Re: Adding Device Fails - Why?

["Christian Völker" <cvoelker@knebb.de>]

Hi Qu Wenro,

thanks for debugging this. What I am wondering- why did it work when 
creating the initial device?
For Debian it looks like there is no other version of btrfs-progs 
available, so I used this version (5.10.1-1) to create the btrfs device.

And may I workaround this issue by manually creating a softlink 
/dev/manualdevice to /dev/mappper/crypt_drbd3? The link will be gone 
after reboot but btrfs should then find the device by it's UUID, 
shouldn't it?

Greetings

/KNEBB


Am 02.03.2021 um 02:18 schrieb Qu Wenruo:
>
>
> On 2021/3/2 上午1:24, Christian Völker wrote:
>> Hi,
>>
>> just a little update on the issue.
>>
>> As soon as I omit the encryption part I can easily add the device to the
>> btrfs filesystem. It does not matter if the crypted device is on top of
>> DRBD or directly on the /dev/sdc. In both cases btrs refuses to add the
>> device when a luks-encrypted device is on top.
>>
>> In case I am swapping my setup (drbd on top of encryption) and add the
>> drbd device to btrfs it works without any issues.
>>
>> However, I prefer the other way round- and as the other two btrfs
>> devices are both encryption on top of drbd it should work...
>>
>> It appears it does not like to add a third device-mapper device...
>>
>> Let me know how I can help in debugging. If i have some time I will
>> setup a machine trying to reproduce this.
>
> Got the problem reproduced here.
>
> And surprisingly, it's something related to btrfs-progs, not the kernel.
>
> I just added one debug info in btrfs-progs, it shows:
>
> $ sudo ./btrfs dev add /dev/test/scratch2  /mnt/btrfs
> cmd_device_add: path=dm-5
> ERROR: error adding device 'dm-5': No such file or directory
>
> See the problem?
>
> The path which should be passed to kernel lacks the "/dev/test/" prefix,
> thus it's not pointing to correct path and cause the ENOENT error, since
> there is no "dm-5" in current path.
>
> Thankfully it's already fixed in devel branch with commit 2347b34af4d8
> ("btrfs-progs: fix device mapper path canonicalization").
>
> The offending patch is 922eaa7b5472 ("btrfs-progs: build: fix linking
> with static libmount"), which is in v5.10.1.
>
> You can revert back to v5.10 to workaroud it.
>
>
> TO David,
>
> Would you consider to add a new v5.10.2 to fix the problem? As it seems
> to affect the end user quite badly.
>
> Thanks,
> Qu
>>
>> any ideas otherwise? Let me know!
>>
>> Thanks!
>>
>> /KNEBB
>>