LVM snapshots for a resetable machine

Fri, 21 Mar 2014 11:16:00 +0000

I have ended up maintaining a few websites which we are hosting on a machine off in Germany somewhere.

I want to get everything automated, so I have less work to do if something goes wrong.

I’m using ansible, which is wonderful, and have a nice set of playbooks I’ve written which take a raw CentOS install, and install everything, (php-fpm, nginx, etc…) set up the virtualhosts, install wordpress & joomla and all that for the sites that need it, etc.

Until today, I’ve been using a virtualbox on my local computer to test on, and it works great. I haven’t bothered with vagrant, as I tried it for a couple of days, and it crashed my whole computer twice, so I gave up. With virtualbox, it’s almost as simple. I have a virtualmachine which I can spin up, install stuff on, and then when I want to go back to a fresh machine, it’s a matter of turning it off, and clicking ‘restore snapshot’ to the snapshot I made when it was clean installed.

It’s practically instant, and just works.

However, running a virtualmachine on my primary work computer all the time does make everything else somewhat sluggish. So I’ve scrouged an old computer that wasn’t doing anything, and am now using that instead.

In order to get snapshots and restore points going well, here’s how I did it:

Install CentOS, leaving a bunch of free space on the LVM primary group.
Make a snapshot when it’s first installed
Restore (merge to) that snapshot whenever I want it back to original settings.
Reboot

To make & restore the snapshots, I’ve written the commands as scripts so I don’t have to remember the lg-whatever stuff. (vg_localtest is the name of the volume group I set up for the HD when I installed):

/usr/sbin/snapshot_make:

#!/bin/sh
lvcreate –size 100G -s -n original_snapshot /dev/vg_localtest/root

/usr/sbin/snapshot_restore

#/bin/sh
lvconvert –merge /dev/vg_localtest/original_snapshot && reboot

It works great so far. One improvement I’m making, since I one time forgot to make a snapshot, and so couldn’t restore to a blank slate without re-installing the whole thing (which, admittedly, only takes half an hour or so):

I’m adding ‘snapshot_make’ into a boot script, and then modifying it to remove itself from the bootscript once it’s made the snapshot. That way as soon as the machine reboots into it’s original snapshot, it will automatically re-create the snapshot.

This looks like:

/usr/sbin/snapshot_make:

#!/bin/sh
lvcreate –size 100G -s -n original_snapshot /dev/vg_localtest/root
sed -ine ‘/snapshot/d’ /etc/rc.local

and then /etc/rc.local will look like:

#… whatever it has
/usr/sbin/snapshot_make

Ansible on Madprof's workshop

LVM snapshots for a resetable machine