Bug#684708: mdadm: support external metadata arrays correctly

[Dmitrijs Ledkovs]

On 13/08/12 16:31, Michael Tokarev wrote:
> On 13.08.2012 18:20, Dmitrijs Ledkovs wrote:
>> On 13/08/12 12:43, Michael Tokarev wrote:
>>> On 13.08.2012 14:10, Miquel van Smoorenburg wrote:
>>>> Package: mdadm
>>>> Version: 3.2.5-1
>>>> Severity: serious
>>>> Tags: patch wheezy sid
>>>>
>>>> The initramfs hook supplied by mdadm doesn't install mdmon. Also, mdmon
>>>> is not included in the .udeb for the installer.
>>>>
>>>> This means that if you have an array with external metadata (ddf or,
>>>> more widely used, imsm - Intel Matrix Raid) that it will come up
>>>> readonly. This causes the installer to hang or the system not being
>>>> able to boot if root is on that array.
>>>
>>> I'm not sure this is the right course of actions -- speaking of the
>>> initramfs part, not about the d-i part.
>>>
>>> Why the system is not being able to boot if root is on such an array?
>>
>> I disagree with Michael, and agree with Miquel. mdmon is needed in the
>> initramfs and I wanted to propose it's inclusion myself as well.
>>
>> Reading mdmon(8) section START UP AND SHUTDOWN, I am convinced that
>> mdmon is required when rootfs is on an external metadata array. Does it
>> make sense to you, Michael?
> 
> Well.  Sort of.  mdmon utility is supposed to be used in the initramfs,
> with support of taking over, etc.  But this is apparently due to defects
> in some filesystems (I guess it is ext[34]fs), which performs writes
> even if asked to do a read-only mount.  I still don't see a reason why
> a _cleanly_ shut down system should have issues booting - I don't know
> which case Miquel referred to above when saying that a system does not
> boot.
> 

Well... based on the patch, my guess is that because the shutdown does
not wait for raid to be synchronised the filesystem ends up being dirty
after a "clean" shut down, hence to reliable boot you should (not must)
have mdmon with rootfs on external metadata raid.

Can we include mdmon conditionally, if rootfs is on external metadata raid?

> BTW, is it still the case that ext4 performs writes behind the scenes
> even if asked for read-only mount?  I think I've seen some fixes in
> this area, but it might be me dreaming.
> 

True. Maybe we should ask Ted? =)

-- 
Regards,
Dmitrijs.