NBD reconnect on open

NBD reconnect on open

[Vladimir Sementsov-Ogievskiy]

Hi Eric!

There is a question to discuss.

We need to make an option to allow nbd-reconnect loop on nbd-open.
For example, add optional nbd blockdev option open-reconnect-delay, to
make it possible to start qemu with specified nbd connection, when nbd
server is down, and make several tries to connect before starting the
guest.

So, we need it for nbd opened from commandline arguments, and this case
seems OK.

But adding option to QAPI, we also allow it for qmp blockdev-add, and
reconnecting in context of qmp command execution is a wrong thing..

I can add new option only to options in block/nbd.c, but this way
-blockdev command line option will not work, it needs QAPI definition.

What do you think about it?

I can detect somehow in nbd_open that we are in qmp monitor context, and
return error if open-reconnect-delay specified.. Is it OK? Is there a
way to do it?


-- 
Best regards,
Vladimir

Re: NBD reconnect on open

[Kevin Wolf]

Am 04.12.2019 um 13:18 hat Vladimir Sementsov-Ogievskiy geschrieben:
> There is a question to discuss.
> 
> We need to make an option to allow nbd-reconnect loop on nbd-open.
> For example, add optional nbd blockdev option open-reconnect-delay, to
> make it possible to start qemu with specified nbd connection, when nbd
> server is down, and make several tries to connect before starting the
> guest.
> 
> So, we need it for nbd opened from commandline arguments, and this case
> seems OK.
> 
> But adding option to QAPI, we also allow it for qmp blockdev-add, and
> reconnecting in context of qmp command execution is a wrong thing..
> 
> I can add new option only to options in block/nbd.c, but this way
> -blockdev command line option will not work, it needs QAPI definition.
> 
> What do you think about it?

I think there is a more general problem here actually. bdrv_open() is a
blocking operation and it shouldn't be. BlockDriver should probably have
a .bdrv_co_open instead, and I think this wouldn't be too hard to do.

However, that's only half of the solution: QMP still takes the BQL while
it's executing a command, so even if we used a coroutine, it would be of
no use if then the QMP command implementation would just call
BDRV_POLL_WHILE() to wait for the completion of the coroutine while
holding the BQL.

This is going in direction of async commands that Marc-André was working
on. I didn't follow this closely, so I'm not sure what the status there
is, but he and Markus should be able to tell more.

> I can detect somehow in nbd_open that we are in qmp monitor context, and
> return error if open-reconnect-delay specified.. Is it OK? Is there a
> way to do it?

The whole point of -blockdev is that it's a direct mapping of
blockdev-add to the command line, so making things behave differently
between them sounds like a bad idea.

Kevin