Re: Hotplugging ethernet cables.

[Kay Sievers <kay.sievers@vrfy.org>]

On Tue, 2005-01-04 at 11:44 -0600, Linas Vepstas wrote:
> A naive question re ethernet hotplug:  what if its iSCSI?
> 
> On Sun, Jan 02, 2005 at 01:43:32AM +0100, Kay Sievers was heard to remark:
> > On Sun, 2005-01-02 at 02:11 +0100, oliver wrote:
> > > They mention ifplugd and even though this seems to work reasonably, I 
> > > was wondering why hotplugging doesn't support ethernet hotplugging 
> > > nativly?
> > 
> > Hotplug supports device add and remove, not state changes of hardware.
> > 
> > > As I understand it the hotplugging daemon responds to events 
> > > created by several devices, where the ifplugd polls and the device.
> > 
> > There is no such daemon involved in the hotplug handling. Hotplug is
> > driven by more or less stateless scripts invoked by the kernel.
> > 
> > The network link state changes can be received on a netlink socket from
> > the kernel. These events are not hotplug events and don't really fit
> > into the model of linux hotplug.
> 
> 
> I can imagine  a scenario where Linux has mounted a filesystem that
> is sitting on an iSCSI disk device.  Thus, in many respects, this is
> analogous to a USB disk being plugged and unplugged.  I also presume
> that iSCSI devices need not be just disks, but could be "anything",
> thus making the situation even more analogous to USB.  

Sure, it's similar in some aspects, but linux-hotplug is only a device
"add" or "remove" notification. Every event corresponds to a created or
removed directory in /sys. There is no such thing for a network link.

If you add/remove the network device you get hotplug events. You don't
get hotplug events if you press a button of an USB device, which is very
similar to the network link state change. :)

Some networking applications produce a very high event traffic for state
changes - like the zebra routing daemon, this is definitely not possible
to do with the event-by-forked-helper-application-hotplug system. They
use the netlink socket for good reason. What do you think is the problem
to get network layer events over netlink? It works nicely this way for a
long time.

For a application level feature rich example, you may look how HAL is
collecting all available kernel events and offers a unified interface to
applications to subscribe to these events:
  http://cvs.freedesktop.org/*checkout*/hal/hal/doc/spec/hal-spec.html?rev=1.36.2.5

> I'm also curious about plans for things like infiniband fabrics,
> which might be plugged/unplugged in various ways, which the OS might
> want to know about.  Again, this seems clearly analogous to USB fabrics

Never seen such a thing. :)

Kay



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