Chapter 5. Librbd (Python)

Creating and writing to an image

1. Connect to RADOS and open an IO context:

   cluster = rados.Rados(conffile='my_ceph.conf')
   cluster.connect()
   ioctx = cluster.open_ioctx('mypool')

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2. Instantiate an :class:rbd.RBD object, which you use to create the image:

   rbd_inst = rbd.RBD()
   size = 4 * 1024**3  # 4 GiB
   rbd_inst.create(ioctx, 'myimage', size)

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3. To perform I/O on the image, instantiate an :class:rbd.Image object:

   image = rbd.Image(ioctx, 'myimage')
   data = 'foo' * 200
   image.write(data, 0)

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   This writes 'foo' to the first 600 bytes of the image. Note that data cannot be :type:unicode - librbd does not know how to deal with characters wider than a :c:type:char.

4. Close the image, the IO context and the connection to RADOS:

   image.close()
   ioctx.close()
   cluster.shutdown()

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   To be safe, each of these calls must to be in a separate :finally block:

   cluster = rados.Rados(conffile='my_ceph_conf')
   try:
       ioctx = cluster.open_ioctx('my_pool')
       try:
           rbd_inst = rbd.RBD()
           size = 4 * 1024**3  # 4 GiB
           rbd_inst.create(ioctx, 'myimage', size)
           image = rbd.Image(ioctx, 'myimage')
           try:
               data = 'foo' * 200
               image.write(data, 0)
           finally:
               image.close()
       finally:
           ioctx.close()
   finally:
       cluster.shutdown()

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   This can be cumbersome, so the Rados, Ioctx, and Image classes can be used as context managers that close or shut down automatically. Using them as context managers, the above example becomes:

   with rados.Rados(conffile='my_ceph.conf') as cluster:
       with cluster.open_ioctx('mypool') as ioctx:
           rbd_inst = rbd.RBD()
           size = 4 * 1024**3  # 4 GiB
           rbd_inst.create(ioctx, 'myimage', size)
           with rbd.Image(ioctx, 'myimage') as image:
               data = 'foo' * 200
               image.write(data, 0)

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