Bug#684708: mdadm: support external metadata arrays correctly
Michael Tokarev
mjt at tls.msk.ru
Mon Aug 13 15:56:59 UTC 2012
On 13.08.2012 19:40, Dmitrijs Ledkovs wrote:
> On 13/08/12 16:31, Michael Tokarev wrote:
[]
>> 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.
Nononono. The _filesystem_ will most likely be clean. The raid array
is not.
> Can we include mdmon conditionally, if rootfs is on external metadata raid?
There's no reason for that. mdmon is automatically started by mdadm
if needed.
>> 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? =)
For this very question - including mdmon into initramfs - i really want
to understand the underlying problem - whenever we're talking about
ext[34]fs which applies journal even for readonly mount and STILL doing
so in 3.2 kernel (wheezy), or something else -- in other words, if
this is really necessary. I guess it wont _hurt_ to include it (*),
after all, initramfs can sometimes be used as a small rescue system
to modify the real root before it gets switched to. But I really want
to see what is the problem we're dealing with.
(*) ofcourse it needs to be verified - the takeover after booting
into real system. I can't do it, since I don't have any spare system
right now to install debian onto imsm array. I can only verify that
the new takeover does not harm regular boot process without imsm
arrays.
This change - mdmon in initramfs - might be included to wheezy, it
isn't a dramatic change. But I still don't think it is a good idea
in general -- using native md arrays is a much better way.
Due to the last statement, I'm not sure the severity of this bugreport
is justified.
/mjt
More information about the pkg-mdadm-devel
mailing list