On Sat, Jul 23, 2016 at 2:37 AM, Hendrik Friedel <hend...@friedels.name> wrote: > Hello, > > this morning I had to face an unusual prompt on my machine. > > I found that the partition table of /dev/sda had vanished. > > I restored it with testdisk. It found one partition, but I am quite sure > there was a /boot partition in front of that which was not found. > > Now, running btrfsck fails: > > root@homeserver:~# fdisk -l /dev/sda > > WARNING: GPT (GUID Partition Table) detected on '/dev/sda'! The util fdisk > doesn't support GPT. Use GNU Parted. > > > Disk /dev/sda: 120.0 GB, 120034123776 bytes > 255 heads, 63 sectors/track, 14593 cylinders, total 234441648 sectors > Units = sectors of 1 * 512 = 512 bytes > Sector size (logical/physical): 512 bytes / 512 bytes > I/O size (minimum/optimal): 512 bytes / 512 bytes > Disk identifier: 0x00000000 > > Device Boot Start End Blocks Id System > /dev/sda1 * 1026048 234440703 116707328 83 Linux > root@homeserver:~# btrfsck /dev/sda1 > checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 > checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 > checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 > checksum verify failed on 20987904 found E4E3BDB6 wanted 00000000 > bytenr mismatch, want=20987904, have=0 > Couldn't read chunk root > Couldn't open file system > > Is there a way to let btrfs search for the start of the partiton? > > I do have a backup; thus it is not fatal. But some data on the disk is more > recent than my back up (of course)
Something like this: [root@f24s ~]# dd if=/dev/sda | hexdump -C | egrep '5f 42 48 52 66 53 5f' 00110040 5f 42 48 52 66 53 5f 4d 8d 4f 04 00 00 00 00 00 |_BHRfS_M.O......| 04100040 5f 42 48 52 66 53 5f 4d 8d 4f 04 00 00 00 00 00 |_BHRfS_M.O......| Since the magic appears 0x40 after the start of the super, and there's a 64KiB, or 0x10000, pad for a bootloader between the start of the partition and the start of the superblock, subtract 0x10040 to find the start of the partition in bytes. 0x110040 - 0x10040 = 0x100000 = 1048576 Divide by 512 to find the LBA to start the partition at 1048576 / 512 = 2048 And in fact that's what my partition is set to start at... [root@f24s ~]# gdisk -l /dev/sda GPT fdisk (gdisk) version 1.0.1 Partition table scan: MBR: protective BSD: not present APM: not present GPT: present Found valid GPT with protective MBR; using GPT. Disk /dev/sda: 1953525168 sectors, 931.5 GiB Logical sector size: 512 bytes Disk identifier (GUID): 3D9880B5-D4DF-41F8-9EFF-40155A1D3DD5 Partition table holds up to 128 entries First usable sector is 34, last usable sector is 1953525134 Partitions will be aligned on 2048-sector boundaries Total free space is 2014 sectors (1007.0 KiB) Number Start (sector) End (sector) Size Code Name 1 2048 377489407 180.0 GiB 8300 fedora-root Note that Btrfs silently tolerates the end of the partition being either shorter or longer than the file system claims it is, so to find the proper ending you'll have to first guess, so that you can point btrfs-show-super at the newly established partition, and then look for dev_item.total_bytes. In my case: dev_item.total_bytes 193273528320 193273528320 / 512 = 377487360 377487360 + 2048 - 1 = 377489407 -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
