Date: Sat, 1 Dec 2007 13:10:14 +0100 (CET)
From: Jan Engelhardt <jengelh@computergmbh.de>
To: Justin Piszcz <jpiszcz@lucidpixels.com>
cc: linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org,
       apiszcz@solarrain.com
Subject: Re: Kernel 2.6.23.9 + mdadm 2.6.2-2 + Auto rebuild RAID1?
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On Dec 1 2007 06:19, Justin Piszcz wrote:

> RAID1, 0.90.03 superblocks (in order to be compatible with LILO, if
> you use 1.x superblocks with LILO you can't boot)

Says who? (Don't use LILO ;-)

>, and then:
>
> /dev/sda1+sdb1 <-> /dev/md0 <-> swap
> /dev/sda2+sdb2 <-> /dev/md1 <-> /boot (ext3)
> /dev/sda3+sdb3 <-> /dev/md2 <-> / (xfs)
>
> All works fine, no issues...
>
> Quick question though, I turned off the machine, disconnected /dev/sda 
> from the machine, boot from /dev/sdb, no problems, shows as degraded 
> RAID1.  Turn the machine off.  Re-attach the first drive.  When I boot 
> my first partition either re-synced by itself or it was not degraded, 
> was is this?

If md0 was not touched (written to) after you disconnected sda, it also 
should not be in a degraded state.

> So two questions:
>
> 1) If it rebuilt by itself, how come it only rebuilt /dev/md0?

So md1/md2 was NOT rebuilt?

> 2) If it did not rebuild, is it because the kernel knows it does not 
>    need to re-calculate parity etc for swap?

Kernel does not know what's inside an md usually. And it should not 
try to be smart.

> I had to:
>
> mdadm /dev/md1 -a /dev/sda2
> and
> mdadm /dev/md2 -a /dev/sda3
>
> To rebuild the /boot and /, which worked fine, I am just curious 
> though why it works like this, I figured it would be all or nothing.

Devices are not automatically readded. Who knows, maybe you inserted a 
different disk into sda which you don't want to be overwritten.

> More info:
>
> Not using ANY initramfs/initrd images, everything is compiled into 1 
> kernel image (makes things MUCH simpler and the expected device layout 
> etc is always the same, unlike initrd/etc).
>
My expected device layout is also always the same, _with_ initrd. Why? 
Simply because mdadm.conf is copied to the initrd, and mdadm will 
use your defined order.
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