Hello,

>> I have a volume group with 20 logical volumes. Only the last one of 
>> these volumes has a strange problem with dmsetup, shown by these 
>> commands and output on the command line:
>> 
>> root@host:/home/linux# /sbin/dmsetup info -c -o name --noheadings 
>> /dev/vg/lv20
>> Device does not exist.
>> Command failed
>> 
>> root@host:/home/linux# lvdisplay -c /dev/vg/lv20
>>    /dev/vg/lv20:vg:3:1:-1:0:4194304:512:-1:0:-1:253:19
>> 
>> root@host:/home/linux# mount /dev/vg/lv20 /mnt
>> root@host:/home/linux# ls /mnt
>> lost+found data1 data2
>> 
>> That is, dmsetup says "Device does not exist" about a logical volume, 
>> though the volume exists and is operating normally. What is the 
>> possible problem here?
>> 

> 
> Have you tried  strace ?

I attached the relevant lines of the strace output. I am really 
wondering what is happening there:

1. A stat() on /dev/mapper/lv20 is performed, though I requested 
/dev/mapper/vg-lv20
2. A stat() on/dev/mapper/vg-lv15-real is performed. What does this have 
to do with lv20 (after I wrote what is at number 3, I know)
3. I do not even know where /dev/mapper/vg-lv15-real is coming from. I 
created a logical volume named lv15, but none named lv15-real. And 
really, 'ls -l /dev/vg/' does not list lv15-real, but 'ls -l 
/dev/mapper' lists vg-lv15-real and shows that is has the same link 
target /dev/dm-18 as lv20.
4. Though stat() found /dev/mapper/vg-lv15-real, ioctl() says that this 
device does not exist.


> Kernel version, lvm version,  distribution... ?

Kernel Debian amd64 4.9.168-1+deb9u2, LVM version 2.03.02(2), Debian 9 
(stretch)

Regards
   Christoph