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SubjectRe: [ANNOUNCE] udev 0.1 release
Antonio Vargas wrote:

> On Fri, Apr 11, 2003 at 06:46:09PM +0100, John Bradford wrote:
>>>>- Performance. What happens if you plug in 4000 disks at once?
>>>You crash your power supply :)
>>Say the power supply had five 5.25" drive power connecters, how many 1
>>into 3 power cable splitters would you need to connect all 4000 disks?

OK, this is all fun and games, but this is a valid point. All it takes for the
driver for a Fibre Channel host adapter to load, and enumerate the devices it
can see. In a matter of seconds many hundreds or thousands of disk devices could
be registered with the kernel.

This is definitely an issue that will need to be addressed, and I think Oliver's
suggestion of using a pipe (i'm going to say it: like devfs did :-) to forward
the events to /sbin/hotplug in a FIFO fashion makes some sense. I have also been
considering this issue from another angle; I am working on userspace partition
discovery, which will be driven by /sbin/hotplug (and udev, probably). I have
concerns that the following scenario will cause problems, if not extreme problems:

- kernel driver finds an IDE drive, registers it and the hotplug event happens
- udev gets called and gives it device node /dev/discs/disc0 (or whatever)
- /sbin/hotplug calls userspace partition discovery, which opens the device and
scans for partitions
- if any partitions are found, they are registered with the kernel using
device-mapper ioctls
- because these new "mapped sections" of the drive are _also_ usable block
devices in their own right, they generate hotplug events
- because these hotplug events are for new block devices, userspace partition
discovery will get called _again_ to handle them (it may not find anything (the
normal case), but this model will support nearly infinite levels of partitioning
on any block device supported by the kernel)

What happens if these secondary hotplug events occur while /sbin/hotplug has not
yet finished processing the first one? Ignoring locking/race issues for the
moment, I'm concerned about memory consumption as many layers of
hotplug/udev/kpartx/etc. are running processing these events.

Of course, another possibility I'll look into this weekend is to actually have
kpartx run as a daemon and receive messages over D-BUS, instead of being invoked
directly by /sbin/hotplug. This would mean it could serialize the events itself
and reduce some of the load (if D-BUS supports message queueing, which I believe
it does).

Actually, here's another thought: have the kernel continue to call /sbin/hotplug
for every event, just as it does now. However, /sbin/hotplug would do _nothing_
but translate that into D-BUS messages and post them. udev, kpartx, etc. would
all just be D-BUS clients that would respond to their messages as they are received.

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