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.
To begin, ensure the system has two disk drives of equal size, these exercises assume they are direct access (da(4)) SCSI disks.
Assuming FreeBSD has been installed on the first, da0 disk device, gmirror(8) should be told to store its primary data there.
Before building the mirror, enable additional debugging information and opening access
to the device by setting the kern.geom.debugflags sysctl(8) option to
the following value:
# sysctl kern.geom.debugflags=17
Now create the mirror. Begin the process by storing meta-data information on the primary disk device, effectively creating the /dev/mirror/gm device using the following command:
Warning: 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 FreeBSD.
# gmirror label -vb round-robin gm0 /dev/da0
The system should respond with:
Metadata value stored on /dev/da0. Done.
Initialize GEOM, this will load the /boot/kernel/geom_mirror.ko kernel module:
# gmirror load
Note: When this command completes successfully, it creates the gm0 device node under the /dev/mirror directory.
Enable loading of the geom_mirror.ko kernel module during system initialization:
# echo 'geom_mirror_load="YES"' >> /boot/loader.conf
Edit the /etc/fstab file, replacing references to the old da0 with the new device nodes of the gm0 mirror device.
Note: If vi(1) is your preferred editor, the following is an easy way to accomplish this task:
# vi /etc/fstabIn vi(1) back up the current contents of fstab by typing :w /etc/fstab.bak. Then replace all old da0 references with gm0 by typing :%s/da/mirror\/gm/g.
The resulting fstab file should look similar to the following. It does not matter if the disk drives are SCSI or ATA, the RAID device will be gm regardless.
# 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
Reboot the system:
# shutdown -r now
During system initialization, the gm0 should be used in place of the da0 device. Once fully initialized, this may be checked by visually inspecting the output from the mount command:
# mount 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
The output looks good, as expected. Finally, to begin synchronization, insert the da1 disk into the mirror using the following command:
# gmirror insert gm0 /dev/da1
As the mirror is built the status may be checked using the following command:
# gmirror status
Once the mirror has been built and all current data has been synchronized, the output from the above command should look like:
Name Status Components
mirror/gm0 COMPLETE da0
da1
If there are any issues, or the mirror is still completing the build process, the example will show DEGRADED in place of COMPLETE.
If the system boots up to a prompt similar to:
ffs_mountroot: can't find rootvp Root mount failed: 6 mountroot>
Reboot the machine using the power or reset button. At the boot menu, select option six (6). This will drop the system to a loader(8) prompt. Load the kernel module manually:
OK? load geom_mirror OK? boot
If this works then for whatever reason the module was not being loaded properly. Check whether the relevant entry in /boot/loader.conf is correct. If the problem persists, place:
options GEOM_MIRROR
in the kernel configuration file, rebuild and reinstall. That should remedy this issue.
The wonderful part about disk mirroring is that when a disk fails, it may be replaced, presumably, without losing any data.
Considering the previous RAID1 configuration, assume that da1 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:
# gmirror forget gm0
# gmirror insert gm0 /dev/da1
Use the gmirror status command to
monitor the progress of the rebuild. It is that simple.