[Bug 1427859] Comment bridged from LTC Bugzilla

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------- Comment From mauricfo at br.ibm.com 2015-03-11 12:13 EDT-------
Hi,

Unfortunately this is not a bug (although it really seems so :).
The rationale..

There are 2 distinct/orthogonal things:
1) Listing the partitions in a disk
2) Creation of the block devices for them (i.e., /dev/something)

They happen to diverge with multipath because of the *option* to choose
the disk-partition separator.

This separator is introduced when the wholedisk device name's last
character is a number/digit (i.e., /dev/mpath0), so you can
differentiate 'mpath1 partition 1' from 'mpath 11 wholedisk' (that,
without a separator, are 'mpath11').

It is relatively unknown because the popular wholedisk device names are
sd[a-z] / vd[a-z] / LVM volume that ends in [a-z]).

Back to the 2 things, when a tool reads a disk's partition table, and then prints it, the *tool* chooses the format.
Notice that /listing/ has *nothing* to do with the actual block devices (/dev/something) of the disk, and must not, because the partitions may not exist (e.g., you're partitioning the disk for the first time, and listing the partitions you created).

In summary, there's no rule saying "print it as it exists/or would exist" in /dev.
With sd[a-z] et al, there's usually no mismatch, because there's no separator.

Notice kpartx has the -p option:
-p     set device name-partition number delimiter

kpartx is one of the tools that gives you that option (which is great, but actually leads to the "problems" noticed)
Most other tools just hard-code 'p' and that's all.
And, on kparx, the /default/  is 'p'., but you can choose.

So, if you list multipath partitions w/ kpartx,  choose the separator:

# kpartx -l -p SEPARATOR /dev/mapper/mpath0
mpath0SEPARATOR1 : 0 14336 /dev/mapper/mpath0 2048
mpath0SEPARATOR2 : 0 64239616 /dev/mapper/mpath0 16384
mpath0SEPARATOR3 : 0 2850816 /dev/mapper/mpath0 64256000

Then, we get to thing #2, the creation of the block devices.

On multipath, again, this is different from other/non-multipath block
devices, because there must be some more talk with the kernel (to create
the device mappings, I guess, and assign them to a given block device,
/dev/something).

This is done with kpartx. See its manpage:

This  tool,  derived  from util-linux' partx, reads partition tables on
specified device  and  create  device  maps  over  partitions  segments
detected.  It  is  called  from  hotplug  upon device maps creation and
deletion.

So, it reads the partition table (which just says 'partition 1, 2, 3'; no separator informed, of course),
and then (different thing) creates block devices in /dev.

This also uses the '-p' option to choose a separator.
If you look at the udev rules, which create the block devices when a multipath disk is detected (i.e., run 'multipath'):

$ grep -H -- -part /lib/udev/rules.d/*kpartx*
/lib/udev/rules.d/95-kpartx.rules:        SYMLINK+="disk/by-id/$env{DM_TYPE}-$env{DM_NAME}-part$env{DM_PART}"
/lib/udev/rules.d/95-kpartx.rules:        RUN+="/sbin/dmsetup ls --target multipath --exec '/sbin/kpartx -a -p -part' -j %M -m %m"
/lib/udev/rules.d/95-kpartx.rules:        RUN+="/sbin/kpartx -a -p -part /dev/$kernel"

Which just says:  use '-part' when creating the block devices for
multipath partitions on /dev/mapper.

Finishing..

The bug title is "Need a common format for partition listing using
kpartx,fdisk.distrib and ls -l "

That's mistaken, because "ls -l" is not listing partitions, it is listing "block devices" in /dev/mapper/.
Sorry, but that's it. :)

It just happens to be consistent w/ partitioning tools for non-multipath
partitions for a coincidence / no need for disk-partitoin separator.
That's all.

And, BTW, *if* you want to *list* existing multipath partitions  in a way that's coherent w/ what's in /dev/mapper,
you have to find what's the separator adopted for udev (in this case, "-part"), and use it in "kpartx -l -p -part".

$ sudo kpartx -l -p -part /dev/mapper/mpath0
mpath0-part1 : 0 14336 /dev/mapper/mpath0 2048
mpath0-part2 : 0 64239616 /dev/mapper/mpath0 16384
mpath0-part3 : 0 2850816 /dev/mapper/mpath0 64256000

But remember, those are different things :)

If some tooling broke because of that, it's not generic enough; and must be fixed.
(for reference, libparted has to check whether a partition/device is on device-mapper for listing its name.)

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Title:
  ISST-SAN:Need a common format for partition listing using
  kpartx,fdisk.distrib and ls -l

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