Adding gmirror to an existing installation

Find the right HDD

Using dmesg I found I had two HDD mounted, ad4 and ad6. Let’s try
mounting them and see what I can find:

Fixit# mount /dev/ad4s1 /mnt
Fixit# ls /mnt
.                               home
..                              lib
.cshrc                          libexec
.profile                        media
.snap                           mnt
COPYRIGHT                       mnt2
THIS.IS.6.3.BLOW.It.away        proc
bin                             rescue
boot                            root
cdrom                           sbin
compat                          sys
dev                             tmp
dist                            usr
entropy                         var
etc

NOTE: that’s not the actual directory listing output. It is made up.
The key is that odd looking file name. It identifies the HDD that
I do not wish to keep. All data on this HDD can be lost.
Therefore, I know that ad4 is the slave HDD.

Now let’s try the other HDD and see what it contains:

Fixit# umount /mnt
Fixit# mount /dev/ad6s1 /mnt
Fixit# ls /mnt
.cshrc          boot/      entropy/     media/   sbin/
.profile        cdrom/     etc/         mnt/     sys@
.snap/          compat@    home@        proc/    tmp/
COPYRIGHT       dev/       lib/         rescue/  usr/
bin/            dist/      libexec/     root/    var/
Fixit#

Again, this is not a copy/paste. It may not represent exactly what I saw.
The key point: there is no special file. I know that ad6 is the master HDD
to use in the gmirror solution.

Setting up the mirror/duplex

I have before me, a copy of The Best of FreeBSD Basics by Dru Lavigne.
I used the section Using Software RAID-1 With FreeBSD. This is not
the first time I’ve used this documentation. I previously upgraded my
gateway/firewall box to use gmirror.

The first step is to create a mirror/duplex. It is critical that you specify
the drive that you wish to mirror, i.e. the master drive. In my case, as
found above, that drive is /dev/ad6.

Fixit# gmirror label -v -b round-robin gm0 /dev/ad6
Metadata value stored on /dev/ad6
Done

This created gm0 and uses /dev/ad6 as the master for this mirror.
At this stage, the data has not yet been mirrored. All we have done is
specify that we want to mirror.

Mount the main HDD for editing

I will now mount /dev/ad6s1 so I can alter a few files.

Fixit# mount /dev/ad6s1 /mnt

Start gmirror at boot

The following line was added to /mnt/boot/loader.conf to ensure gmirror
started at boot time:

geom_mirror_load="YES"

Alter fstab

Instead of mounting /dev/ad6* at the next boot, we want to mount /dev/mirror/gm0.
This is how FreeBSD will access your HDD, not directly, but through the mirror.
This is similarly to how hardware RAID works; the RAID cards presents a device
to the OS, and the OS accesses that device, e.g. twed0.

I will now alter /mnt/etc/fstab. I will change all
ad6 references to mirror/gm0

Fixit# vi /mnt/etc/fstab

This vi command does a global replacement:

:%s/ad6/gmirror\/gm0/g

Here is what fstab looks like now:

# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/mirror/gm0s1b      none            swap    sw              0       0
/dev/mirror/gm0s1a      /               ufs     rw              1       1
/dev/mirror/gm0s1e      /tmp            ufs     rw              2       2
/dev/mirror/gm0s1f      /usr            ufs     rw              2       2
/dev/mirror/gm0s1d      /var            ufs     rw              2       2
/dev/acd0               /cdrom          cd9660  ro,noauto       0       0

Booting

I then rebooted the system. However, it booted from ad4 instead of ad6.
That’s a problem. It needs to boot from ad6. I was able to fix this in
the BIOS. But not before playing around and trying to find a solution
from the boot loader prompt. I found this would work:

boot disk2s1a:/boot/kernel

After booting

After getting the system booted from the correct HDD, things went much smoother.

So far, we have set up the mirror and specified the source for the mirror. Now we
want to add the other half of the mirror into the system. That other drive is
/dev/ad4. I used this command:

gmirror insert gm0 /dev/ad4

[My thanks to manolis for pointing this out.

After adding the new disk to the mirror, here is the list of the gmirror devices:

# gmirror list
Geom name: gm0
State: DEGRADED
Components: 2
Balance: round-robin
Slice: 4096
Flags: NONE
GenID: 0
SyncID: 1
ID: 4226236689
Providers:
1. Name: mirror/gm0
   Mediasize: 500107861504 (466G)
   Sectorsize: 512
   Mode: r6w5e6
Consumers:
1. Name: ad6
   Mediasize: 500107862016 (466G)
   Sectorsize: 512
   Mode: r1w1e1
   State: ACTIVE
   Priority: 0
   Flags: DIRTY
   GenID: 0
   SyncID: 1
   ID: 533080279
2. Name: ad4
   Mediasize: 500107862016 (466G)
   Sectorsize: 512
   Mode: r1w1e1
   State: SYNCHRONIZING
   Priority: 0
   Flags: DIRTY, SYNCHRONIZING
   GenID: 0
   SyncID: 1
   Synchronized: 0%
   ID: 438208982

#

Yes, ad6 is dirty. That’s to be expected. At any given point, there is
probably some data to be written to disk. That’s all that means. Wait a
while. Try again. Perhaps it won’t be dirty.

You will see that ad4 is synchronizing. That took a few hours. These
are 500GB HDD.

The status command gives a more succinct output

# gmirror status
      Name    Status  Components
mirror/gm0  DEGRADED  ad6
                      ad4 (6%)

After the sync

Once everything is synced up, and the initial mirroring process has
completed, this is what I saw:

# gmirror status
      Name    Status  Components
mirror/gm0  COMPLETE  ad4
                      ad6

Cheap, simple, reliable

gmirror is cheap, simple, and reliable. I’m using it on a number of my machines
at home. It is relatively easy to set up and, more importantly, is stable.

Enjoy.