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 metadata from a previous mirror.

GPT metadata can be erased with gpart(8). This example erases both primary and backup GPT partition tables from disk ada8:

# gpart destroy -F ada8

A disk can be removed from an active mirror and the metadata erased in one step using gmirror(8). Here, the example disk ada8 is removed from the active mirror gm4:

# gmirror remove gm4 ada8

If the mirror is not running, but old mirror metadata is still on the disk, use gmirror clear to remove it:

# gmirror clear ada8

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 entire GPT disks with 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 the mirror metadata.

In this example, FreeBSD has already been installed on a single disk, ada0. Two new disks, ada1 and ada2, have been connected to the system. A new mirror will be created on these two disks and used to replace the old single disk.

The geom_mirror.ko kernel module must either be built into the kernel or loaded at boot- or run-time. Manually load the kernel module now:

# gmirror load

Create the mirror with the two new drives:

# gmirror label -v gm0 /dev/ada1 /dev/ada2

gm0 is a user-chosen device name assigned to the new mirror. After the mirror has been started, this device name appears in /dev/mirror/.

MBR and bsdlabel partition tables can now be created on the mirror with gpart(8). This example uses a traditional file system layout, with partitions for /, swap, /var, /tmp, and /usr. A single / and a swap partition will also work.

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 ada0.

# gpart create -s MBR mirror/gm0
# gpart add -t freebsd -a 4k mirror/gm0
# gpart show mirror/gm0
=>       63  156301423  mirror/gm0  MBR  (74G)
         63         63                    - free -  (31k)
        126  156301299                 1  freebsd  (74G)
  156301425         61                    - free -  (30k)

# gpart create -s BSD mirror/gm0s1
# gpart add -t freebsd-ufs  -a 4k -s 2g mirror/gm0s1
# gpart add -t freebsd-swap -a 4k -s 4g mirror/gm0s1
# gpart add -t freebsd-ufs  -a 4k -s 2g mirror/gm0s1
# gpart add -t freebsd-ufs  -a 4k -s 1g mirror/gm0s1
# gpart add -t freebsd-ufs  -a 4k       mirror/gm0s1
# gpart show mirror/gm0s1
=>        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)

Make the mirror bootable by installing bootcode in the MBR and bsdlabel and setting the active slice:

# gpart bootcode -b /boot/mbr mirror/gm0
# gpart set -a active -i 1 mirror/gm0
# gpart bootcode -b /boot/boot mirror/gm0s1

Format the file systems on the new mirror, enabling soft-updates.

# newfs -U /dev/mirror/gm0s1a
# newfs -U /dev/mirror/gm0s1d
# newfs -U /dev/mirror/gm0s1e
# newfs -U /dev/mirror/gm0s1f

File systems from the original ada0 disk can now be copied onto the mirror with dump(8) and restore(8).

# mount /dev/mirror/gm0s1a /mnt
# dump -C16 -b64 -0aL -f - / | (cd /mnt && restore -rf -)
# mount /dev/mirror/gm0s1d /mnt/var
# mount /dev/mirror/gm0s1e /mnt/tmp
# mount /dev/mirror/gm0s1f /mnt/usr
# dump -C16 -b64 -0aL -f - /var | (cd /mnt/var && restore -rf -)
# dump -C16 -b64 -0aL -f - /tmp | (cd /mnt/tmp && restore -rf -)
# dump -C16 -b64 -0aL -f - /usr | (cd /mnt/usr && restore -rf -)

Edit /mnt/etc/fstab to point to the new mirror file systems:

# 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

If the geom_mirror.ko kernel module has not been built into the kernel, /mnt/boot/loader.conf is edited to load the module at boot:

geom_mirror_load="YES"

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.

See Section 18.3.4, “Troubleshooting” if there are problems booting. Powering down and disconnecting the original ada0 disk will allow it to be kept as an offline backup.

In use, the mirror will behave just like the original single drive.

