New features of Solaris: Al...

Using snapshots for boot environments

One of the nice features of ZFS is the fact, that you get snapshots for free. The reason lies in the copy-on-write nature of ZFS. You can freeze the filesystem by not freeing the old blocks. as new data is written is written to new blocks, you don't even have to copy the blocks (in this sense the COW of ZFS is more like a ROW ... redirect on write).

ZFS boot enables the system to work with such snapshots, as you can use one of these to boot from. You can establish multiple boot environments just by snapshoting the bootfilesystems, clonimg them and promoting them to real filesystems. This are features inherent to ZFS.

A practical example

A warning at first: Don´t try this example without a backup of your system. Or use a test system or test VM. We will fsck up the system during this example. Okay....

I've updated my system, so i have alread two boot environments on my system:

jmoekamp@glamdring:~# beadm list

BE            Active Active on Mountpoint Space 
Name                 reboot               Used 
----          ------ --------- ---------- -----
opensolaris-1 yes    yes       legacy     2.31G
opensolaris   no     no        -          62.72M

This mirrors the actual state in your ZFS pools. You will find filesystems with accordings names.

NAME                                                        USED  AVAIL  REFER  MOUNTPOINT
rpool                                                      2.39G   142G  56.5K  /rpool
rpool@install                                              18.5K      -    55K  -
rpool/ROOT                                                 2.37G   142G    18K  /rpool/ROOT
rpool/ROOT@install                                             0      -    18K  -
rpool/ROOT/opensolaris                                     62.7M   142G  2.23G  legacy
rpool/ROOT/opensolaris-1                                   2.31G   142G  2.24G  legacy
rpool/ROOT/opensolaris-1@install                           4.66M      -  2.22G  -
rpool/ROOT/opensolaris-1@static:-:2008-04-29-17:59:13      5.49M      -  2.23G  -
rpool/ROOT/opensolaris-1/opt                               3.60M   142G  3.60M  /opt
rpool/ROOT/opensolaris-1/opt@install                           0      -  3.60M  -
rpool/ROOT/opensolaris-1/opt@static:-:2008-04-29-17:59:13      0      -  3.60M  -
rpool/ROOT/opensolaris/opt                                     0   142G  3.60M  /opt
rpool/export                                               18.9M   142G    19K  /export
rpool/export@install                                         15K      -    19K  -
rpool/export/home                                          18.9M   142G  18.9M  /export/home
rpool/export/home@install                                    18K      -    21K  -

After doing some configuration, you can create an boot environment called opensolaris-baseline:
It's really easy. You just have to create a new boot environment:

# beadm create -e opensolaris-1 opensolaris-baseline

But we will not work with this environment. We use it as a baseline, as a last resort when we destroy our running environment. To run the system we will create another snapshot:

# beadm create -e opensolaris-1 opensolaris-work

Now let´s look into the list of our boot environments.

jmoekamp@glamdring:~# beadm list

BE                   Active Active on Mountpoint Space 
Name                        reboot               Used 
----                 ------ --------- ---------- -----
opensolaris-baseline no     no        -          53.5K
opensolaris-1        yes    yes       legacy     2.31G
opensolaris          no     no        -          62.72M
opensolaris-work     no     no        -          53.5K

Okay, now we activate the opensolaris-work boot environment:

jmoekamp@glamdring:~# beadm activate opensolaris-work

Okay, let´s look at the list of boot environments again.

jmoekamp@glamdring:~# beadm list

BE                   Active Active on Mountpoint Space 
Name                        reboot               Used 
----                 ------ --------- ---------- -----
opensolaris-baseline no     no        -          53.5K
opensolaris-1        yes    no        legacy     24.5K
opensolaris          no     no        -          62.72M
opensolaris-work     no     yes       -          2.31G
jmoekamp@glamdring:~#

You will see that the opensolaris-1 snapshot is still active, but that the opensolaris-work will be active at the next reboot. Okay, now reboot:

jmoekamp@glamdring:~# beadm list

BE                   Active Active on Mountpoint Space 
Name                        reboot               Used 
----                 ------ --------- ---------- -----
opensolaris-baseline no     no        -          53.5K
opensolaris-1        no     no        -          54.39M
opensolaris          no     no        -          62.72M
opensolaris-work     yes    yes       legacy     2.36G

Okay, you see ... the boot environment opensolaris-work is now active and it´s activated for the next reboot (until you activate another boot environment).

Now we can reboot the system. The GRUB comes up and it will default to the opensolaris-work environment. Please remember on whicht position you find opensolaris-baseline in the boot menu. You need this position in a few moments. After a few seconds, you can log into the system and work with it.

Okay ... now let's drop the atomic bomb of administrative mishaps to your system. Log into your system, assume the root role and do the following stuff:

# cd /
# rm -rf *

You known what happens. Depending from how fast you are able to interrupt this run to get an slightly damaged system up to a system fscked up beyond any recognition. Normaly the system would send you to the tapes now. But remember. You have some alternate boot environments.

Reboot the system, wait for the grub. You may have an garbeled output, so it's hard to read the output from the grub. Choose opensolaris-baseline. The system will boot up quite normaly.

You need a terminal window now. How you get such a terminal window depends from incurred damage. The boot environment snapshots doesn't cover the home directories. So you may have no home directory any longer. I will assume this for this example: You can get a terminal window by clicking on "Options", then "Change Session" and choose "Failsafe Terminal" there.

Okay, login via the graphical login manager, a xterm will appear. At first we delete the defunct boot environment:

# beadm destroy opensolaris-work1
Are you sure you want to destroy opensolaris-work1? This action cannot be undone (y/[n]):
y

Okay, now we clone the opensolaris-baseline environment to form a new opensolaris-work environment.

# beadm create -e opensolaris-baseline opensolaris-work

We reactivate the opensolaris-work boot environment:

# beadm activate opensolaris-work

Now check, if you still have a homedirectory for your user:

# ls -l /export/home/jmoekamp
/export/home/jmoekamp: No such file or directory

If your home directory doesn't exist any longer, create a new one:

# mkdir -p /export/home/jmoekamp
# chown jmoekamp:staff /export/home/jmoekamp<(code>

Now reboot the system:

# reboot

Wait a few moments. The system starts up. The GRUB defaults to opensolaris-work and the system starts up normaly without any problem in that condition the system had, when you create the opensolaris-baseline boot environment.

# beadm list

BE                   Active Active on Mountpoint Space 
Name                        reboot               Used 
----                 ------ --------- ---------- -----
opensolaris-baseline no     no        -          3.18M
opensolaris-1        no     no        -          54.42M
opensolaris          no     no        -          62.72M
opensolaris-work     yes    yes       legacy     2.36G

Obviously you may have to recover your directory with data. It's a best practice to make snapshots of this directories on a regular schedule. So you can simply promote a snapshot to your actual version of the directory.

Conclusion

You see, this is a really neat feature. Recovering from a disaster in a minute or two. Snapshotting opens a completely new way to recover from errors. Unlike with Liveupgrade you don't need extra disks or extra partitions, and as ZFS snapshots are really fast, creating alternate boot environments on zfs are extremly fast as well.

At the moment this feature is available on Opensolaris 2008.05 only. But with future updates it will find it´s way into Solaris as well.