[linux-lvm] dmsetup says "Device does not exist", though it exists

[Zdenek Kabelac]

Dne 14. 08. 19 v 9:22 Christoph Pleger napsal(a):
> 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 at host:/home/linux# /sbin/dmsetup info -c -o name --noheadings /dev/vg/lv20
>>> Device does not exist.
>>> Command failed
>>>
>>> root at host:/home/linux# lvdisplay -c /dev/vg/lv20
>>>    /dev/vg/lv20:vg:3:1:-1:0:4194304:512:-1:0:-1:253:19
>>>
>>> root at host:/home/linux# mount /dev/vg/lv20 /mnt
>>> root at 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)

Hi

So are you actually trying to access not a 'normal' LV - but an LV under 
snapshot ?

Debian is using it's own modified udev-rules - so I can't really tell without
exactly looking what sort of rules are running there.

You can try to provide   'lvchange -ayvvvv' debug trace when you activate such 
LV and you can enable verification of udev rule processing with lvm.conf option:

         verify_udev_operations = 1

That should 'create'  missing links if they were not created by udev.

NOTE: this is a cruel hack - nothing I'd advice to use normally - since udev 
rules should handle this case normally - but it can give us some hints,
where the issue could be buried-in.

So try above steps and check if the  LV links are in  /dev/vgname dir.

Regards

Zdenek