In this example, FreeBSD has already been installed on a single disk, ada0. A new disk, ada1, 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 the old disk will be inserted into the mirror. This slightly complex procedure is required because gmirror needs to put a 512-byte block of metadata at the end of each disk, and the existing ada0 has usually had all of its space already allocated.

Load the geom_mirror.ko kernel module:

# gmirror load

Check the media size of the original disk with diskinfo:

# diskinfo -v ada0 | head -n3
/dev/ada0
	512             # sectorsize
	1000204821504   # mediasize in bytes (931G)

Create a mirror on the new disk. To make certain that the mirror capacity is not any larger than the original ada0 drive, 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 gmirror(8) creates the mirror, it will restrict the capacity to the size of gzero.nop, even if the new ada1 drive has more space. Note that the 1000204821504 in the second line is equal to ada0's media size as shown by diskinfo above.

# geom zero load
# gnop create -s 1000204821504 gzero
# gmirror label -v gm0 gzero.nop ada1
# gmirror forget gm0

Since gzero.nop does not store any data, the mirror does not see it as connected. The mirror is told to “forget” unconnected components, removing references to gzero.nop. The result is a mirror device containing only a single disk, ada1.

After creating gm0, view the partition table on ada0. 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 ada0 to the new mirror.

However, if the output shows that all of the space on the disk is allocated, as in the following listing, there is no space available for the 512-byte mirror metadata at the end of the disk.

# gpart show ada0
=>        63  1953525105        ada0  MBR  (931G)
          63  1953525105           1  freebsd  [active]  (931G)

In this case, the partition table must be edited to reduce the capacity by one sector on mirror/gm0. The procedure will be explained later.

In either case, partition tables on the primary disk should be first copied using gpart backup and gpart restore.

# gpart backup ada0 > table.ada0
# gpart backup ada0s1 > table.ada0s1

These commands create two files, table.ada0 and table.ada0s1. This example is from a 1 TB drive:

# cat table.ada0
MBR 4
1 freebsd         63 1953525105   [active]

# cat table.ada0s1
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

If no free space is shown at the end of the disk, the size of both the slice and the last partition 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.

# cat table.ada0
MBR 4
1 freebsd         63 1953525104   [active]

# cat table.ada0s1
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  984640880

If at least one sector was unallocated at the end of the disk, these two files can be used without modification.

Now restore the partition table into mirror/gm0:

# gpart restore mirror/gm0 < table.ada0
# gpart restore mirror/gm0s1 < table.ada0s1

Check the partition table with gpart show. This example has gm0s1a for /, gm0s1d for /var, gm0s1e for /usr, gm0s1f for /data1, and gm0s1g for /data2.

# gpart show mirror/gm0
=>        63  1953525104  mirror/gm0  MBR  (931G)
          63  1953525042           1  freebsd  [active]  (931G)
  1953525105          62              - free -  (31k)

# gpart show mirror/gm0s1
=>         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)

Both the slice and the last partition must have at least one free block at the end of the disk.

Create file systems on these new partitions. The number of partitions will vary to match the original disk, ada0.

# newfs -U /dev/mirror/gm0s1a
# newfs -U /dev/mirror/gm0s1d
# newfs -U /dev/mirror/gm0s1e
# newfs -U /dev/mirror/gm0s1f
# newfs -U /dev/mirror/gm0s1g

Make the mirror bootable by installing bootcode in the MBR and bsdlabel and setting the active slice:

# gpart bootcode -b /boot/mbr mirror/gm0
# gpart set -a active -i 1 mirror/gm0
# gpart bootcode -b /boot/boot mirror/gm0s1

Adjust /etc/fstab to use the new partitions on the mirror. Back up this file first by copying it to /etc/fstab.orig.

# cp /etc/fstab /etc/fstab.orig

Edit /etc/fstab, replacing /dev/ada0 with mirror/gm0.

# 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

If the geom_mirror.ko kernel module has not been built into the kernel, edit /boot/loader.conf to load it at boot:

geom_mirror_load="YES"

File systems from the original disk can now be copied onto the mirror with dump(8) and restore(8). Each file system dumped with dump -L will create a snapshot first, which can take some time.

