufacturers use somewhat
+ arbitrary values when rating drives in gigabytes, and the
+ only way to really be sure is to compare the total count of
+ sectors shown by <command>diskinfo -v</command>. A drive with
+ larger capacity than the mirror will work, although the extra
+ space on the new drive will not be used.</para>
+
+ <para>After the computer is powered back up, the mirror will be
+ running in a <quote>degraded</quote> mode with only one drive.
+ The mirror is told to forget drives that are not currently
+ connected:</para>
<screen>&prompt.root; <userinput>gmirror forget gm0</userinput></screen>
- <screen>&prompt.root; <userinput>gmirror insert gm0 /dev/da1</userinput></screen>
-
- <para>Use the <command>gmirror</command> <option>status</option>
- command to monitor the progress of the rebuild. It is that
- simple.</para>
+ <para>Any old metadata should be <link
+ linkend="GEOM-mirror-metadata">cleared from the replacement
+ disk</link>. Then the disk, <devicename>ada4</devicename>
+ for this example, is inserted into the mirror:</para>
+
+ <screen>&prompt.root; <userinput>gmirror insert gm0 /dev/ada4</userinput></screen>
+
+ <para>Resynchronization begins when the new drive is inserted
+ into the mirror. This process of copying mirror data to a new
+ drive can take a while. Performance of the mirror will be
+ greatly reduced during the copy, so inserting new drives is
+ best done when there is low demand on the computer.</para>
+
+ <para>Progress can be monitored with <command>gmirror
+ status</command>, which shows drives that are being
+ synchronized and the percentage of completion. During
+ resynchronization, the status will be
+ <computeroutput>DEGRADED</computeroutput>, changing to
+ <computeroutput>COMPLETE</computeroutput> when the process is
+ finished.</para>
</sect2>
</sect1>
@@ -489,7 +877,7 @@ OK? <userinput>boot</userinput></screen>
fault tolerance of 1 drive, while providing a capacity of 1 - 1/n
times the total capacity of all drives in the array, where n is the
number of hard drives in the array. Such a configuration is
- mostly suitable for storing data of larger sizes, e.g.
+ mostly suitable for storing data of larger sizes, e.g.,
multimedia files.</para>
<para>At least 3 physical hard drives are required to build a
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