VMA

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Revision as of 10:59, 26 February 2015 by M ardito (talk | contribs)
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PVE has a new format for vm backups (since 2.3): VMA

Since the 2.3 release PVE has a new format for its very powerful vm backup feature: .vma, replacing the old one, the common .tar format.

As with the old .tar, also .vma files can be compressed in .lzo or .gz format.

You can read more about Backup and Restore in http://pve.proxmox.com/wiki/Backup_and_Restore

What you will find below are informations about the new .vma format.

The reasons for the switch from tar

PVE nowadays supports various storage models (some of them still in experimental flavor): lvm, sheepdog, ceph, local, nfs, iscsi, and can allow for usage of raw, qcow, vmdk, images and so on.

see, amongst the other wiki pages:

The main reason for developing a brand new format for vm backup, was to being able to use just one format, and efficiently, in particular when it comes to snapshots usage, which can also be done in a number of ways depending on the storage model, which showed drawbacks with the plain old standard .tar format, and .vma try to address and efficiently allow one single backup behavior.

There is a very detailed explanation of those reasons available here. Here is a short summary:

  • Most VM backup solutions use some kind of snapshot to get a consistent VM view at a specific point in time (eg: LVM snapshots, qcow2 snapshot, qemu livebackup), but they can involve considerable overhead, in different ways.
  • Some storage types/formats supports internal snapshots using some kind of reference counting (rados, sheepdog, dm-thin, qcow2). It would be possible to use that for backups, but for now we want to be storage-independent.
  • There is the need of a way to be more efficient, and avoid any unnecessary step, but to make that work, the backup archive need to be able to store image data 'out of order', although this will not work with traditional archive formats like tar.
  • The new method/format allows for very good performance, works on any storage type and image format, doe not need temporary storage, it is simple archive format, which is able to store sparse files efficiently.

So it's easy to understand that the aim is to get a good, simple, efficient and consistent backup behavior, more specifically suited to the vm backup, unlike the old traditional file/folder tools!

The VMA format specification

The format details can be found here.

Command line utility

~# vma
usage: vma command [command options]

vma list <filename>
vma create <filename> [-c config] <archive> pathname ...
vma extract <filename> [-v] [-r <fifo>] <targetdir>
vma verify <filename> [-v]

Things to be aware of

Using well-known archive formats like the old tar.gz allowed (mainly) linux users to take advantage of a number of tools already available, such as rdiff to do off-site incremental backups, but now everyone should be aware that the new vma backup file is unique with no similarity to the vma from the old backup, so always the full file needs to be moved to the remote location. This means that rdiff will not be able anymore to easily spot "diffs" between two similar vma files, and will therefore produce a very big (not really) "incremental" file. See this post for more info and an example: http://forum.proxmox.com/threads/13475-Proxmox-2-3-new-backup-methode-vma-not-rdiff-friendly

It is possible to extract VMA outside of proxmox, too even if it could be cumbersome and overkill in some way: see (all) this thread: http://forum.proxmox.com/threads/14226-VMA-archive-restore-outside-of-Proxmox