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:
> > Am Sonntag 30 November 2008 schrieb Justin Piszcz:
> >> On Sun, 30 Nov 2008, Wilhelm Meier wrote:
> >
> > 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.
>
> From the docs, when you remove and re-add it would appear to be a
> 'reocover' whereas an unclean shutdown would be resyncing (or
> repair if you do not have a bitmap).
>
> linux-2.6.27.7/Documentation/md.txt (actually a good doc to read
> btw)
>
>         resync        - redundancy is being recalculated after
> unclean shutdown or creation
>         recover       - a hot spare is being built to replace a
>                         failed/missing device
>         repair        - A full check and repair is happening.  This
> is similar to 'resync', but was requested by the user, and the
> write-intent bitmap is NOT used to optimise the process.

Oh yes, I mean "recover", thats what is happening after the manual 
re-add if I unplug und then replug the usb-disk.

>
> > 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
>
> Yeah I suppose you could do something like this.  Is the purpose
> more or less to have a portable raid1 array?

Yes, exactly.
I want to have a portable usb-raid of 2 disks (or 2x2 with lvm on top) 
and I want to plug and unplug the disks freely.

-- 
Wilhelm