linux-kernel - Re: Linux mdadm superblock question.

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Message-ID: <4B7AD4A4.7070900@anonymous.org.uk>
Date:	Tue, 16 Feb 2010 17:23:48 +0000
From:	John Robinson <john.robinson@...nymous.org.uk>
To:	Linux RAID <linux-raid@...r.kernel.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Linux mdadm superblock question.

On 16/02/2010 14:37, Volker Armin Hemmann wrote:
> On Dienstag 16 Februar 2010, John Robinson wrote:
>> On 14/02/2010 19:13, Volker Armin Hemmann wrote:
>>> On Sonntag 14 Februar 2010, you wrote:
>>>> On 14/02/2010 18:40, Volker Armin Hemmann wrote:
>>>>> On Sonntag 14 Februar 2010, you wrote:
>>>>>> In other words, 'auto-detection' for 1.x format devices is using an
>>>>>> initrd/initramfs.
>>>>> which makes 1.x format useless for everybody who does not want to deal
>>>>> with initrd/initramfs.
>>>> True, but afaik every distro uses an initrd/initramfs and bundles tools
>>>> making it easy to manage and customise them, so what's the problem?
>>> and distros do it because of all the drivers they have to ship. But for
>>> example I am not bound by such limitations. Why should I deal with that?
>>> It is hard enough not to forget 'make modules_install'. And now add
>>> initrd. Autodetecting just works - but if you use an initrd an it
>>> doesn't. Where do you start?
>>>
>>> Initrd's maybe great for distro packagers, but are they really usefull
>>> for anybody else?
>> Not just for distro packagers, they're useful for distro users, which
>> are presumably 99% of Linux users these days, including the vast
>> majority of enterprise users who like tested, supported systems.
>>
>> But even for people building their own kernels, initrd/initramfs are
>> useful if you're using LVM, or indeed trying to boot off anything that's
>> not a simple device.
> 
> so assume you have an initrd and metadata 1.x without auto assembling.
> 
> You do some changes to the raid and screw up something else. Next boot nothing 
> works. Mostly because the mdadm.conf in your initrd is not correct.
> 
> You whip out your trusty usb stick with a resuce system - and you are stuck. 
> If autoassembling would work, you would have working md devices you could 
> mount and edit the files you have to. But you don't and the mdadm.conf in the 
> initrd is outdated.
> 
> Sounds like 'you are screwed'.

No; mdadm can assemble arrays without needing a conf file (at least 
arrays which have superblocks).

And if you have otherwise screwed something up with the RAID, no amount 
of in-kernel autoassembly is going to help, in fact it's more likely to 
get it wrong and make things worse; you need a command line and mdadm to 
sort it out.

Cheers,

John.

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