On 2020/1/18 9:43 下午, Clodoaldo Neto wrote: > On Sat, Jan 18, 2020 at 9:35 AM Coly Li <colyli@xxxxxxx> wrote: >> >> 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@xxxxxxxxx> wrote: >>>> >>>> On Thu, Jan 16, 2020 at 9:59 PM Coly Li <colyli@xxxxxxx> wrote: >>>>> >>>>> On 2020/1/17 5:52 上午, Clodoaldo Neto wrote: >>>>>> >>>>>> Em seg, 13 de jan de 2020 11:19, Coly Li <colyli@xxxxxxx >>>>>> <mailto:colyli@xxxxxxx>> 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@xxxxxxx <mailto:colyli@xxxxxxx>>: >>>>>>>>> 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 > > Still no luck > > # echo '0 3774578656 linear /dev/mapper/backing-device 16' | dmsetup create dmb > # 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. The tricky part is to calculate the correct linear mapping size. For example, I have a 500G hard drive, from fdisk I see its size is 1048576000 sectors. I use it as a bcache backing device and format an Ext4 file system on it. Then I use the following mistaken command line to setup a linear target to skip the first 8 sectors, # echo '0 1048575000 linear /dev/sdc 16' | dmsetup create dmb Then mount /dev/mapper/dmb to /mnt fails, the kmesg hints me, [1884572.477316] EXT4-fs (dm-0): bad geometry: block count 131071998 exceeds size of device (131071875 blocks) So I realize Ext4 file system will check the block device size, so I need to provide an exact accurate length number for the linear target. >From the kmesg I see the correct sector size should be (131071998*8=)1048575984, then I re-create the linear target by, # echo '0 1048575984 linear /dev/sdc 16' | dmsetup create dmb Then I mount /dev/mapper/dmb to /mnt, it works and I have the following line from command 'mount', /dev/mapper/dmb on /mnt type ext4 (rw,relatime) I don't use the encrpyt file system and I don't know the excact detail about it. But I guess, it might because the incorrect length number for your dm linear target as I show by the above example. -- Coly Li
