Lets take a look how rbd object are store on rados and the difference between format 1 and format 2.

Format 1

$ rbd create myrbd --size=10 
$ rados ls -p rbd      
myrbd.rbd
rbd_directory

$ rbd map myrbd
$ dd if=/dev/zero of=/dev/rbd/rbd/myrbd
$ rados ls -p rbd
myrbd.rbd
rb.0.1286.74b0dc51.000000000000
rb.0.1286.74b0dc51.000000000001
rb.0.1286.74b0dc51.000000000002
rbd_directory

1. $image_name.rbd : Contain the id of the image ( rb.0.1286.74b0dc51 )
2. $rbd_id.$fragment : Raw data image
3. rbd_directory : Rbd image list for the current pool

Format 2

$ rbd create myrbd --size=10 --image-format=2
$   rados ls -p rbd      
rbd_directory
rbd_header.134a74b0dc51
rbd_id.myrbd

$ rbd map myrbd
$ dd if=/dev/zero of=/dev/rbd/rbd/myrbd
$ rados ls -p rbd
rbd_data.134a74b0dc51.0000000000000000
rbd_data.134a74b0dc51.0000000000000001
rbd_data.134a74b0dc51.0000000000000002
rbd_directory
rbd_header.134a74b0dc51
rbd_id.myrbd

1. rbd_data.$rbd_id.$fragment : Raw data image
2. rbd_directory : Rbd image list for the current pool
3. rbd_header.$rbd_id : Metadata about the image
4. rbd_id.$image_name : Contain the id of the image ( 134a74b0dc51 )