Re: Question: How to change backing file ?

[Vladimir Sementsov-Ogievskiy]

Hi Kevin!

28.12.2020 21:03, Kevin Nguetchouang wrote:
> Hello everyone, in a class project, i would like to change the backing file of 
> the current image opened with a particular path file.
>
> I try differents functions i saw in the source code
> - bdrv_change_backing_file
> - bdrv_open
> - bdrv_open_child
>
> but no one work... from segmentation fault error to bdrv_attach_backing passing 
> through parent->blocking_error, i don't know how to achieve what i want.
>
> --
> /Kevin Nguetchouang./

First, you should understand, that there are two different thing in "changing the 
backing file":

1. Change backing link in the block nodes graph
2. Change backing file name in the metadata of the image (for example in qcow2 
header

For example, look at the bdrv_drop_intermediate() (block.c) function. it's aim 
is to drop all nodes in the chain between top and base, and make base to be new 
backing file for the top.

Key things in the function:

- bdrv_subtree_drained_begin/end - to pause any io operations during graph 
update

- bdrv_replace_node_common() - which actually replace backing child of top node

- c->klass->update_filename() - which will update backing-file in the metadata 
of top image


Another example in block/stream.c, in stream_prepare():

- bdrv_set_backing_hd() to change backing child
- bdrv_change_backing_file() to change backing-file in the metadata of top image


(hmm, why don't we have drained begin/end here? why we don't use 
bdrv_drop_intermediate()? - just good questions).


Hope, it helps a bit.


--
Best regards,
Vladimir