Programatically run a QEMU VM instance from snapshot

Question

I have a QEMU image with a snapshot stored in it. Right now I'm using libvirt to start it.

However, I want to be able to run more than one instance out of the same image/snapshot.

I guess I can do that by cloning the virtual-hd and installing/creating a new domain (virsh) and then running revert from snapshot. But I want to be able to do that pretty much "on-the-fly" with as little as possible latency from the time I decide I need to run another instance of image X to the time that instance is running from the stored snapshot.

Anyone did anything like that? I started thinking maybe libvirt is not low-level enough for this ?

[EDIT: Sorry if this wasn't clear - I'm talking about a RAM+HD snapshot, not just HD snapshot, which I already know how to create...]

Thanks

This is both a programming and a system question, so I hope this is the right place. — Ofir Israel
– Ofir Israel, Commented Jun 1, 2014 at 16:18
Have you ever managed to get this to work with libvirt and not by using the -snapshot directly on qemu? — Grim
– Grim, Commented Jun 26, 2016 at 16:10

6EQUJ5 · Accepted Answer · 2014-06-05 01:23:07Z

3

+25

I was able to run multiple concurrent qemu from the same snapshot using a command like the following. (Obviously the -arm, -kernel, -cpu etc. arguments will be different in your case)

qemu-system-arm -hda snapshot.qcow2 -snapshot -kernel some_vmlinux \
                -serial stdio -append 'root=/dev/sda2 rootfstype=ext4 rw'\
                -cpu arm1176 -m 256 -M versatilepb

The important argument here is -snapshot, so that temporary memory is used for disk writes.

What I havent tried is forcing a writeback to the underlying device, I suspect all manner of havoc could occur if this is possible and it happened... basically, don't forget the -snapshot argument!

If you want some writable storage that is different per instance you probably need to add a second virtual hard disk and have the common snapshot mount that somehow:

qemu-system-arm -hda snapshot.qcow2 -snapshot -kernel some_vmlinux \
                -serial stdio -append 'root=/dev/sda2 rootfstype=ext4 rw'\
                -cpu arm1176 -m 256 -M versatilepb -hdb drive_system1.img &
qemu-system-arm -hda snapshot.qcow2 -snapshot -kernel some_vmlinux \
                -serial stdio -append 'root=/dev/sda2 rootfstype=ext4 rw'\
                -cpu arm1176 -m 256 -M versatilepb -hdb drive_system2.img &

If you are using networking, dont forget to make the MAC address and any hostfw port values different.

answered Jun 5, 2014 at 1:23

6EQUJ5

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3 Comments

Ofir Israel Over a year ago

Thanks, but it's not exactly what I wanted. I already have a snapshot stored in the qcow2 image (can be watched using qemu-img info <image_file>. I want to be able to revert from THAT snapshot (also includes RAM state)

6EQUJ5 Over a year ago

the solution above is running from a previously created snapshot... admittedly, it doesnt include RAM state, perhaps you could add that requirement to the question?

6EQUJ5 Over a year ago

No worries. Havent tried that with qemu, I usually use virtual box for vms that need memory saved so I'll be watching this with interest....

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