[Qemu-devel] block-layer: questions on manipulation of internal nodes

[Stefano Panella <stefano.panella@citrix.com>]

Hi everybody,

I am a relatively new user of qemu block layer. I am interested in it mainly because it looks very powerful and general and I am hoping to integrate it on our product and to contribute to it for new usecases.

I have existing use cases where we work with a model of a disk process per VM disk and I am experimenting with qemu and qmp to  build something similar.

At the moment I have managed to build a new binary, called qemu-dp (probably should be called qemu-bl for block layer) which is basically starting as a qmp server and accepting qmp block layer commands to operate on disks.

just to give you an example this is the kind of thing I am doing:

EXTERNALLY:
/usr/lib64/xen/bin/qemu-img create -f qcow2 -o size=1M /root/a
/usr/lib64/xen/bin/qemu-img create -f qcow2 -b /root/a -o size=1M /root/b
/usr/lib64/xen/bin/qemu-img create -f qcow2 -b /root/b -o size=1M /root/c
/usr/lib64/xen/bin/qemu-img create -f qcow2 -b /root/c -o size=1M /root/d
/usr/lib64/xen/bin/qemu-img create -f qcow2 -b /root/d -o size=1M /root/e

let's assume there were some data in every layer....

Than:

USING QMP:
{ "execute": "qmp_capabilities" }
{
	"execute": "blockdev-add",  
	"arguments": {
		"driver": "qcow2", 
		"node-name": "qemu_node",
		"discard": "unmap",
		"cache": {
			"direct": true
		},
		"file": {
			"driver": "file",
			"filename": "/root/e"
		}
	}
}

{
	"execute": "nbd-server-start",
	"arguments": { 
		"addr": {
			"type": "unix",
			"data": {
				"path": "/tmp/nbd.test1"
			}
		}
	}
}

{
	"execute": "nbd-server-add",
	"arguments": { 
		"device": "qemu_node",
		"writable": true
	}
}

after this the chain looks like:

a < b < c < d < e < NBD_server

now I make a full copy of b and c which I call b1 and c1 and for example I run externally qemu-img commit c1 -> b1 while qemu-dp has still the chain opened.

I would now like to send a qmp command to tell qemu-dp to hold any IO from the NBD_server and forget about a, b, c and insert b1 as d's child, like this:

a < b1 < d < e < NBD_server

I have tried to implement this qmp command and looked at 

qmp_change_backing_file()
qmp_x_blockdev_change()
https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg02660.html

but did not figure out a way of doing that yet...

I suspect my problem is that I am still very confused about the semantics of the object model in the block layer, the ref counting, the graph manipulation, the monitor etc. etc.

I have tried to have some interactive chats on irc and they have been very useful so far (thanks again stefanha, kwolf, berto, eblake) but maybe a proper email would be a good starting point as stefanha has suggested.

Please if somebody could point me to a bit of code to achieve my example that would be great, otherwise if there is no code for that kind of functionality, it would be good to have a little guide on the sequence of block primiteve I should call and on which node, including refs, locking, drain, caches, reopen etc...

Thanks a lot,

Stefano