Re: [Qemu-devel] New option for snapshot_blkdev to avoid image creation

[Federico Simoncelli]

----- Original Message -----
> From: "Stefan Hajnoczi" <address@hidden>
> To: "Federico Simoncelli" <address@hidden>
> Cc: address@hidden, address@hidden, address@hidden
> Sent: Tuesday, October 4, 2011 9:33:48 AM
> Subject: Re: [Qemu-devel] New option for snapshot_blkdev to avoid image 
> creation
> 
> On Mon, Oct 03, 2011 at 04:09:01PM +0000, Federico Simoncelli wrote:
> > In some situations might be useful to let qemu use an image that
> > was
> > prepared for a live snapshot.
> > The advantage is that creating the snapshot file outside of the
> > qemu
> > process we can use the whole range of options provided by the
> > format
> > (eg for qcow2: encryption, cluster_size and preallocation).
> > It is also possible to pre-set a relative path to the backing file
> > (now it is created by default as absolute path).
> > In the long run it can also avoid the danger of reimplementing
> > qemu-img
> > inside qemu (if we wanted to expose such options when a snapshot is
> > requested).
> 
> When the image file is created based on the backing file size:
> 
> $ qemu-img create -f qcow2 -o backing_file=master.img vm001.qcow2
> 
> It turns out that bdrv_img_create() opens the backing file with
> read/write permissions.  This is generally a bad idea but especially
> dangerous when the VM currently has the image file open already since
> image formats are not designed for multiple initiators (clustering).

Hi Stefan, are you sure that bdrv_img_create opens the backing file
with read/write permissions?
After a quick check I can't see that happening:

$ ./qemu-img create -f qcow2 /tmp/master.img 1G
Formatting '/tmp/master.img', fmt=qcow2 size=1073741824 encryption=off 
cluster_size=65536

$ strace -e trace=open ./qemu-img create -f qcow2 -o 
backing_file=/tmp/master.img /tmp/vm001.qcow2
[...]
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = -1 ENOENT (No such file or 
directory)
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = -1 ENOENT (No such file or 
directory)
open("/tmp/master.img", O_RDONLY|O_NONBLOCK) = 3
open("/tmp/master.img", O_RDONLY|O_NONBLOCK) = 3
open("/tmp/master.img", O_RDONLY|O_DSYNC|O_CLOEXEC) = 3
open("/tmp/master.img", O_RDONLY|O_NONBLOCK) = 3
open("/tmp/master.img", O_RDONLY|O_NONBLOCK) = 3
open("/tmp/master.img", O_RDONLY|O_CLOEXEC) = 3
Formatting '/tmp/vm001.qcow2', fmt=qcow2 size=1073741824 
backing_file='/tmp/master.img' encryption=off cluster_size=65536 
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = -1 ENOENT (No such file or 
directory)
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = -1 ENOENT (No such file or 
directory)
open("/tmp/vm001.qcow2", O_WRONLY|O_CREAT|O_TRUNC, 0644) = 6
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = 6
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = 6
open("/tmp/vm001.qcow2", O_RDWR|O_DSYNC|O_CLOEXEC) = 6
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = 6
open("/tmp/vm001.qcow2", O_RDONLY|O_NONBLOCK) = 6
open("/tmp/vm001.qcow2", O_RDWR|O_CLOEXEC) = 6

-- 
Federico