Re: RFC - device names and mdadm with some reference to udev.

On Monday November 3, dledford@redhat.com wrote:

The in-kernel names are set by the kernel md code.  Right now, it has a
simplistic test that checks if the device is partitionable, then sets
the kobject name to either md%d or md_d%d.  The key point being that the
md code gets to set the kobject name, and it's the kobject name that is
used my udev.  Don't get me wrong, I know changing this setup now would
break udev horribly, this being because udev current does
subsystem==block,kernel=="md*" to match all md devices.  In order to
break from this, we would need to do something like
subsystem==block,subtype==md and skip any name check tests.  Then we
could in fact use arbitrary names and udev and the rest of the system
would be fine.  So, I'm not saying it would work today, but that doesn't
mean it couldn't be designed for and then implemented with a coordinated
change to the kernel and udev.

That's an interesting idea.

   echo md_fred > /sys/modules/md_mod/parameters/new_array
   # /sys/block/md_fred magically appears with some random minor number
   # udev ignores it because md/array_state is 'clear'
   cd /sys/block/md_fred/md
   echo raid5 > level
   .....
   echo active > array_state
   # udev gets a CHANGE event and creates /dev/md_fred, and/or
   #  /dev/md/fred plus any partitions that are found.

I think that we need to have the "md_" prefix.  I don't want someone
ever to be able to create and md array called "sda"!
And I think this would work with udev today, though mdadm wouldn't
make use of it, and could get confused if some other code did.

Any code which parses /proc/mdstat and expects to see "md%d" there
will also get confused.

But this is the first idea I have seen that really gives some value in
an alternate way for creating md arrays (alternate to just opening the
block-special device).

Thanks for the suggestion.

NeilBrown