Re: [Nbd] NBD wishlist items?

[Paul C. <pau...@st...>]

Wouter Verhelst wrote:

> A protocol extension that lets both the server and the client initiate
> this keepalive probe would deal with both issues (provided we do not
> somehow swap out nbd-client).

Do we need the server to do this keepalive stuff, though? I mean, it's 
really the client that cares if the connection goes down. The server 
ought to be happy to just sit there -- if it's not getting any requests, 
it shouldn't matter.


>>> Implementing this backwards-compatibly is going to be the hardest part,
>>> I guess. Perhaps opening one of the NBD devices to call a specific ioctl
>>> to verify whether it supports this interface, and then setting a bit in
>>> the field of 'reserved' bits in the handshake could work, but I'm not
>>> sure how this would be best done.
>> There'd probably be an ioctl to enable this ping packet, so you could 
>> just call the ioctl and ignore failure (for older kernels). Then there'd 
>> be a new packet flag for this ping packet (or maybe just use a 
>> zero-length read or write, assuming the nbd-server just ignores those 
>> and returns a response). That way it would almost completely backward 
>> compatible.
> 
> Sorry, I'm not sure I understand that bit. Could you elaborate?

I was just thinking out loud. Basically, an easy and backward compatible 
way to do this (client-side) ping is to periodically send a zero-length 
read or write, and as long as the server just ignores it (i.e., doesn't 
crash or do something otherwise stupid) and sends a response, everything 
will work and the protocol doesn't have to change.

--
Paul