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FreeBSD Basics

Using Software RAID-1 with FreeBSD

by Dru Lavigne
11/10/2005

Have you ever needed a software RAID solution for a low-end server install? Perhaps you've wanted your workstation to take advantage of the redundancy provided by a disk mirror without investing in a hardware RAID controller. Has a prior painful configuration experience turned you off software RAID altogether on Unix systems?

Since 5.3-Release, FreeBSD comes with gmirror(8), which allows you to easily configure a software RAID 1 solution. While tutorials on gmirror exist, I found them to require either manual calculations of partition sizes with bsdlabel or the use of a fix-it floppy on an existing system.

It made more sense to me to configure RAID during the install of the operating system. I also wanted a procedure that was easy to follow and didn't introduce human error in the form of a math miscalculation. After cobbling together the available documentation and experimenting my way through various configurations, I came across a procedure that has worked well for me on several different systems. I also received valuable feedback from Pawel Jakub Dawidek, the author of gmirror, who gave some insight into some of the not yet documented features of gmirror.

Some GEOM Background

Related Reading

The Complete FreeBSD
Documentation from the Source
By Greg Lehey

Before demonstrating the configuration, it is useful to understand a bit about GEOM. GEOM is the modular disk framework introduced in FreeBSD 5.0. This modularity allows the creation of programs to manipulate disks. The best examples are the software RAID programs introduced with FreeBSD 5.3:

The initial g indicates that each of these programs takes advantage of GEOM.

Note: if you're totally new to RAID, Webopedia has a good definition of each RAID level.

man 4 geom describes the terms it uses to refer to disks, some of which you'll see when setting up gmirror. These terms include the following:

Configuring the Mirror/Duplex During the Install

If you're going to use RAID 1, make your life easy and purchase two identical disks (of the same model and size). You can complicate things by insisting on different disks with different sizes, but in the end you just end up with a harder configuration that wastes the extra disk space on the larger disk. Cable the identical drives so that one is the primary master and the other is the secondary master. Before installing the operating system, double-check that your CMOS recognizes both disks.

Using your favorite installation method, start a FreeBSD install of any version (5.3 or higher). When you get to the Select Drives menu, it should show ad0 and ad2. Select ad0, as you will be installing the operating system on the primary master.

Within the fdisk utility, remove any existing partitions and then select "Use entire disk." When asked about the boot menu, choose "Standard MBR."

In the disklabel editor, set up the partitions on ad0 according to your requirements. If in doubt, choose a for automatic. Then choose your install sets and your install media, and let the operating system install as usual.

When finished, go through the postinstall configurations and set your time zone, create a user account, set the root password, and so on.

However, don't reboot when you end up back at the sysinstall main menu. Instead, press Alt-F4, which will take you to a command prompt. The first command I type is csh so I can get a shell with history (the default shell is Bourne).

Creating a mirror/duplex is as simple as typing:

# gmirror label -v -b round-robin gm0 /dev/ad0

where gmirror label creates the mirror; -v enables verbose mode; -b round-robin chooses a balance algorithm (at the moment, round-robin is the algorithm with the best performance); gm0 is the name of mirror/duplex (this name represents the first GEOM mirror); and /dev/ad0 represents the disk containing the data to mirror.

However, you'll be disappointed if you try the command now:

# gmirror label -v -b round-robin gm0 /dev/ad0
Can't store metadata on /dev/ad0: Operation not permitted

This is a security feature that indicates that the disk is currently mounted for writing and therefore is unavailable. However, you can get around this chicken-and-egg problem and temporarily force gmirror to bypass this measure in order to create the mirror/duplex by setting a sysctl MIB:

# sysctl kern.geom.debugflags=16
kern.geom.debugflags: 0 -> 16

Don't worry; this MIB will return to 0 when you reboot (which I'll have you do in just a few minutes). Try again:

# gmirror label -v -b round-robin gm0 /dev/ad0
Metadata value stored on /dev/ad0

That's it; you now have a RAID 1 system.

It is, however, useful to tell the operating system to load it whenever you boot. This requires edits to two files. The first one is currently empty, so just echo over the required line:

# echo geom_mirror_load="YES" > /boot/loader.conf

However, /etc/fstab is not empty, so I recommend making a backup copy before editing it:

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

Change each ad to a gm, and insert a mirror after /dev. For example, /dev/ad0s1a becomes /dev/mirror/gm0s1a. Unless you've made extra partitions, you'll have ad0s1 devices ending in a, b, d, e, and f and will need to edit each of those lines.

When finished, triple-check your changes to both /etc/fstab and /boot/loader.conf. While it is fixable, it sucks not being able to boot into a new system because of a typo.

Note: some tutorials indicate you also need to add a swapoff option to /etc/rc.conf. This is no longer necessary, and neither is using shutdown -r now instead of reboot.

Once you're sure you don't have any typos, return to Alt-F1 and exit the installation menu after removing your installation media.

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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