I'm playing around with using Qemu external snapshots for KVM (because they are so fast!) on Ubuntu 16.04 and have the following setup:
- All files for the VM are within a single directory (named after the guest).
- There is a
head.qcow2symlink that always points to the top "head" qcow2 image. This way I don't have to keep updating the VM config. (virsh edit).
- The original/bottom image is called
I use the following script to take a snapshot (create a new qcow2 image and make it the head). The script is only executed when the guest is not running.
#!/bin/bash UNIX_TIMESTAMP=$(date +%s) CURRENT_BACKING_FILE=`readlink head.qcow2` NEW_HEAD_FILE="`pwd`/`echo $UNIX_TIMESTAMP`.qcow2" # Create the new "head" disk image where all future changes are made. sudo qemu-img create \ -f qcow2 \ -b $CURRENT_BACKING_FILE \ $NEW_HEAD_FILE # Update the symlink rm head.qcow2 ln -s $NEW_HEAD_FILE head.qcow2
This will work fine when head.qcow2 points to base.qcow2, or I run the script once so there is only a single backing file. However, If there is a chain of backing files (e.g. a backing file also has a backing file), then I will get the following error:
error: Failed to start domain template-ubuntu-docker2 error: internal error: early end of file from monitor, possible problem: 2018-04-05T19:04:55.597373Z qemu-system-x86_64: -drive file=/media/ssd_storage2/kvm/vms/templates/template-ubuntu-docker2/head.qcow2,format=qcow2,if=none,id=drive-virtio-disk0: Could not open backing file: Could not open backing file: Could not open '/media/ssd_storage2/kvm/vms/templates/template-ubuntu-docker2/base.qcow2': Permission denied
The output of
qemu-img --backing-chain head.qcow2 is:
image: head.qcow2 file format: qcow2 virtual size: 20G (21474836480 bytes) disk size: 196K cluster_size: 65536 backing file: /media/ssd_storage2/kvm/vms/templates/template-ubuntu-docker2/1522954330.qcow2 Format specific information: compat: 1.1 lazy refcounts: false refcount bits: 16 corrupt: false image: /media/ssd_storage2/kvm/vms/templates/template-ubuntu-docker2/1522954330.qcow2 file format: qcow2 virtual size: 20G (21474836480 bytes) disk size: 4.1M cluster_size: 65536 backing file: base.qcow2 (actual path: /media/ssd_storage2/kvm/vms/templates/template-ubuntu-docker2/base.qcow2) Format specific information: compat: 1.1 lazy refcounts: false refcount bits: 16 corrupt: false image: /media/ssd_storage2/kvm/vms/templates/template-ubuntu-docker2/base.qcow2 file format: qcow2 virtual size: 20G (21474836480 bytes) disk size: 8.8G cluster_size: 65536 Format specific information: compat: 1.1 lazy refcounts: true refcount bits: 16 corrupt: false
I've played around with deleting images or using qemu-img commit to merge them, and as soon as I go down to just one or two images the guest will start.
Since it looked like a permission error with "could not open" I tried setting permissions to 777 and I still get the same error message, so it shouldn't be a permissions issue.
What do I need to do?