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
