Using Software RAID-1 with FreeBSD
Pages: 1, 2

Booting into the Mirror/Duplex

If you watch your boot-up messages, you should see this in bold white text right after the disks are probed:

GEOM_MIRROR: Device gm0 created (id=2125638583).
GEOM_MIRROR: Device gm0: provider ad0 detected.
GEOM_MIRROR: Device gm0: provider ad0 activated.
GEOM_MIRROR: Device gm0: provider mirror/gm0 launched.
GEOM_MIRROR: Device gm0 already configured.
Mounting root from ufs:/dev/mirror/gm0s1a

and the system will continue to boot. However, if you have a typo in /etc/fstab, the boot will stop at this point and wait for you to type something meaningful. In this example, I forgot to insert mirror when I edited /etc/fstab, meaning /dev/gm0s1a should have been /dev/mirror/gm0s1a so that FreeBSD could find my root filesystem:

Mounting root from ufs:/dev/gm0s1a
setrootbyname failed
ffs_mountroot: can't find rootvp
Root mount failed: 6

Manual root filesystem specification:
  <fstype>:<device>  Mount <device> using filesystem <fstype>
            e.g. ufs:da0s1a
  ?             List valid disk boot devices
  <empty line>       Abort manual input

mountroot>

Fortunately, that's not as scary as it looks. Start by listing your valid disk boot devices:

mountroot> ?

List of GEOM managed disk devices:
  mirror/gm0s1f mirror/gm0s1e mirror/gm0s1d mirror/gm0s1c mirror/gm0s1b 
mirror/gm0s1a mirror/gm0s1 ad2s1 mirror/gm0 ad0s1 ad2 acd0 ad0 fd0

If you type in the correct location of the / filesystem, the system will continue to reboot:

mountroot> ufs:/dev/mirror/gm0s1a
Mounting root from /dev/mirror/gm0s1a

After logging in, be sure to edit the offending line in /etc/fstab and try rebooting again. When you can boot up and log in successfully, verify that each partition on the mirror mounted successfully with:

% df -h
Filesystem            Size    Used    Avail    Capacity    Mounted on
/dev/mirror/gm0s1a    248M     35M     193M       15%        /
devfs                 1.0K    1.0K       0B      100%        /dev
/dev/mirror/gm0s1e    248M     12K     228M        0%        /tmp
/dev/mirror/gm0s1f    7.3G     99M     6.7G        1%        /usr
/dev/mirror/gm0s1d    248M    196K     228M        0%        /var

df won't show your swap partition; you can verify it with:

% swapinfo
Device                1K-blocks    Used    Avail    Capacity
/dev/mirror/gm0s1b       629544       0   629544        0%

Synchronizing the Mirror/Duplex

The only thing left to do is to synchronize the data on both hard drives. This will happen automatically as soon as you issue the command to insert the second drive into the mirror:

# gmirror insert gm0 /dev/ad2
GEOM_MIRROR: Device gm0: provider ad2 detected.
GEOM_MIRROR: Device gm0: rebuilding provider ad2.

To see what's happening:

# gmirror list | more
Geom name: gm0
State: DEGRADED
Components: 2
Balance: round-robin
Slice: 4096
Flags: NONE
GenID: 0
SyncID: 1
ID: 2125638583
Providers:
1. Name: mirror/gm0
   Mediasize: 10262568448 (9.6G)
   Sectorsize: 512
   Mode: r6w5e2
Consumers:
1. Name: ad0
   Mediasize: 10262568448 (9.6G)
   Sectorsize: 512
   Mode: r1w1e1
   State: ACTIVE
   Priority: 0
   Flags: DIRTY
   GenID: 0
   SyncID: 1
   ID: 3986018406
2. Name: ad2
   Mediasize: 10262568448 (9.6G)
   Sectorsize: 512
   Mode: r1w1e1
   State: SYNCHRONIZING
   Priority: 0
   Flags: DIRTY, SYNCHRONIZING
   GenID: 0
   SyncID: 1
   Synchronized: 1%
   ID: 1946262342

Note the SYNCHRONIZING on the Flags line. It will take a while for these two drives to synchronize, as it is currently at 1 percent. I've seen times ranging from about 30 minutes for a 10GB drive to about two and a half hours for a 75GB drive. If you're curious, check the progress with:

# gmirror status
Name    Status    Components
mirror/gm0    DEGRADED    ad0
            ad2 (2%)

You'll see a status message in bold white text when the synchronization finishes:

GEOM_MIRROR: Device gm0: rebuilding provider ad2 finished.
GEOM_MIRROR: Device gm0: provider ad2 activated.

If you repeat gmirror list, you'll note that the State has changed from DEGRADED to COMPLETE and the Synchronized line is now gone. Don't worry if you see DIRTY on the Flags line, as it simply indicates that the system has written new data to the disk but hasn't mirrored it yet. If you were to wait a few seconds on a quiet disk, you would see the Flags line change to NONE.

For the final test, reboot the system.

This time your startup messages should include:

GEOM_MIRROR: Device gm0 created (id=2125638583).
GEOM_MIRROR: Device gm0: provider ad0 detected.
GEOM_MIRROR: Device gm0: provider ad2 detected.
GEOM_MIRROR: Device gm0: provider ad0 activated.
GEOM_MIRROR: Device gm0: provider ad2 activated.
GEOM_MIRROR: Device gm0: provider mirror/gm0 launched.
Mounting root from ufs:/dev/mirror/gm0s1a

Final Notes

GEOM utilities are works in progress, and the developers constantly add new features and updates to the man pages. It's well worth your while to keep your favorite version of FreeBSD up-to-date using cvsup or to choose a newer release when deciding which version of FreeBSD to install.

If you wish to gather performance statistics on your mirror/duplex, try gstat(8). A good read through gmirror(8) is also in order, especially if you want an overview of the procedure for replacing a failed disk.

Dru Lavigne is a network and systems administrator, IT instructor, author and international speaker. She has over a decade of experience administering and teaching Netware, Microsoft, Cisco, Checkpoint, SCO, Solaris, Linux, and BSD systems. A prolific author, she pens the popular FreeBSD Basics column for O'Reilly and is author of BSD Hacks and The Best of FreeBSD Basics.

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