Re: Asynchronous scsi scanning, version 9

[Patrick Mansfield <patmans@us.ibm.com>]

[adding hotplug-devel ... maybe Marco or Kay can comment]

On Mon, May 29, 2006 at 07:05:15AM -0600, Matthew Wilcox wrote:
> On Mon, May 29, 2006 at 10:38:13AM +0200, Stefan Richter wrote:
> > Matthew Wilcox wrote:
> > > Add the scsi_mod.scan kernel parameter to determine how scsi busses
> > > are scanned.  "sync" is the current behaviour.  "none" punts scanning
> > > scsi busses to userspace.  "async" is the new default.
> > 
> > This parameter is only relevant with LLDDs which use scsi_scan_host, right?
> 
> Not entirely.  If you set it to "none", scsi_scan_target() also returns
> without doing anything.  If you use the scsi_prep_async_scan() and
> scsi_finish_async_scan() API, you can also use this infrastructure to
> make scanning sbp2 synchronised with other scsi hosts.  Then the setting
> of sync vs async also triggers old vs new behaviour.
> 
> > Furthermore, "sync|async" basically means "serialized|parallelized
> > across host adapters". Does it also mean "finishing before|after driver
> > initialization"? (With LLDDs which use scsi_scan_host.)
> 
> That's what scsi_complete_async_scans() is for.  If you have a built-in
> module, it will wait for the async scans to finish before we get as far
> as trying to mount root.  It does change observable behaviour in that
> sys_module_init() will return before scans are complete.  However, I
> believe most distros userspace copes with this these days.  For example,
> Debian has:
> 
>     # wait for the udevd childs to finish
>     log_action_begin_msg "Waiting for /dev to be fully populated"
>     while [ -d /dev/.udev/queue/ ]; do
>         sleep 1
>         udevd_timeout=$(($udevd_timeout - 1))
> [...]

Not sure where that is, but AFAIR that is to process the cold plug case,
where udev starts up, the hotplug/netlink events are replayed, and we
don't want to continue until all those events have been processed.

SLES 10 has similar code, but a sleep of 0.1 (see their /etc/init.d/boot.udev,
and I think /sbin/mkinitrd).

> Since the scsi scan is going to be finding new devices the entire time,
> the queue directory is going to not empty.

It won't always be finding new devices, there could be glitches like a
timeout, or some read (partitition check) that happens to take more than a
second, and the udev queue becomes empty even though the scsi /sd scan is
still in progress.

You really want some udev rule that mounts root or such and then the boot
continues from there ... rather than waiting for an unrelated sets of
events, and then trying to mount root unconditionally (and possibly
failing). I thought Hannes or someone had posted an example udev rule or
such for this. Maybe it is even in SLES 10?

Same for applications - you want them to start after a dev (or set of
devs) shows up, though if we wait for the root dev it is even less likely
that an app's dev will be unavailable.

Of course if you aren't using udev in your init{rd|ramfs}, udev rules
and such can't fix the problem :-(

-- Patrick Mansfield


-------------------------------------------------------
All the advantages of Linux Managed Hosting--Without the Cost and Risk!
Fully trained technicians. The highest number of Red Hat certifications in
the hosting industry. Fanatical Support. Click to learn more
http://sel.as-us.falkag.net/sel?cmd=lnk&kid\x107521&bid$8729&dat\x121642
_______________________________________________
Linux-hotplug-devel mailing list  http://linux-hotplug.sourceforge.net
Linux-hotplug-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel