Re: [Qemu-devel] qcow2: Can create qcow2 image format on rbd server

[Jun Li <junmuzi@gmail.com>]

On Mon, 12/08 15:49, Kevin Wolf wrote:
> Am 08.12.2014 um 14:50 hat Jun Li geschrieben:
> > On Mon, 12/08 11:07, Kevin Wolf wrote:
> > > Am 05.12.2014 um 16:32 hat Jun Li geschrieben:
> > > > Currently, qemu-img can not create qcow2 image format on rbd server. Analysis
> > > > the code as followings:
> > > > when create qcow2 format image:
> > > > qcow2_create2
> > > >   bdrv_create_file(filename, opts, &local_err);  --> Here will create a 0 size
> > > >   file(e.g: file1) on rbd server.
> > > >   ...
> > > >   ret = bdrv_pwrite(bs, 0, header, cluster_size); --> So here can not write
> > > >   qcow2 header into file1 due to the file1's length is 0. Seems
> > > >   qemu_rbd_aio_writev can not write beyond EOF.
> > > >   ...
> > > > 
> > > > As above analysis, there are two methods to solve the above bz as followings:
> > > > 1, When create file1, just create a fixed-size file1 on rbd server(not 0 size).
> > > 
> > > This is not a solution. Even if you might be able to create an image
> > > successfully, using qcow2 without a backend that allows the image file
> > > to grow is bound to fail sooner or later.
> > > 
> > > So yes, you'll want to extend the rbd block driver to grow the file
> > > asynchronously when writing beyond EOF.
> > > 
> > 
> > Hi Kevin,
> > 
> >   Although rbd_resize is synchronous, I just want to create a new child-thread
> > to realize asynchronous(Maybe just like userspace aio: libaio) rbd_resize.
> > Just like following:
> > 
> > qemu_rbd_aio_writev()
> > {
> >     if (BlockDriverState->file->growable == 1) {
> >         pthread_create(&thread_id, NULL, child_thread, NULL);
> >     }
> > 
> >     rbd_start_aio();
> > }
> > 
> > child_thread()
> > {
> >     ...
> >     rbd_resize();
> >     rbd_start_aio();
> >     ...
> > }
> > 
> > Currently, seems do not have original asynchronous rbd_resize. Besides, rbd block
> > driver do not support growable file. So I want to use above method to realize
> > asynchronous rbd_resize() in our qemu level.
> > 
> > What's your opinion?
> 
> Are you sure that the rbd libraries are thread-safe and can be used in
> this way?
> 

I have tested this method, seems it's not thread-safe. Gdb debug info just as
followings:
# gdb --args /opt/qemu-git-arm/bin/qemu-img create -f qcow2
  rbd:qemu-kvm-pool/juli-test44.qcow2:mon_host=$IP 10M
(gdb) bt
#0  librbd::resize (ictx=0x48fffe948fe80574, size=120095921237475840,
				prog_ctx=...) at librbd/internal.cc:1492
#1  0x00007ffff7478bb2 in rbd_resize (image=<optimized out>, size=<optimized
				out>) at librbd/librbd.cc:726
#2  0x00005555555cfa9c in qemu_rbd_truncate (bs=0x7ffff7f9ca00,
				offset=120095921237475840) at block/rbd.c:798
#3  0x00005555555cf797 in thread_info (t_parameters=0x7ffff7f9c9d0) at
block/rbd.c:706
#4  0x00007ffff1c5bee5 in start_thread (arg=0x7fffd5ffb700) at
pthread_create.c:309
#5  0x00007ffff198ab8d in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:111


Regards,
Jun Li