linux-kernel - Re: Raid 10 question/problem [ot]

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Date:	Sun, 28 Jan 2007 22:44:16 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
CC:	Marc Perkel <mperkel@...oo.com>, linux-kernel@...r.kernel.org
Subject: Re: Raid 10 question/problem [ot]

Jan Engelhardt wrote:
> On Jan 28 2007 12:05, Michael Tokarev wrote:
[]
>> Mdadm creates those nodes automa[tg]ically - man mdadm, search for --auto.
> 
> Note that `mdadm -As` _is_ run on FC6 boot.

See above -- man mdadm, search for --auto.  -A = --assemble, -s = --scan.

>> In order for an md array to be started up on boot, it has to be specified
>> in /etc/mdadm.conf.  With proper DEVICE line in there.  That's all.
> 
> That's how it is, and it does not work.

Sure.  Because of this missing --auto flag.

> openSUSE 10.2:
> no mdadm.conf _at all_, /etc/init.d/boot.d/boot.md is chkconfig'ed _out_,
> _no_ md kernel module is loaded, and I still have all the /dev/md nodes.

And no udev.  Or, alternatively, *all* md devices are created by some other
script or somesuch.  There's No Magic (tm).

[]
> # mdadm -C /dev/md1 -e 1.0 -l 1 -n 2 /dev/sdb2 /dev/sdc2
> mdadm: error opening /dev/md1: No such file or directory
> 
> Showstopper.

Nonsense.  See above again, man mdadm, search for --auto.

[]
> You see, I have all the reason to be confused.

Yeah, this is quite... confusing.

It's all due to the way how mdadm iteracts with the kernel and how
udev works - all together.  The thing is - in order to assemble an
array, proper device node has to be in place.  But udev wont create
it until array is assembled.  Chicken&eggs problem.

Exactly due to this, the node(s) can be created with mdadm (with --auto
option, whicih can be specified in mdadm.conf too), AND/OR with some
startup script before invoking mdadm, AND/OR when the system isn't
broken by udevd (with ol'good static /dev).

>> But in any case, this has exactly nothing to do with kernel.
>> It's 100% userspace issues, I'd say distribution-specific issues.

..And each distribution uses its own kludge/workaround/solution for
this stuff - which you demonstrated... ;)

/mjt
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