[systemd-devel] Need help with a systemd/mdadm interaction.

[NeilBrown]

On Tue, 12 Nov 2013 18:49:34 +0100 Colin Guthrie <gmane at colin.guthr.ie> wrote:

> 'Twas brillig, and NeilBrown at 12/11/13 11:17 did gyre and gimble:
> > On Tue, 12 Nov 2013 18:16:24 +0900 Greg KH <gregkh at linuxfoundation.org> wrote:
> > 
> >> On Tue, Nov 12, 2013 at 07:54:42PM +1100, NeilBrown wrote:
> >>> On Tue, 12 Nov 2013 00:10:28 -0800 Greg KH <gregkh at linuxfoundation.org> wrote:
> >>>
> >>>> On Tue, Nov 12, 2013 at 11:31:45AM +1100, NeilBrown wrote:
> >>>>> Alternately, is there some "all devices have been probed, nothing new will
> >>>>> appear unless it is hot-plugged" event.  That would be equally useful (and
> >>>>> probably mirrors what hardware-RAID cards do).
> >>>>
> >>>> No, there's no way to ever know this in a hotplug world, sorry.
> >>>> Especially with USB devices, they show up when they show up, there's no
> >>>> "oh look, the bus is all scanned now and all devices currently plugged
> >>>> in are found" type knowledge at all.
> >>>>
> >>>> Then there are hotplug PCI systems where people slam in PCI cards
> >>>> whenever they feel like it (remember, thunderbolt is PCI express...)
> >>>>
> >>>> Sorry,
> >>>>
> >>>> greg k-h
> >>>
> >>> Surely something must be possible.
> >>
> >> For USB, nope, there isn't, sorry.
> >>
> >>> Clearly a physical hot-plug event will cause more devices to appear, but
> >>> there must come a point at which no more (non-virtual) devices will appear
> >>> unless a physical event happens?
> >>
> >> Not for USB, sorry.
> >>
> >> The USB bus just announces devices when it finds them, there is no "all
> >> is quiet" type signal or detection.
> >>
> >> Same for PCI hotplug, devices can show up at any point in time, you
> >> never know when, and you don't know when all devices are "found".
> >>
> >> sorry,
> >>
> >> greg k-h
> > 
> > 
> > Hmmm... OK.  USB doesn't bother me a lot, but PCI is important.
> > 
> > I guess I'll just have to settle for a timeout much like the current
> > device-discovery timeout that systemd has.
> > Still hoping someone can tell me how to plug into that though...
> 
> Wouldn't it be nicer to work on a nice text-UI/wizard type thing that
> would allow an admin to manually say "yes, I appreciate that the raid
> array is degraded and I would like to start it anyway"?
> 
> It seems to me that doing this automatically is a bad idea if someone
> simply forgot to plug in a drive... or, and this has happened to me
> (tho' I stress not *by* me!), removes the wrong drive. I guess I'm in
> two minds on this one as I can see the usefulness of just coping and
> carrying on but the prospect of a multi-day resync with modern large
> disks for a simple mistake isn't too nice either!
> 
> There doesn't appear to be any generic way to get user approval for such
> interactive questions, but perhaps this is the kind of infrastructure
> that should be provided - perhaps similar to the whole password agent thing?
> 
> Maybe this is just all a bit too much tho'...

Thanks for the suggestion.  However lots of machines are expected to boot
unattended and that is the case that I really want to work.
I think boot should only fail if it cannot possibly succeed (without risking
serious corruption).

Thanks,
NeilBrown

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