Bug#684708: mdadm: support external metadata arrays correctly

[Michael Tokarev]

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.

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.

Thanks,

/mjt