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

[Ritika Srivastava <ritika.srivastava@oracle.com>]

On 2/28/22, 2:44 PM, "Benjamin Marzinski" wrote:

    > So unless I'm missing something, we'd only really want this for removing
    > a kpartx device, in the case where somehow you have /dev/loopXpY
    > partitions without the LO_FLAGS_PARTSCAN flag set on the disk. That

That's correct. We only want this option so that once PARTSCAN flag is set, 
Kpartx -d can delete /dev/loopXpY.

    > seems like it shouldn't happen in the first place.  Obviously, you
    > showed that it can with parted.  But I would argue that this is a bug in
    > parted.  If parted is creating partitions, it should set
    > LO_FLAGS_PARTSCAN so the partition nodes get cleaned up.
    > I suppose kpartx could check if there are partition devices for the loop
    > device, and if so, it could set LO_FLAGS_PARTSCAN before doing the

Would removing all partition nodes (/dev/loop0pY) on kpartx -d be a better solution.?

    > remove. But setting it unilaterally would just cause it to create an
    > extra set of devices that would only serve to confuse people (and lvm).

-P should be used only when partition scan needs to be enabled - only on need basis.

    > Also, the actual partition /dev/loopXpY will always get removed. It's
    > just the devnode that stays around, and that won't confuse lvm. This

The example below shows that /dev/loop0p1 is not removed which is confusing the LVM.

    > isn't that odd for loop devices. The /dev/loopX devnodes will stay
    > around once you're done with them, regardless of whether you create the
    > loop device will kpartx or losetup.

That's correct.
But loop device when setup with losetup -P option does remove /dev/loopXpY on detach
Hoping to achieve the same functionality in kpartx.


    >> 
    >> // workaround - losetup -P
    >> # kpartx -a -v test.img                                                                             // First kpartx
    >> # 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                                                                         //Second kpartx
    >> # 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.

    > So this example shows exactly why I don't want both /dev/loopXpY and
    > /dev/mapper/loopXpY. Whenever we use your -P option we can run into
    > this situation, right?

In the above example, the first `kpartx -a` could benefit from `-P` option and would remove /dev/loop0p1 on kpartx -d.
This is to avoid having both /dev/loop0p1 and /dev/mapper/loop0p1.

The second `kpartx -a` does not need a partition scan and `-P` option should not be provided.
However, yes, you are right - if it is specified in this case, it would create both /dev/loop0p1 and /dev/mapper/loop0p1 
- which then would have to be deleted and recreated without the `-P` option.
Maybe a warning message here would help - something like `-P` option should be used only while creating new partitions. 

Thanks,
Ritika


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