Initial configuration. "Current image" pointer is an abstraction for the image currently used by qemu to run a given domain.
Current image
|
v
+-----------------+
| disk.qcow2 |
+-----------------+
Run fi-backup.
Current image
|
v
+------------------+ +-------------------------+
| disk.qcow2 |<--| disk.qcow2-<timestamp1> |
+------------------+ +-------------------------+
After the first run, disk.qcow2 is the backing file of disk.qcow2-<timestamp1>. That means that disk.qcow2 is used in read-only mode by qemu. Thus, it can be safely copied to perform our backup.
Current image
|
v
+------------------+ +-------------------------+ +-------------------------+
| disk.qcow2 |<--| disk.qcow2-<timestamp1> |<--| disk.qcow2-<timestamp2> |
+------------------+ +-------------------------+ +-------------------------+
After the second run, disk.qcow2 has not changed, so we can just copy disk.qcow2-<timestamp1>: this is the "key" of forward incremental backup.
Consolidation copies the content of backing files into the current image, allowing us to remove them. This reduces the number of image files, reduces the size of disk images and improves domain performances.
Current image
|
v
+------------------+ +-------------------------+ +-------------------------+
| disk.qcow2 |<--| disk.qcow2-<timestamp1> |<--| disk.qcow2-<timestamp2> |
+------------------+ +-------------------------+ +-------------------------+
Running in consolidation mode will "copy" backing images content back into the current image.
Current image
+-----bpull----+ |
| v v
+---------------+ +-------------------------+ +-------------------------+
| disk.qcow2 |<--| disk.qcow2-<timestamp1> | X | disk.qcow2-<timestamp2> |
+---------------+ +-------------------------+ +-------------------------+
After that, old backing files can be deleted.
Current image
|
v
+-------------------------+
| disk.qcow2-<timestamp2> |
+-------------------------+
Restore procedure for a domain A, with images in directory B, backup directory C is the following. Thanks to svorobyov for these steps.
- clean up the directory B from all images for domain A;
- copy the desired chains of the backing images for domain A from C to B (although it is not recommended, chains can be of different lengths, e.g., recover disk 1 to the state two days back and disk 2 to the state three days);
- specify (using
virsh,virt-manager,virt-install, ...) that the disks for A are the last images of the chains in 2 (this is important, otherwise the VM state description may be inconsistent) - optionally, consolidate for domain A (which will use chains of images in B)
Thanks a lot, and a good description, I think.
A few corrections:
When consolidation finishes, all intermediate backing files are deleted automatically, and only two images remain: disk.qcow2 and disk. (a warning is printed).
After that disk.qcow2 may be deleted, and the current state becomes:
... your last drawing ...