What exactly is 'private-cow.img' in appvms?

[Stickstoff]

Hello everyone,

there is documentation about 'root-cow.img' online [1], but nothing
about 'private-cow.img'.
Am I right to assume that the 'private.img' is the writable part the VM
sees, with the changes the VM wrote saved on 'private-cow.img', which is
rotated with 'private-img.old.img' after each VM restart for rollback
possibility? This is how I understand 'root-cow.img' at least.

If this is correct, and I backup only 'private.img' of a running VM, I
would miss on all changes the VM did since its current start?
Would 'private.img' be enough when the VM is shut down? It does sound
like 'root.img' need one of the two .old as "binary patches" in any case.

And just to be sure: 'root-cow.img' and 'volatile.img' shouldn't contain
any backup-worthy data of a VM, right?


Thank you,

Stickstoff

[1] https://www.qubes-os.org/doc/template-implementation/

[Rusty Bird]

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Stickstoff:

> there is documentation about 'root-cow.img' online [1], but nothing
> about 'private-cow.img'.
> Am I right to assume that the 'private.img' is the writable part the VM

> sees, with the changes the VM wrote saved on 'private-cow.img' [...]

It's kind of the other way around - foo.img stores the most current
live data for volume foo, and foo-cow.img stores differing old data
blocks that allow the corresponding device-mapper snapshot* device to
present a virtual view of the contents of volume foo from the time it
was snapshotted, i.e. before the live data started to diverge.

> If [..] I backup only 'private.img' of a running VM

This would result in inconsistent/damaged data.

Rusty


* https://www.kernel.org/doc/Documentation/device-mapper/snapshot.txt
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[Rusty Bird]

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Rusty Bird:

> Stickstoff:
> > there is documentation about 'root-cow.img' online [1], but nothing
> > about 'private-cow.img'.
> > Am I right to assume that the 'private.img' is the writable part the VM
> > sees, with the changes the VM wrote saved on 'private-cow.img' [...]
>
> It's kind of the other way around - foo.img stores the most current
> live data for volume foo, and foo-cow.img stores differing old data
> blocks that allow the corresponding device-mapper snapshot* device to
> present a virtual view of the contents of volume foo from the time it
> was snapshotted, i.e. before the live data started to diverge.
>
> > If [..] I backup only 'private.img' of a running VM
>
> This would result in inconsistent/damaged data.

To be clear - what I meant by inconsistent is that that when the VM is
running, some data blocks in private.img will change while your manual
backup operation is copying that file.

Rusty


> * https://www.kernel.org/doc/Documentation/device-mapper/snapshot.txt
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