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 > > > > > # echo '8 3774578664 linear /dev/mapper/backing-device 0' | dmsetup > > create dmb > > device-mapper: reload ioctl on dmb failed: Invalid argument > > Command failed. > > > > I'm not sure about how it works. Is it not 8 sectors for 4k bytes? > > > > It works on my side, my kernel is Linux v5.5-rc2, lvm2 version is > 2.02.180, dmsetup version is 1.02.149. The difference is the device > size, my disk size is much less. > > -- > > Coly Li
