Re: [Patch] mdadm: move mdadm.map file into /dev/md

["NeilBrown" <neilb@suse.de>]

On Tue, April 7, 2009 10:16 pm, Doug Ledford wrote:
> On Apr 7, 2009, at 2:05 AM, Goswin von Brederlow wrote:
>> Doug Ledford <dledford@redhat.com> writes:
>>
>>> Originally, mdadm used /var/run/mdadm/mdadm.map file to store the
>>> temporary mappings of incrementally added devices to device names.
>>> Unfortunately, this breaks incremental assembly if used early in the
>>> booting process.  Specifically, root may still be read only.  Since
>>> incremental assembly is largely a udev specific feature, and udev
>>> needs a writable /dev tmpfs mount even when root is still read only,
>>> it's safer to put our mdadm.map file in /dev/md so that we can write
>>> to the map file no matter how early in the boot process we are
>>> attempting to use incremental assembly.
>>
>> What about /lib/init/rw?
>
> Never heard of it.  But, if / is read-only, why wouldn't that also be
> read-only?

because a tmpfs was mounted there.  It's probably a Debian-specific
thing.

Why /var/run cannot be mounted from tmpfs nice and early too I don't know..
I understand that some people think /var/run needs to persist across
reboots to preserve ownership of directories, but they are wrong :-)
/var/run should be a tmpfs mount point very early.

I have to say that putting the map file in /dev feels very icky
to me.  It isn't a device, and so doesn't belong in /dev.
If we go around putting things somewhere convenient rather than
where they belong, we quickly end up with a mess.

So while it might be very pragmatic, I am currently dis-inclined
to take the patch.   Can you try asking your boot-script people
to make /var/run be an early tmpfs ???

NeilBrown