Re: handling mdmon in the initramfs

[Hans de Goede <hdegoede-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Hi,

On 10/02/2009 05:31 AM, Neil Brown wrote:

<snip>

>>>> Problem 2: Discovery / Assembly
>>>> Several issues have forced dracut to punt on using mdadm -I.  Instead
>>>> dracut copies mdadm.conf to the initramfs and uses mdadm -As after a
>>>> udevadm --settle.  One low hanging issue is the fact that non-rootfs
>>>> arrays may only be partially assembled when dracut discovers and
>>>> switches to the final rootfs.  Upon switching the in-progress map file
>>>> is lost.  Moving /var/run/mdadm/map to /dev/.mdadm/map would appear to
>>>> solve this issue.
>>>
>>> mdadm already uses /dev/.mdadm.map if /var/run is not writable.  So is
>>> this a solved problem, or do we need some other way to force mdadm not
>>> to use /var/run too early.
>>
>> So you are talking about commit cf3a3d78 [2].  I think the problem is
>> that /var/run/mdadm/map is writable in dracut so we never fall back to
>> /dev/.mdadm.map.  Any reason to not make /dev/.mdadm.map the first
>> location to try?
>
> Because (damn it) the file is not a device file, it is a
> run-time-management file and so it belongs in /var/run.  If we just
> put things wherever was convenient instead of where they belonged we
> would end up with something like /proc!
>
> If there is a writable /var/run in early boot, presumably it is a
> tmpfs filesystem.
> Would it not be simple to
>     mount --bind /var/run /root/var/run
> before the pivot_root, or maybe "mount --move"?
> Then everything would benefit from a stable /var/run and nothing would
> have to put silly inappropriate files in /dev.
>
> On the other hand, if /var/run is not a tmpfs filesystem and is
> normally part of the real /var, what is the justification for having a
> writable /var/run in early boot?  Sounds like a recipe for confusion.
>

There is no such thing in the initrd (a writable /var/run), but there is
a writable / (initrd == ramdisk), and mdadm will happily create /var/run
itself, if it would not do that, there would no issue.

Although I do wonder how later, when we do have a writable /var/run, mdadm
decides which file to use. Once it has used /dev/.mdadm.map once it should
keep on using that.

Regards,

Hans
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