mirror of
git://git.proxmox.com/git/pve-qemu.git
synced 2024-12-23 21:35:23 +03:00
117 lines
4.0 KiB
Plaintext
117 lines
4.0 KiB
Plaintext
Efficient VM backup for qemu
|
|
|
|
=Requirements=
|
|
|
|
* Backup to a single archive file
|
|
* Backup needs to contain all data to restore VM (full backup)
|
|
* Do not depend on storage type or image format
|
|
* Avoid use of temporary storage
|
|
* store sparse images efficiently
|
|
|
|
=Introduction=
|
|
|
|
Most VM backup solutions use some kind of snapshot to get a consistent
|
|
VM view at a specific point in time. For example, we previously used
|
|
LVM to create a snapshot of all used VM images, which are then copied
|
|
into a tar file.
|
|
|
|
That basically means that any data written during backup involve
|
|
considerable overhead. For LVM we get the following steps:
|
|
|
|
1.) read original data (VM write)
|
|
2.) write original data into snapshot (VM write)
|
|
3.) write new data (VM write)
|
|
4.) read data from snapshot (backup)
|
|
5.) write data from snapshot into tar file (backup)
|
|
|
|
Another approach to backup VM images is to create a new qcow2 image
|
|
which use the old image as base. During backup, writes are redirected
|
|
to the new image, so the old image represents a 'snapshot'. After
|
|
backup, data need to be copied back from new image into the old
|
|
one (commit). So a simple write during backup triggers the following
|
|
steps:
|
|
|
|
1.) write new data to new image (VM write)
|
|
2.) read data from old image (backup)
|
|
3.) write data from old image into tar file (backup)
|
|
|
|
4.) read data from new image (commit)
|
|
5.) write data to old image (commit)
|
|
|
|
This is in fact the same overhead as before. Other tools like qemu
|
|
livebackup produces similar overhead (2 reads, 3 writes).
|
|
|
|
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.
|
|
|
|
=Make it more efficient=
|
|
|
|
The be more efficient, we simply need to avoid unnecessary steps. The
|
|
following steps are always required:
|
|
|
|
1.) read old data before it gets overwritten
|
|
2.) write that data into the backup archive
|
|
3.) write new data (VM write)
|
|
|
|
As you can see, this involves only one read, and two writes.
|
|
|
|
To make that work, our backup archive need to be able to store image
|
|
data 'out of order'. It is important to notice that this will not work
|
|
with traditional archive formats like tar.
|
|
|
|
During backup we simply intercept writes, then read existing data and
|
|
store that directly into the archive. After that we can continue the
|
|
write.
|
|
|
|
==Advantages==
|
|
|
|
* very good performance (1 read, 2 writes)
|
|
* works on any storage type and image format.
|
|
* avoid usage of temporary storage
|
|
* we can define a new and simple archive format, which is able to
|
|
store sparse files efficiently.
|
|
|
|
Note: Storing sparse files is a mess with existing archive
|
|
formats. For example, tar requires information about holes at the
|
|
beginning of the archive.
|
|
|
|
==Disadvantages==
|
|
|
|
* we need to define a new archive format
|
|
|
|
Note: Most existing archive formats are optimized to store small files
|
|
including file attributes. We simply do not need that for VM archives.
|
|
|
|
* archive contains data 'out of order'
|
|
|
|
If you want to access image data in sequential order, you need to
|
|
re-order archive data. It would be possible to to that on the fly,
|
|
using temporary files.
|
|
|
|
Fortunately, a normal restore/extract works perfectly with 'out of
|
|
order' data, because the target files are seekable.
|
|
|
|
* slow backup storage can slow down VM during backup
|
|
|
|
It is important to note that we only do sequential writes to the
|
|
backup storage. Furthermore one can compress the backup stream. IMHO,
|
|
it is better to slow down the VM a bit. All other solutions creates
|
|
large amounts of temporary data during backup.
|
|
|
|
=Archive format requirements=
|
|
|
|
The basic requirement for such new format is that we can store image
|
|
date 'out of order'. It is also very likely that we have less than 256
|
|
drives/images per VM, and we want to be able to store VM configuration
|
|
files.
|
|
|
|
We have defined a very simply format with those properties, see:
|
|
|
|
https://git.proxmox.com/?p=pve-qemu.git;a=blob;f=vma_spec.txt;
|
|
|
|
Please let us know if you know an existing format which provides the
|
|
same functionality.
|
|
|
|
|