Libvirt Snapshot Question

[Elias Mobery]

Hey man, I tried both suggestions, thanks again.

Option 1.) Create/attach a second qcow2 disk & unplug when wiping.
This did not work unfortunately. After wiping the overlay filesystem on the
host (where the snapshots are sitting), I cannot create another snapshot, a
similar error message like before appears.

Option 2.) Enabling Discard

Now I think this worked, but I'm not sure. I read all the docs, and enabled
discard like so on the VM domain:

<driver name='qemu' type='qcow2' discard='unmap'>

I tried both discard=unmap and discard=on (same thing)

Then I enabled fstrim in the guest like this:
(Not entirely sure if I am missing smth)

sudo fstrim -av

Or permanently

sudo systemctl enable --now fstrim.timer


On Tue, Nov 30, 2021, 4:08 PM Elias Mobery <eliasmobery at gmail.com> wrote:

> Hey Peter, thanks for the super quick reply, you're awesome!
>
> So you mean the first option would be to create a second disk instead of
> using snapshot for write, when it gets too big I can then unplug the disk,
> delete it (or wipe entire overlay fs) and recreate it to start fresh?
>
> OK I will try that, thanks for the tip!
>
> Option 2 with trim/discard I have to do some research, this seems a bit
> more challenging.
>
> Thank you SO much for the great advice. I will try everything out and
> report back. :-)
>
>
> On Tue, Nov 30, 2021, 3:25 PM Peter Krempa <pkrempa at redhat.com> wrote:
>
>> On Tue, Nov 30, 2021 at 14:51:54 +0100, Elias Mobery wrote:
>> > Hello Peter, thank you so much for that detailed info!
>> >
>> > Sorry, you were right, when trying to delete my external snapshot via
>> > snapshot-delete, the error says "deletion of external snapshots
>> unsupported"
>> >
>> > I can't merge the snapshot because the VM image is in a read-only
>> > filesystem. Sorry I should've said, it's a live system. So the image is
>> in
>> > the read-only squashfs and external snapshot in overlay is used for
>> writing.
>>
>> Okay, that changes the situation quite a bit:
>>
>> >
>> > Now I would like the snapshot emptied or deleted/recreated when it
>> reaches
>> > 4GB.
>> >
>> > Is there even a way to do this with the image being read-only?
>>
>> If the base image is on a read-only filesystem you obviously can't
>> commit to it.
>>
>> Now the question is what should happen to the data in the overlay.
>>
>> Discarding/recreating the overlay image is possible only once you turn
>> off the VM because it basically rolls back the state of the disk back to
>> the time when the overlay was created. This means that everything
>> written to the disk will be lost.
>>
>> Filesystems obviously can't handle that so that's why it simply won't be
>> possible to do live with the root image.
>>
>> You can have a second disk, which you hot-unplug, wipe the overlay and
>> plug it back.
>>
>> Another possibility is to enable trim/discard and just simply delete the
>> data in the VM which was added after the overlay was created. When
>> trim/discard is enabled on all layers incluging the guest filesystem,
>> then deleting stuff inside the VM will also mean that the overlay will
>> shrink again.
>>
>>
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