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

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

[Christoph Pleger]

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?

Regards
   Christoph

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

[Zdenek Kabelac]

Dne 13. 08. 19 v 9:11 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@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?
>

Hi

Have you tried  strace ?

Are you using this only in singly machine with properly running udev ?
Kernel version, lvm version,  distribution... ?


Zdenek

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

[Bernd Eckenfels]

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Hello,

I did not know dmsetup works with lvm file names, did you try to run `dmsetup ls` to see the known devices and the dm compatible name?

Gruss
Bernd


--
http://bernd.eckenfels.net

________________________________
Von: linux-lvm-bounces@redhat.com im Auftrag von Zdenek Kabelac <zkabelac@redhat.com>
Gesendet: Dienstag, August 13, 2019 3:54 PM
An: LVM general discussion and development; Christoph Pleger
Betreff: Re: [linux-lvm] dmsetup says "Device does not exist", though it exists

Dne 13. 08. 19 v 9:11 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@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?
>

Hi

Have you tried strace ?

Are you using this only in singly machine with properly running udev ?
Kernel version, lvm version, distribution... ?


Zdenek

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

[Christoph Pleger]

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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

[-- Attachment #2: strace.txt --]
[-- Type: text/plain, Size: 1152 bytes --]

ioctl(3, DM_VERSION, {version=4.0.0, data_size=16384, flags=DM_EXISTS_FLAG} => {version=4.35.0, data_size=16384, flags=DM_EXISTS_FLAG}) = 0
stat("/dev/vg/lv20", {st_mode=S_IFBLK|0660, st_rdev=makedev(253, 18), ...}) = 0
stat("/dev/mapper/lv20", 0x7ffd1b0ce0d0) = -1 ENOENT (No such file or directory)
open("/dev/mapper", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFDIR|0755, st_size=460, ...}) = 0
getdents(4, /* 23 entries */, 32768)    = 992
stat("/dev/mapper/vg-lv15-real", {st_mode=S_IFBLK|0660, st_rdev=makedev(253, 18), ...}) = 0
close(4)                                = 0
ioctl(3, DM_DEV_STATUS, {version=4.0.0, data_size=16384, name="vg-lv15-real", flags=DM_EXISTS_FLAG} => {version=4.35.0, data_size=16384, name="vg-lv15-real", flags=DM_EXISTS_FLAG}) = -1 ENXIO (No such device or address)
write(2, "Device does not exist.", 22Device does not exist.)  = 22
write(2, "\n", 1
)                       = 1
write(2, "Command failed.", 15Command failed.)         = 15
write(2, "\n", 1
)                       = 1
close(3)                                = 0
exit_group(1)                           = ?
+++ exited with 1 +++

Re: [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@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)

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

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

[Zdenek Kabelac]

Dne 13. 08. 19 v 23:25 Bernd Eckenfels napsal(a):
> Hello,
> 
> I did not know dmsetup works with lvm file names, did you try to run `dmsetup 
> ls` to see the known devices and the dm compatible name?

Hi

Of course it has to work.

lvm2 names are just symlinks to /dev/dm-XXX real device nodes.

If symlinks are missing - something is broken in the setting configuration...


Regards

Zdenek

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

[Christoph Pleger]

Hello,

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

No, /dev/vg/lv20 is a normal logical volume. But /dev/vg/lv15 has been 
under snapshot before.

Now, I created a snapshot manually (before. it was created by an 
automatic backup mechanism) and saw what happened:

1. I created the snapshot with 'lvcreate -s -L 2G /dev/vg/lv15'
2. Afterwards,  a symbolic link /dev/vg/lvol0 existed and pointed to 
/dev/dm-21. Additionally, a link /dev/mapper/vg-lv15-real existed, 
pointing to /dev/dm-19
3. I deleted the snapshot with lvremove
4. Afterwards, /dev/vg/lvol0 had disappeared. /dev/mapper/vg-lv15-real 
was still present, and it did not point to /dev/dm-19 any more, but to 
/dev/dm-18, the same as /dev/vg/lv20

Regards
   Christoph

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

[Christoph Pleger]

Hello,

> and you can enable verification of udev rule
> processing with lvm.conf option:
> 
>         verify_udev_operations = 1

after setting this, I got the following message from lvremove:

Node /dev/mapper/vg-lv15-real was not removed by udev. Falling back to 
direct node removal.

Regards
   Christoph