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

[Stefan Hajnoczi]

On Wed, Mar 14, 2018 at 03:07:21PM +0000, Stefano Panella wrote:
> 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

Holding I/O is done via bdrv_drained_begin/end() and most block-related
monitor commands should use it.

> 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...

blockdev-add and related APIs are still under development and incomplete.

Manos' block-insert-node is probably closest to what you need, so I
would start there.

I'm not clear enough on what exactly is necessary on top of Manos' patch
without doing the work myself, so I'm afraid I can't give a detailed
sequence of steps you need to take.

Stefan

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