adding multipath device without reboot?

[Herta Van den Eynde <herta.vandeneynde@cc.kuleuven.be>]

I'm trying to add a new SAN LUN to a system, create a multipath mdadm 
device on it, partition it, and create a new filesystem on it, all 
without taking the system down.

All goes well, up to partitioning the md device:

   # fdisk /dev/md12
   Device contains neither a valid DOS partition table, nor Sun, SGI
   or OSF disklabel
   Building a new DOS disklabel. Changes will remain in memory only,
   until you decide to write them. After that, of course, the
   previous content won't be recoverable.

   The number of cylinders for this disk is set to 8569312.
   There is nothing wrong with that, but this is larger than 1024,
   and could in certain setups cause problems with:
   1) software that runs at boot time (e.g., old versions of LILO)
   2) booting and partitioning software from other OSs
      (e.g., DOS FDISK, OS/2 FDISK)
   Warning: invalid flag 0x0000 of partition table 4 will be
   corrected by w(rite)

   Command (m for help): p

   Disk /dev/md12: 35.0 GB, 35099901952 bytes
   2 heads, 4 sectors/track, 8569312 cylinders
   Units = cylinders of 8 * 512 = 4096 bytes

        Device Boot      Start         End      Blocks   Id  System

   Command (m for help): n
   Command action
      e   extended
      p   primary partition (1-4)
   p
   Partition number (1-4): 1
   First cylinder (1-8569312, default 1):
   Using default value 1
   Last cylinder or +size or +sizeM or +sizeK (1-8569312, default
   8569312):
   Using default value 8569312

   Command (m for help): w
   The partition table has been altered!

   Calling ioctl() to re-read partition table.

   WARNING: Re-reading the partition table failed with error 22:
   Invalid argument.
   The kernel still uses the old table.
   The new table will be used at the next reboot.
   Syncing disks.

I know a reboot will read the new table, but is there any way to clear 
the in-memory table and replace it with the newly written one?
The entire point of this exercise is to be able to add diskspace without 
having to reboot.

Kind regards,

Herta

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