[dm-devel] [PATCH] kpartx: Add -P option for partition scanning

[Ritika Srivastava]

Ben,

Thank you for reviewing this.

On 2/22/22, 11:32 AM, "Benjamin Marzinski" wrote:

   > And I'm trying to figure out why you want to create /dev/loop0p1 if you
   > already have /dev/mapper/loop0p1?  The ability of kpartx to create
   > partitioned loop devices predates the existance "kpartx -P". So perhaps
   > the better question is, why do you want to run

   > # kpartx -av test.img

   > instead of just running

   > # loasetup -fP test.img

   > They both get you partition devices, and you don't need both of them.
   > Or do you, for some reason I don't understand?

My previous update may not have been clear regarding the supporting use case behind this patch.
It highlighted what the -P option would add to kpartx.

The idea is to avoid having both /dev/loop0p1 and /dev/mapper/loop0p1 and support the following use case - 
After detaching via kpartx -d, /dev/loop0p1 is still present which on subsequent kpartx -a operation results in both /dev/loop0p1 and /dev/mapper/loop0p1 being present.

Yes, you are right, workaround could be to use losetup -P the first time around when creating the loop device.
Though, having this functionality within kpartx would be nice since kpartx is also used extensively.

// workaround - losetup -P
# kpartx -a -v test.img
# ls -l /dev/loop0*
brw-rw----. 1 root disk  7,   0 Feb 22 20:05 /dev/loop0

# parted -a none -s /dev/loop0 mkpart primary 64s 100000s
# parted -a none -s /dev/loop0 set 1 lvm on
# kpartx -d test.img  
# ls -l /dev/loop0*
brw-rw----. 1 root disk   7,   0 Feb 22 20:05 /dev/loop0
brw-rw----. 1 root disk 259,   0 Feb 22 20:05 /dev/loop0p1

# kpartx -av test.img
# ls -l /dev/mapper/loop0*
lrwxrwxrwx. 1 root root       7 Feb 22 20:53 loop0p1 -> ../dm-2

# pvcreate /dev/mapper/loop0p1
# pvscan
  WARNING: Not using device /dev/loop0p1 for PV <UUID>.
  WARNING: PV <UUID> prefers device /dev/mapper/loop0p1 because device is in dm subsystem.
--                
Thanks,
Ritika