Re: [ANNOUNCE] udev 0.1 release

Steven Dake
2003-04-11T23:21:08Z

Greg KH wrote:

>On Fri, Apr 11, 2003 at 03:30:21PM -0700, Steven Dake wrote:
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>>There is no "spec" that states this is a requirement, however, telecom
>>customers require the elapsed time from the time they request the disk
>>to be used, to the disk being usable by the operating system to be 20 msec.
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>What defines the term, "request the disk to be used"? Slamming it into
>the SCSI tray? Mounting the device on the command line? I don't think
>you can spin up a scsi disk in 20msec today :)
>
I should have been more clear.

What I actually mean is:
disk is in the bus/loop/etc, powered on, ready to be enumerated.
The user then tells the OS "please insert the disk" This is the request
which starts the clock.
The point where a device entry is in /dev ready to be used stops the clock.

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>>Its even more helpful for their applications if the call that hotswap
>>inserts blocks until the device is actually ready to use and available
>>in the filesystem.
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>What would it block from happening? The kernel? Userspace? I'm
>confused.
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An insert operation would block until the following sequence occurs:
insert a disk, disk added to subsystem (scsi, etc), hotplug event,
hotplug handler creates device node.

Without blocking until the device is created and ready to be used, it
becomes difficult to actually "hotswap insert" and then immediatly use
the device, requiring polling. Most users would like to wait for the
event to complete, or have a select()able fd to wait on to know when the
event has been completed.

This might be possible to emulate through dnotify, but would still
require a rescan of the dev directory, causing poor performance.

>thanks,
>
>greg k-h
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