Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu

[Kevin Wolf]

Am 13.05.2020 um 15:48 hat Dr. David Alan Gilbert geschrieben:
> * Daniel P. Berrangé (address@hidden) wrote:
> > On Wed, May 13, 2020 at 01:56:24PM +0100, Dr. David Alan Gilbert wrote:
> > > * Kevin Wolf (address@hidden) wrote:
> > > > I guess it would be nice to have a single namespace for everything in
> > > > QEMU, but the reality is that we have a few separate ones. As long as we
> > > > consistently add a prefix that identifies the namespace in question, I
> > > > think that would work.
> > > 
> > > > This means that if we're using node-name to identify the NBD connection,
> > > > the namespace should be 'block' rather than 'nbd'.
> > > > 
> > > > One more thing to consider is, what if a single object has multiple
> > > > connections? In the case of node-names, we have a limited set of allowed
> > > > characters, so we can use one of the remaining characters as a separator
> > > > and then suffix a counter. In other places, the identifier isn't
> > > > restricted, so suffixing doesn't work. Maybe prefixing does, but it
> > > > would have to be there from the beginning then.
> > > 
> > > Yeh I worry about whether on nbd if you can have multiple nbd
> > > connections to one block device.
> > 
> > The kernel NBD driver now supports multiple parallel connections.
> > QEMU hasn't implemented this in its NBD code yet, but I certainly
> > see that being in scope for future.
> 
> It's not parallel for performance that worries me, it's more about
> separateq connections for separate uses - e.g. if we're serving the same
> read-only disk to multiple separate things.

That would be a concern for the NBD server. I'm not sure if anything in
QEMU ever waits for NBD servers (except for the client on the other side
of the connection, of course), so there might be no use case for yanking
their connections.

Anyway, here we were talking about the NBD client, which always accesses
one disk. If you access a second disk, you have a second NBD block node.

Kevin