There is no fix for internal snapshots even in year 2022. The only solution would be to use external snapshots. For more details visit KVM: creating and reverting libvirt external snapshots. All the credits for the external snapshots examples go to Fabien Lee.
External snapshot for simple guest domains
Creating external snapshot
If your guest domain is a simple VM where all the <disk> are of type="file", then creating an external snapshot of a powered-on VM’s state (excluding the RAM) looks like below. For this example, assume there is a hard drive at hda and CD-ROM at hdb.
# name of domain, snapshot, and target disk device
thedomain="mysimpledomain"
snapshotname="simple-test"
targetdisk="hda"
# look at '<disk>' types, should be just 'file' types
virsh dumpxml $thedomain | grep '<disk' -A5
# show block level devices and qcow2 paths (hda,hdb,..etc)
virsh domblklist $thedomain
# create snapshot in default pool location
# file name is $thedomain.$snapshotname
virsh snapshot-create-as $thedomain --name $snapshotname --disk-only
# list snapshot
virsh snapshot-list $thedomain
Reverting external snapshot
To see the mechanism underneath and prepare variables for the revert of the snapshot, execute the commands below.
# notice path to hda has now changed to snapshot file
virsh domblklist $thedomain
# <source> has changed to snapshot file
virsh dumpxml $thedomain | grep '<disk' -A5
# pull default pool path from xml
pooldir=$(virsh pool-dumpxml default | grep -Po "(?<=path\>)[^<]+")
echo "default pool dir: $pooldir"
# should see two files starting with $thedomain
# the one named $thedomain.$snapshotname is the snapshot
cd $pooldir
ls -latr $thedomain*
# snapshot points to backing file, which is original disk
sudo qemu-img info $thedomain.$snapshotname -U --backing-chain
# capture original backing file name so we can revert
backingfile=$(qemu-img info $thedomain.$snapshotname -U | grep -Po 'backing file:\s\K(.*)')
echo "backing file: $backingfile"
To do the revert we need to modify the domain xml back to the original qcow2 file, delete the snapshot metadata, and finally the snapshot file.
# stop VM
virsh destroy $thedomain
# edit hda path back to original qcow2 disk
virt-xml $thedomain --edit target=$targetdisk --disk path=$backingfile --update
# validate that we are now pointing back at original qcow2 disk
virsh domblklist $thedomain
# delete snapshot metadata
virsh snapshot-delete --metadata $thedomain $snapshotname
# delete snapshot qcow2 file
sudo rm $pooldir/$thedomain.$snapshotname
# start guest domain
virsh start $thedomain
The guest domain should now be in the original state. For more details about external snapshots, visit the link above.
Why the internal snapshots have been disabled?
Why the internal snapshots have been disabled you can read below. One of thee best answer I found when searching this topis is this:
On Mon, Sep 04, 2017 at 08:30:23AM -0700, ovirt@xxxxxxxxxxxxxxxxx
wrote:
o/s: Fedora 26 + Virt-Manager v1.42
vm: Win 10 UEFI + Q35
Error creating snapshot: Operation not supported:
Internal snapshots of a VM with pflash based firmware are not supported.
Is there a fix?
Hi, unfortunately no, and it will probably take some time to support
snapshots for guests with UEFI.
The issue is that internal snapshots are obsolete and misses a lot of
features that external snapshots have. They are basically usable only
for guests with only one disk, where everything including the guest
state is stored in one file.
In order to get this fixed and to get snapshots for guests with UEFI
virt-manager will have to switch to use external snapshots, there is
already a BUG for it 3. However, it cannot be currently done
because libvirt still lacks some mandatory features for external
snapshots, for example you cannot revert to external snapshot which
makes them unusable. There is also a BUG for libvirt to implement
this 4.
Pavel
3 https://bugzilla.redhat.com/show_bug.cgi?id=1403951 4
https://bugzilla.redhat.com/show_bug.cgi?id=1402581
The reason for this is here:
Re: [libvirt] [PATCH v2] qemu: snapshot: Forbid internal snapshots
with pflash firmware
From: Laszlo Ersek <lersek redhat com>
To: Peter Krempa <pkrempa redhat com>
Cc: libvir-list redhat com
Subject: Re: [libvirt] [PATCH v2] qemu: snapshot: Forbid internal snapshots with pflash firmware
Date: Thu, 23 Mar 2017 17:49:56 +0100
On 03/23/17 15:07, Peter Krempa wrote:
On Thu, Mar 23, 2017 at 11:03:02 +0100, Laszlo Ersek wrote:
On 03/23/17 10:54, Peter Krempa wrote:
On Thu, Mar 23, 2017 at 10:48:01 +0100, Laszlo Ersek wrote:
On 03/23/17 10:29, Peter Krempa wrote:
If the variable store () file is raw qemu can't do a snapshot of
it and thus the snapshot would be incomplete. QEMU does no reject such
snapshot.
