* [Qemu-devel] block-layer: questions on manipulation of internal nodes
@ 2018-03-14 15:07 Stefano Panella
2018-03-16 15:55 ` Stefan Hajnoczi
0 siblings, 1 reply; 2+ messages in thread
From: Stefano Panella @ 2018-03-14 15:07 UTC (permalink / raw)
To: qemu-devel@nongnu.org
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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [Qemu-devel] block-layer: questions on manipulation of internal nodes
2018-03-14 15:07 [Qemu-devel] block-layer: questions on manipulation of internal nodes Stefano Panella
@ 2018-03-16 15:55 ` Stefan Hajnoczi
0 siblings, 0 replies; 2+ messages in thread
From: Stefan Hajnoczi @ 2018-03-16 15:55 UTC (permalink / raw)
To: Stefano Panella; +Cc: qemu-devel@nongnu.org
[-- Attachment #1: Type: text/plain, Size: 3742 bytes --]
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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