On 09/13/2012 01:47 AM, Eric Blake wrote:
On 09/12/2012 09:22 AM, Guannan Ren wrote:
After failure of qemu transaction command, the snapshot file still be there with non-zero in size. In order to unlink the file, the patch removes the file size checking. Can you give some exact steps to reproduce this, so that I know who is making the file have non-zero size? I'm worried that unlinking a non-empty file is discarding data, so the commit message needs a lot more details about how we are proving that the only way the file can be non-zero size is because qemu happened to put data into a previously empty file prior to the failed 'transaction' attempt.
qemu left non-empty file. Steps: 1, Create a qemu instance with two drive images of qcow2 type (root user) /usr/libexec/qemu-kvm -m 1024 -smp 1 -name "rhel6u1" \ -drive file=/var/lib/libvirt/images/firstqcow2,if=none,id=drive-virtio-disk0,format=qcow2,cache=none -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 \ -drive file=/var/lib/libvirt/images/secondqcow2,if=none,id=drive-virtio-disk1,format=qcow2,cache=none \ -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=drive-virtio-disk1,id=virtio-disk1 -qmp stdio 2, Initialize qemu qmp {"execute":"qmp_capabilities"} 3, Remove the second drive image file rm -f /var/lib/libvirt/images/secondqcow2 4, Run 'transaction' command with snapshot qemu commands in. {"execute":"transaction","arguments": {"actions": [{"type":"blockdev-snapshot-sync","data":{"device":"drive-virtio-disk0","snapshot-file":"/var/lib/libvirt/images/firstqcow2-snapshot.img","format":"qcow2"}}, {"type":"blockdev-snapshot-sync","data":{"device":"drive-virtio-disk1","snapshot-file":"/var/lib/libvirt/images/secondqcow2-snapshot.img","format":"qcow2"}}] }, "id":"libvirt-6"} 5, Got the error as follows: {"id": "libvirt-6", "error": {"class": "OpenFileFailed", "desc": "Could not open '/var/lib/libvirt/images/secondqcow2-snapshot.img'", "data": {"filename": "/var/lib/libvirt/images/secondqcow2-snapshot.img"}}} 6, List first newly-created snapshot file: -rw-r--r--. 1 root root 262144 Sep 13 11:43 firstqcow2-snapshot.img
That is, after re-reading context code just now, I'm fairly confident that this code can only be reached when qemu supports the 'transaction' monitor command, and libvirt's --reuse-ext flag was not specified, and therefore libvirt must have just created the file. But I want that in the commit message rather than having to re-read the code every time I visit this commit in future reads of the git log.
That's ok, I will add this information into commit log in v2.
It may also be that qemu has a bug in that the 'transaction' command is modifying files even when it fails, so even while this works around the bug, I'm cc'ing Jeff to see if qemu also needs a bug fix.