# mount /dev/mirror/gm0s1a /mnt
# dump -C16 -b64 -0aL -f - /    | (cd /mnt && restore -rf -)
# mount /dev/mirror/gm0s1d /mnt/var
# mount /dev/mirror/gm0s1e /mnt/usr
# mount /dev/mirror/gm0s1f /mnt/data1
# mount /dev/mirror/gm0s1g /mnt/data2
# dump -C16 -b64 -0aL -f - /usr | (cd /mnt/usr && restore -rf -)
# dump -C16 -b64 -0aL -f - /var | (cd /mnt/var && restore -rf -)
# dump -C16 -b64 -0aL -f - /data1 | (cd /mnt/data1 && restore -rf -)
# dump -C16 -b64 -0aL -f - /data2 | (cd /mnt/data2 && restore -rf -)

Restart the system, booting from ada1. If everything is working, the system will boot from mirror/gm0, which now contains the same data as ada0 had previously. See Section 18.3.4, “Troubleshooting” if there are problems booting.

At this point, the mirror still consists of only the single ada1 disk.

After booting from mirror/gm0 successfully, the final step is inserting ada0 into the mirror.

# gmirror insert gm0 ada0
GEOM_MIRROR: Device gm0: rebuilding provider ada0

Synchronization between the two disks will start immediately. Use gmirror status to view the progress.

# gmirror status
      Name    Status  Components
mirror/gm0  DEGRADED  ada1 (ACTIVE)
                      ada0 (SYNCHRONIZING, 64%)

After a while, synchronization will finish.

GEOM_MIRROR: Device gm0: rebuilding provider ada0 finished.
# gmirror status
      Name    Status  Components
mirror/gm0  COMPLETE  ada1 (ACTIVE)
                      ada0 (ACTIVE)

mirror/gm0 now consists of the two disks ada0 and ada1, and the contents are automatically synchronized with each other. In use, mirror/gm0 will behave just like the original single drive.

If the system no longer boots, 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 they contain identical data.

If the boot stops with this message, something is wrong with the mirror device:

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>

Forgetting to load the geom_mirror.ko module in /boot/loader.conf can cause this problem. To fix it, boot from a FreeBSD installation media and choose Shell at the first prompt. Then load the mirror module and mount the mirror device:

# gmirror load
# mount /dev/mirror/gm0s1a /mnt

Edit /mnt/boot/loader.conf, adding a line to load the mirror module:

geom_mirror_load="YES"

Save the file and reboot.

Other problems that cause error 19 require more effort to fix. Although the system should boot from ada0, another prompt to select a shell will appear if /etc/fstab is incorrect. Enter ufs:/dev/ada0s1a at the boot loader prompt and press Enter. Undo the edits in /etc/fstab then mount the file systems from the original disk (ada0) instead of the mirror. Reboot the system and try the procedure again.

Enter full pathname of shell or RETURN for /bin/sh:
# cp /etc/fstab.orig /etc/fstab
# reboot

The benefit of disk mirroring is that an individual disk can fail without causing the mirror to lose any data. In the above example, if ada0 fails, the mirror will continue to work, providing data from the remaining working drive, ada1.

To replace the failed drive, shut down the system and physically replace the failed drive 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 diskinfo -v. A drive with larger capacity than the mirror will work, although the extra space on the new drive will not be used.

After the computer is powered back up, the mirror will be running in a “degraded” mode with only one drive. The mirror is told to forget drives that are not currently connected:

# gmirror forget gm0

Any old metadata should be cleared from the replacement disk using the instructions in Section 18.3.1, “Metadata Issues”. Then the replacement disk, ada4 for this example, is inserted into the mirror:

# gmirror insert gm0 /dev/ada4

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.

Progress can be monitored with gmirror status, which shows drives that are being synchronized and the percentage of completion. During resynchronization, the status will be DEGRADED, changing to COMPLETE when the process is finished.