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 -- ============================================================= Alain Péan - LPP/CNRS Attention !! : Nouveau numéro de Telephone : 01-44-27-92-39 Administrateur Système/Réseau Laboratoire de Physique des Plasmas - UMR 7648 Observatoire de Saint-Maur 4, av de Neptune, Bat. A 94100 Saint-Maur des Fossés =============================================================