Re: removed disk && md-device

[Bernd Schubert <bs@q-leap.de>]

On Thursday 10 May 2007 09:12:54 Neil Brown wrote:
> On Wednesday May 9, bs@q-leap.de wrote:
> > Neil Brown <neilb@suse.de> [2007.04.02.0953 +0200]:
> > >Hmmm... this is somewhat awkward.  You could argue that udev should be
> > >taught to remove the device from the array before removing the device
> > >from /dev.  But I'm not convinced that you always want to 'fail' the
> > >device.   It is possible in this case that the array is quiescent and
> > >you might like to shut it down without registering a device failure...
> >
> > Hmm, the the kernel advised hotplug to remove the device from /dev, but
> > you don't want to remove it from md? Do you have an example for that
> > case?
>
> Until there is known to be an inconsistency among the devices in an
> array, you don't want to record that there is.
>
> Suppose I have two USB drives with a mounted but quiescent filesystem
> on a raid1 across them.
> I pull them both out, one after the other, to take them to my friends
> place.
>
> I plug them both in and find that the array is degraded, because as
> soon as I unplugged on, the other was told that it was now the only
> one.
> Not good.  Best to wait for an IO request that actually returns an
> errors.

Ok, keeping the raid working in this case would be a good idea, so we would 
need it configurable if it should degrade or not.
However, have you tested if pulling and hotplugging the drive works? Actually 
thats what our customer did. As long as md keeps the old device information, 
the re-plugged-in device will get another device name (and of course also 
another major number) and so the md-device will still keeps the old device 
information and it will never automagically add the new device.  
Probably thats even a good idea, how should the md-layer know if it is really 
the very same device and even if it would know that, how should it know that 
no data have been modified on it, while it was plugged out?

>
> > >Maybe an mdadm command that will do that for a given device, or for
> > >all components of a given array if the 'dev' link is 'broken', or even
> > >for all devices for all array.
> > >
> > >   mdadm --fail-unplugged --scan
> > >or
> > >   mdadm --fail-unplugged /dev/md3
> >
> > Ok, so one could run this as cron script. Neil, may I ask if you already
> > started to work on this? Since we have the problem on a customer system,
> > we should fix it ASAP, but at least within the next 2 or 3 weeks. If you
> > didn't start work on it yet, I will do...
>
> No, I haven't, but it is getting near the top of my list.
> If you want a script that does this automatically for every array,
> something like:

I have never looked into the mdadm sources before, but I will try during the 
weekend (without any promises).

>
>   for a in /sys/block/md*/md/dev-*
>   do
>     if [ -f $a/block/dev ]
>     then : still there
>     else
> 	echo faulty > $a/state
> 	echo remove > $a/state
>     fi
>   done
>
> should do what you want. (I haven't tested it though).

Thanks a lot, we will test that here. Do you propose the same logic for mdadm?


Thanks,
Bernd


-- 
Bernd Schubert
Q-Leap Networks GmbH