[linux-lvm] Cannot create PV on /dev/dm-NN (/dev/dm-N works)

[Roger Heflin]

I suggest you should never directly be touching the /dev/dm* devices
via that name.

Always touch the actual named device (/dev/mpath, /dev/vgname/lv  A
given dm device can be a number of other devices (lv,s, mpath, mpath
components, encrypted device, md raid, and probably some others).
>From just the dm* name you really don't know which kind of device it
is, and there is a lot of risk of confusion and/or data loss.

I have debugged situations were someone went in and partitioned all of
their /dev/dm* devices and some of those devices were actually the
LV's and others were they created PV's on other LV's.   Without a few
commands you cannot easily know what the dm* device is, so access it
via the more sensibly named device.



On Wed, Mar 28, 2018 at 3:27 AM, Zdenek Kabelac <zkabelac at redhat.com> wrote:
> Dne 27.3.2018 v 12:38 Michael Fladischer napsal(a):
>>
>> Hi,
>>
>> I'm unable to create PVs on Multipath-Volumes that are available at
>> /dev/dm-NN where N~[0-9] but I can create them on single digit devices
>> like /dev/dm-9:
>>
>> # pvcreate /dev/dm-6
>>    Physical volume "/dev/dm-6" successfully created.
>>
>> # pvcreate /dev/dm-16
>>    Device /dev/dm-16 not found (or ignored by filtering).
>>
>> My Version of LVM is 2.02.168 from Debian 9.
>> The filter ist set to a permissive value:
>>    filter = [ "a|.*|" ]
>>    global_filter = [ "a|.*|" ]
>>
>> Any ideas why two or more digits in the device path can cause it to be
>> filtered by pvcreate?
>>
>
>
>
> Hi
>
>
> lvm2 is detecting  multipath 'component' devices - those are not allowed to
> be used for pvcreate.
>
> You can use  'pvcreate -vvv' to get more information - there will be printed
> the reason why devices are rejected....
>
> Eventually if you can't figure this out yourself - provide this trace in
> attachment.
>
>
> Regards
>
> Zdenek
>
>
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