[CentOS-virt] LVM-snapshot based KVM VMs
Alain Péan
alain.pean at lpp.polytechnique.fr
Thu Apr 26 03:38:02 EDT 2012
Le 25/04/2012 23:21, Paul Basov a écrit :
>
> Good m/d/n guys, wanted to put idea through you before trying to
> implement this.
>
> So afaik LVM snapshots basically don't take any "real" space on hard
> drives.
>
> I'm using KVM virtualization on my hosts and let's say i create an LVM
> volume for my linux VM, then install it properly, shut it down and
> take LVM snapshot of it.
>
> Basically i've just cloned it (LVM2 snapshots are r/w).
>
> I feed it to KVM and that's it? Can i snapshot this volume N number
> times and get N number of VMs that take zero actual space on actual
> hard drive?
>
> And when cloned VMs start writing, they will be actually writing only
> diffs from "master" VM. This looks like an interesting idea to manage
> a lot of "template" VMs while saving huge amounts of disk space.
>
> Or maybe i'm just inventing the bicycle. What do you think? Is this
> possible?
>
Hi Paul,
Indeed, your feeling is wrong. LVM snapshots do take some space. You
need to reserve some space on your LVM volume group to store the
snpashots (about 5 GB could be a good estimate, it depends on the size
of the snapshot). It appears as Free extent when you display
informations about your volume group. For exemple (in this case it a
Proxmox virtualization server) :
# vgdisplay
....
--- Volume group ---
VG Name pve
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 8512
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 3
Open LV 3
Max PV 0
Cur PV 1
Act PV 1
VG Size 1,36 TiB
PE Size 4,00 MiB
Total PE 357182
Alloc PE / Size 356160 / 1,36 TiB
Free PE / Size 1022 / 3,99 GiB
VG UUID UcKh7L-rSyq-NYd5-Q12V-EP8I-7QbE-RD5z4M
Without Free PE (Free space), you will not be able to create a snapshot.
What the snapshot do is to create a temporary logical volume in this
free space to store the snapshot (at least the differences with the
actual VM volume). Snapshots are mainly used to backup a live VM,
without stopping it. When it is done, you remove the snapshot, hence the
temporary logical volume.
See for example :
http://tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html
Alain
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Alain Péan - LPP/CNRS
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