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Re: [Qemu-devel] block-layer: questions on manipulation of internal node
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] block-layer: questions on manipulation of internal nodes |
Date: |
Fri, 16 Mar 2018 15:55:27 +0000 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
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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