Re: mdadm and automatic re-add / incremental mode with usb-disk

[Wilhelm Meier <wilhelm.meier@fh-kl.de>]

Am Sonntag 30 November 2008 schrieb Justin Piszcz:
> On Sun, 30 Nov 2008, Wilhelm Meier wrote:
> > Hi,
> >
> > I'm using debian etch with mdadm 2.5.6-9.
> >
> > I have a md-device /dev/md1000 with two usb-disks as raid1. The
> > array is assembled well if the system boots, if I unplug one of
> > the disks, the array goes to degraded. Thats all ok.
> >
> > If I re-plug the usb-disk, udev discovers the device fine, but
> > mdadm doesn't start the re-add to the md-array. I have to do this
> > but hand.
> >
> > Is there something missing to make this work automatically?
> >
> > I tried the mdm-2.6.2 from etch-backports too. Same effect.
> >
> > Here, if I try to use the --incremental mode, it constructs a new
> > (!) array /dev/md/d_1000  instead of adding it to /dev/md1000.
> > Thats strange to me.
> >
> > I thought I got it working some weeks ago (maybe with earlier /
> > other versions of mdadm or somme missing other tool), but I can't
> > get the puzzle right now.
> >
> > Any hints?
> > --
> > Wilhelm
> > --
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>
> Wilhelm,
>
> As far as I know mdadm never will re-add a
> broken/failed/disconnected disk back into the array.  Perhaps what
> you saw before is on an un-clean shutdown or something similar,
> upon reboot the array is checking/re-initializing.

That might be, but what is the difference between doing the re-add and 
re-sync on boot (that's what happens!) or if the drive comes back on 
a running system.

mdadm can surely determine the state/uuid and do this - the same as on 
reboot.

> When you have a failed drive and you re-attach it, it will stay as
> a removed unit and you need to remove it and add it manually as you
> stated.

the only thing I have to do is e.g.

mdadm --re-add /dev/md1000 /dev/sdg1

then it starts reconstructing the right way.

So, my thought was to do this as part of a udev-rule. 
But I think this is a common case and therefore there should be a 
well-known solution

> Were you seeing something other than this before?

I might be wrong, but I thought it was on debian ...

> Justin.
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-- 
Wilhelm