RAID1, or mirroring, 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.
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.
Traditionally, the two drives in a mirror are identical in model and capacity, but 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.
Warning: The mirroring procedures shown here are non-destructive, but as with any major disk operation, make a full backup first.
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 gmirror(8) 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
gmirror(8) can remove a disk from an active mirror and erase the metadata in one step. 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 full 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 gmirror(8).
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.
gmirror(8) requires a kernel module, geom_mirror.ko, either 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 will appear in the /dev/mirror/ directory.
MBR and bsdlabel partition tables can now be created on the mirror with gpart(8). Here we show a traditional split-filesystem layout, with partitions for /, swap, /var, /tmp, and /usr. A single / filesystem 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 filesystems 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
Filesystems from the original disk (ada0) 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 -)
/mnt/etc/fstab must be edited to point to the new mirror filesystems:
# 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 gmirror(8) 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 the Troubleshooting section 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(8) 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 gmirror(8) kernel module.
# gmirror load
Check the media size of the original disk with diskinfo(8).
# 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 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 drive (ada1) has more space. Note that the 1000204821504 in the second line should be equal to ada0's media size as shown by diskinfo(8) above.
# geom zero load # gnop create -s 1000204821504 gzero # gmirror label -v gm0 gzero.nop ada1 # gmirror forget gm0
gzero.nop does not store any data, so 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 like the following listing, there is no space available for the 512-byte gmirror(8) 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 copied first. It can be done by using gpart(8) backup and restore subcommands.
# 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 the whole disk was used in the output of gpart(8) show, 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.
# 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 the gpart(8) 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 should have some free space at the end of each disk.
Create filesystems on these new partitions. The number of partitions will vary, matching the partitions on 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 gmirror(8) kernel module has not been built into the kernel, edit /boot/loader.conf to load it:
geom_mirror_load="YES"
Filesystems from the original disk can now be copied onto the mirror with dump(8) and restore(8). Note that it may take some time to create a snapshot for each filesystem dumped with dump -L.
# 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 the Troubleshooting section 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.
Important: When ada0 is inserted into the mirror, its former contents will be overwritten by data on the mirror. Make certain that mirror/gm0 has the same contents as ada0 before adding ada0 to the mirror. If there is something wrong with the contents copied by dump(8) and restore(8), revert /etc/fstab to mount the filesystems on ada0, reboot, and try the whole procedure again.
# gmirror insert gm0 ada0 GEOM_MIRROR: Device gm0: rebuilding provider ada0
Synchronization between the two disks will start immediately. gmirror(8) status shows 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.
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.
If the boot stopped 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 module in /boot/loader.conf can cause this problem. To fix it, boot from a FreeBSD-9 or later CD or USB stick 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. Enter ufs:/dev/ada0s1a at the prompt. Although the system should boot from ada0, another prompt to select a shell appears because /etc/fstab is incorrect. Press the Enter key at the prompt. Undo the modifications so far by reverting /etc/fstab, mounting filesystems 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 wonderful part about disk mirroring is that an individual disk can fail without causing the mirror to lose any data.
ada0 is one of two drives making up the mirror in the previous example. If ada0 fails, the mirror will continue to work, providing data from the remaining working drive, ada1.
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 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. Then the 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.