Re: undo make-bcache (was: Re: Can't mount an encrypted backing device)

[Coly Li <colyli@suse.de>]

On 2020/1/18 8:22 下午, Clodoaldo Neto wrote:
> On Sat, Jan 18, 2020 at 7:54 AM Clodoaldo Neto
> <clodoaldo.pinto.neto@gmail.com> wrote:
>>
>> On Thu, Jan 16, 2020 at 9:59 PM Coly Li <colyli@suse.de> wrote:
>>>
>>> On 2020/1/17 5:52 上午, Clodoaldo Neto wrote:
>>>>
>>>> Em seg, 13 de jan de 2020 11:19, Coly Li <colyli@suse.de
>>>> <mailto:colyli@suse.de>> escreveu:
>>>>>
>>>>> On 2020/1/13 8:44 下午, Jens-U. Mozdzen wrote:
>>>>>> Hi Coly,
>>>>>>
>>>>>> jumping in here, because I was looking for a way to revert from bcache
>>>>>> to plain device:
>>>>>>
>>>>>> Zitat von Coly Li <colyli@suse.de <mailto:colyli@suse.de>>:
>>>>>>> The super block location of the backing disk is occupied by bcache. You
>>>>>>> cannot mount the file system directly from the backing disk which is
>>>>>>> formated as bcache backing device [...] (bcache offset all I/Os on
>>>>>>> bcache device 4KB behind the requesting
>>>>>>> LBA on backing disk).
>>>>>>
>>>>>> Assuming that no caching device is associated with a backing device (so
>>>>>> the backing device is "clean" as in "containing all data blocks with the
>>>>>> current content"), could one convert the content of a backing device to
>>>>>> a "non-bcached device" by removing the first 4096 octets of the backing
>>>>>> device content?
>>>>>>
>>>>>> Something like "dd if=backingdev of=newdev skip_bytes=4096 ..."?
>>>>>
>>>>> Hi Jens-U,
>>>>>
>>>>> you may try dmsetup to setup a linear device mapper target, and the map
>>>>> table just skipping the first 4KB (bcache superblock area). If you are
>>>>> lucky, I mean the real file system is not corrupted, the created device
>>>>> mapper target can be mounted directly.
>>>>
>>>>
>>>> I'm trying dmsetup but it does not accept anything other than 0 and 0
>>>> at the beginning and end of the table:
>>>>
>>>> # echo '0 3774578672 linear /dev/mapper/backing-device 8' | dmsetup
>>>> create dmb
>>>> device-mapper: reload ioctl on dmb  failed: Invalid argument
>>>> Command failed.
>>>
>>> The above line should work, if 3774578672 is a correct size number in
>>> sectors.
>>
>> I took it from the original map:
>>
>> # dmsetup table /dev/mapper/backing-device
>> 0 3774578672 crypt aes-xts-plain64
>> :64:logon:cryptsetup:7e2c0b40-8dec-4b13-8d00-b53b55160775-d0 0 251:0
>> 32768
> 
> It works like this:
> 
> # echo '0 3774578664 linear /dev/mapper/backing-device 8' | dmsetup create dmb
> 
> But then I can't mount it:
> 
> # mount /dev/mapper/dmb /r
> mount: /r: wrong fs type, bad option, bad superblock on
> /dev/mapper/dmb, missing codepage or helper program, or other error.

It might be my fault, from bcache-tools, it seems the offset is
BDEV_DATA_START_DEFAULT (16 sectors). How about:
# echo '0 3774578656 linear /dev/mapper/backing-device 16' | dmsetup
create dmb


-- 

Coly Li