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

[Colin Guthrie]

'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'...


Col


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Colin Guthrie
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http://colin.guthr.ie/

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