[Libguestfs] [PATCH libnbd] api: Synchronous connect waits til all connections are connected.

[Eric Blake]

On 5/22/19 4:28 AM, Richard W.M. Jones wrote:

>> Should this be a bool parameter for the caller to opt-in to?  In one
>> direction, returning as soon as possible allows the caller to issue
>> further synchronous commands which will pick the ready connections and
>> skip the ones still initializing, to at least get lower latency on the
>> first commands and better performance later as more connections come up;
>> in the other direction, waiting to return until all connections are made
>> makes it easier to issue async commands on any connection without
>> worrying if the connection is up. The choice of the parameter to pass in
>> depends, then, on whether the caller plans to make future sync calls or
>> async calls.
> 
> It could be.  On the other hand if people want to do complicated stuff
> then there is the AIO interface.

True enough. A caller that wants to avoid latency should be using AIO
rather than sync. So this change makes sense to me overall.


>>> For ‘nbd_connect_command’, it essentially ignored multi-conn anyway,
>>> and I changed it so that it waits for conn[0] to get connected and
>>> returns, the other connections (if they exist) being ignored.  It
>>> should probably be an error for the user to enable multi-conn on the
>>> handle and then use nbd_connect_command, but I did not make that
>>> change yet.
>>
>> Yes, that change (as a separate patch) makes sense.
> 
> Even more radical, what if we prevent the use of multi-conn at all
> with the synchronous interface?  (I don't think this is a good idea,
> just putting it out there for discussion).

multi-conn + synchronous still makes sense (the fact that you
round-robin between connections is already nice); it's only the command
interface (where you can't make more connections, because you are
limited to stdin/stdout) where an error would be appropriate.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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