Category Archives: openvz

How to partition an openvz host to take snapshots

/dev/sda1 - /boot - 250 MB (because kernel cannot be on LVM)
/dev/sda2 - LVM - rest of space
volume group=vps
root - / - 102400 MB (100 GB) (logical volume name "root")
swap - N/A - 4096 MB (4 GB) (logical volume name "swap")
/vz - /vs - (logical volume name "vz")
(here you take remaining space and subtract it from amount of space you want for snapshots)

Now you have 3 LVM’s in /dev/vps/*, and you have remaining space for snapshots. You should keep at least 10GB free for snapshots depending how much your data changes. If alot changes you would be better off with 100GB of free space. In my examples for snapshots I am going to assume I left 10GB free of space for snapshots.

History on snapshots:
#snapshot–snapshots only require enough space for how much data you think will be changed(copy on write)
# –longer a snapshot left sitting there, more snapshot may grow with changes to take out
# –ie: you create a new 2G file after snapshot is taken, snapshot grows 2G so it can take that file out after
Lets begin:
Lets say I want to take a snapshot of /vz(where all my openvz hosts reside)
I know I have 10GB left of free space, a way to check( do a “pvdisplay” will show you the “PV Size”, then do a “lvdisplay” which shows you all LV’s and their sizes, add up all the “LV Size” of each, then use that to subtract from “PV Size” for remaining space for snapshots. WARNING: never attempt to shrink a LV to get extra space for snapshots, backup the LV , delete it, recreate it, and copy data back)
# lvcreate --snapshot --name snap --size 10GB /dev/vps/vz
(in this example I just created a file /dev/vps/snap, that will be good for 10GB worth of changes to the /dev/vps/vz logical volume.)
(Now let’s mount it for example if we wanted to back it up)
# mkdir /mnt; mount /dev/vps/snap /mnt; ls -al /mnt
(now we have /mnt available for a backup with ie: rsync)
(to unmount and remove snapshot to get our 10GB of free space back for another snapshot:)
# umount /mnt; lvremove /dev/vps/snap
(simple as that, now you are an expert at snapshots)

Now lets get more advanced, lets say I want to take that 10GB of space and take snapshots of /dev/vps/vz daily and once a month, as well as take a monthly snapshot of host itself: /dev/vps/root, so that we have ability to get back anything we accidently deleted anywhere.

Lets setup a cronjob:
#crontab -e
(lets add the following:)
#10 Gigabytes available for snapshots, 7 days in a week=7GB+2GB monthly snapshot+1GB host snapshot=10GB
#daily snapshots
0 0 * * * (lvremove -f /dev/vps/snap-`/bin/date +\%A`; lvcreate --snapshot --name snap-`/bin/date +\%A` --size 1GB /dev/vps/vz) > /dev/null 2>&1
#monthly snapshot
0 0 1 * * (lvremove -f /dev/vps/snap-Monthly; lvcreate --snapshot --name snap-Monthly --size 2GB /dev/vps/vz) > /dev/null 2>&1
#host snapshot once a month
0 0 1 * * (lvremove -f /dev/vps/snap-host-Monthly; lvcreate --snapshot --name snap-host-Monthly --size 1GB /dev/vps/root) > /dev/null 2>&1

(and there you have it, everyday we will get /dev/vps/snap-Thursday for example day of week snapshots, 1 monthly snapshot, and 1 monthly snapshot of host itself we can mount anytime
to /mnt and do any recovery or backups needed)

My recommendation if you have the space is have a backup LVM the same size of /vz. My preference for backups personally is ZFS filesystem using freebsd, which you can easily
install as a VPS under KVM virtualization, share the disk to the VM then have freebsd do a ZFS raid on them, but thats for another article.

As far as LVM goes, what I personally do is rsync the /vz directory each night to a ZFS filesystem, and take snapshots there, ZFS is much nicer to work with when dealing with snapshots, but where LVM comes in handy is to snapshot the LVM(because we use ext4 on linux) for something like a mysql database, mount it, then rsync it off to a ZFS filesystem.