Additionally allowing to use a qcow2 variable store backing file would
solve this issue but then it would become eligible to become target of
the memory dump.
Offline internal snapshot would be incomplete too with either storage
format since libvirt does not handle the pflash file in this case.
Forbid such snapshot so that we can avoid problems.
[...]
@@ -13873,8 +13873,14 @@ qemuDomainSnapshotPrepare(virConnectPtr conn,
goto cleanup;
}
- /* internal snapshots + pflash based loader have the following problems:
-
* - if the variable store is raw, the snapshot is incomplete
-
* - alowing a qcow2 image as the varstore would make it eligible to receive
-
* the vmstate dump, which would make it huge
-
* - offline snapshot would not snapshot the varstore at all
-
*
-
* Avoid the issues by forbidding this completely.
-
*/
I thought about this a bit more and I think that while there are the
above problems we still can have users of snapshots + OVMF which use it
successfully. Forbiding it would create a regression for them since they
did not observe anything bad despite the problems mentioned above:
The reasons are following:
- internal snapshots are the default in virt-manager
- guests usually don't re-write the varstore very often, usually only
at install
- OSes usually don't modify anything besides the boot entry
- snapshot of an online VM carries the varstore in the memory image
- OSes are pretty good at restoring the boot entry if it fails
Due to the facts above I think that there are users that legitimately
think that snapshots with pflash loaders work as expected. It's mostly
due to the fact that the data are pretty static and OSes don't store
anything important there and are able to self-heal some of the problems.
I think we should not disallow this to avoid usability regressions. We
can add documentation that states that it's unsafe to do snapshots.
Additionally we will need to add support for external snapshots, which
currently have similar kind of problems, although fixable.
The tradeoff is between a seemingly working, but inherently unsafe
operation, and a clear error message that keeps things safe.
The UEFI variable store is used for more and different things than you
mention, such as (in roughly decreasing order of importance):
Some UEFI variables (the authenticated ones) have security impact. This covers the standardized ones used for Secure Boot (Platform Key,
Key Exchange Keys, white-listed certificates and signatures (DB) and
black-listed certificates and signatures (DBX)).
To my knowledge, it also includes some similar security-related variables used by shim / MokManager (where "MOK" is short for Machine
Owner Key); that is, non-volatile variables to which shim delegates
the EFI binaries' verification, from the standardized Secure Boot
interfaces.
UEFI variables can serve as the backend for the linux "pstore" (persistent store) file system. Pstore in turn can be used to save the
last part of dmesg on a crash. The messages can be re-read at a new
boot.
Firmware uses (reads/writes) a number of variables internally at each boot. These may not be critical. One example is a variable that
helps reduce UEFI memory map fragmentation over a series of boots.
OVMF manipulates UEFI boot options on each boot, according to the bootindex properties (or more directly, according to the "bootorder"
fw_cfg file). Although, admittedly, this is likely the least risky
category of varstore contents.
While I have myself successfully used -- offline only -- internal
snapshots with OVMF guests, hand-waving away the knowledge that the
varstore was never actually snapshotted, I feel real uncomfortable
about silently performing an inherently lossy operation, especially
when the varstore may well have security impact.
Users will not read the documentation (they never do), and I would
rather not field future bug reports about obscure Secure Boot
misbehavior.
It is ultimately up to libvirt developers, but IMO, if we continue to
allow this unsafe operation, then the minimum would be:
in virt-manager, pop up an extra confirmation dialog, with clear indication that the operation will be lossy and could have security
impact,
in virsh, reject the operation with a similar error message, unless "--force" or something similar were specified.
And, because there are other (independent) libvirt client applications, this would likely require a new flag on the libvirt API
level, so that libvirt itself can reject unsafe snapshotting requests,
regardless of the client application that submits it.
I agree that usability regressions are not nice and will likely
generate bug reports; however, if they are in direct conflict with
security improvements, then security improvements trump usability
regressions. I guess we can allow users to ignore security, but they
need to be informed on the spot, and they have to opt in.
I would prefer if we went ahead with this patch; but, again, it's up
to you in the end.
Thanks! Laszlo
