Mounting a filesystem with dmsetup

Using TestDisk to repair the filesystem
Forum rules
When asking for technical support:
- Search for posts on the same topic before posting a new question.
- Give clear, specific information in the title of your post.
- Include as many details as you can, MOST POSTS WILL GET ONLY ONE OR TWO ANSWERS.
- Post a follow up with a "Thank you" or "This worked!"
- When you learn something, use that knowledge to HELP ANOTHER USER LATER.
Before posting, please read https://www.cgsecurity.org/testdisk.pdf
Post Reply
Message
Author
hollymcr
Posts: 2
Joined: 18 Nov 2023, 18:21

Mounting a filesystem with dmsetup

#1 Post by hollymcr »

I have a disk removed from an mdadm RAID1 array which mdadm refuses to work with. However testdisk can find the EXT4 filesystem and browse the files.

There are several TB of files to recover which I need to rsync to a new disk so I would like to mount the filesystem using dmsetup but I'm not having any success. (I'm a bit out of my depth: I have very little experience of dmsetup or calculating offsets etc.)

The second dmsetup sample command in the /debug logs - which I am assuming is for the ext4 filesystem (the first being for the RAID partition), when run, gives:
echo "0 7813771264 linear /dev/sdf 266240" | sudo dmsetup create test1
device-mapper: reload ioctl on test1 (253:0) failed: Invalid argument
Command failed.
Various attempts at the maths to calculate the table myself have generally resulted in a successful dmsetup command but I've not then been able to mount the filesystem so I'm obviously getting something wrong. (My best guess was "0 976721360 linear /dev/sdf 258048")

testdisk /debug log:
OS: Linux, kernel 6.2.0-36-generic (#37~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Mon Oct 9 15:34:04 UTC 2) x86_64
Compiler: GCC 11.2
ext2fs lib: 1.46.5, ntfs lib: libntfs-3g, reiserfs lib: none, ewf lib: none, curses lib: ncurses 6.3
Hard disk list
Disk /dev/sdf - 4000 GB / 3726 GiB - CHS 476930 64 32, sector size=4096 - ST4000DM 004-2CV104, FW:0001

Partition table type (auto): Intel
Disk /dev/sdf - 4000 GB / 3726 GiB - ST4000DM 004-2CV104
Partition table type: Intel

Analyse Disk /dev/sdf - 4000 GB / 3726 GiB - CHS 476930 64 32
Geometry from i386 MBR: head=64 sector=32

Raid magic value at 0/8/1
Raid apparent size: 3403207372 sectors
check_MD 1.2
fileserver:2 md 1.x L.Endian Raid 1 - Array Slot : 1 (empty, 0)
get_geometry_from_list_part_aux head=8 nbr=2
Current partition structure:
1 * Linux RAID 0 8 1 476914 55 32 976721408 [fileserver:2]

search_part()
Disk /dev/sdf - 4000 GB / 3726 GiB - CHS 476930 64 32

Raid magic value at 0/8/1
Raid apparent size: 3403207372 sectors
fileserver:2 md 1.x L.Endian Raid 1 - Array Slot : 1 (empty, 0)
Linux RAID 0 8 1 476914 54 17 976721361 [fileserver:2]
md 1.x L.Endian Raid 1 - Array Slot : 1 (empty, 0), 4000 GB / 3725 GiB
get_geometry_from_list_part_aux head=8 nbr=1
Warning: the current number of heads per cylinder is 64 but the correct value may be 8.

Results
* Linux RAID 0 8 1 476914 55 32 976721408 [fileserver:2]
md 1.x L.Endian Raid 1 - Array Slot : 1 (empty, 0), 4000 GB / 3725 GiB

Hint for advanced users: dmsetup may be used if you prefer to avoid rewriting the partition table for the moment:
echo "0 7813771264 linear /dev/sdf 2048" | dmsetup create test0

interface_write()
1 * Linux RAID 0 8 1 476914 55 32 976721408 [fileserver:2]

search_part()
Disk /dev/sdf - 4000 GB / 3726 GiB - CHS 476930 64 32

Raid magic value at 0/8/1
Raid apparent size: 3403207372 sectors
fileserver:2 md 1.x L.Endian Raid 1 - Array Slot : 1 (empty, 0)
Linux RAID 0 8 1 476914 54 17 976721361 [fileserver:2]
md 1.x L.Endian Raid 1 - Array Slot : 1 (empty, 0), 4000 GB / 3725 GiB

recover_EXT2: s_block_group_nr=0/29807, s_mnt_count=49/4294967295, s_blocks_per_group=32768, s_inodes_per_group=8192
recover_EXT2: s_blocksize=4096
recover_EXT2: s_blocks_count 976721360
recover_EXT2: part_size 976721360
Filesystem created: Tue Sep 18 09:20:54 2018
Last mount time: Fri Nov 10 15:52:00 2023
Linux 16 16 1 476930 62 16 976721360
ext4 blocksize=4096 Large_file Sparse_SB, 4000 GB / 3725 GiB

block_group_nr 1

recover_EXT2: "e2fsck -b 32768 -B 4096 device" may be needed
recover_EXT2: s_block_group_nr=1/29807, s_mnt_count=0/4294967295, s_blocks_per_group=32768, s_inodes_per_group=8192
recover_EXT2: s_blocksize=4096
recover_EXT2: s_blocks_count 976721360
recover_EXT2: part_size 976721360
Filesystem created: Tue Sep 18 09:20:54 2018
Linux 16 16 1 476930 62 16 976721360
ext4 blocksize=4096 Large_file Sparse_SB Backup_SB, 4000 GB / 3725 GiB

block_group_nr 3

recover_EXT2: "e2fsck -b 98304 -B 4096 device" may be needed
recover_EXT2: s_block_group_nr=3/29807, s_mnt_count=0/4294967295, s_blocks_per_group=32768, s_inodes_per_group=8192
recover_EXT2: s_blocksize=4096
recover_EXT2: s_blocks_count 976721360
recover_EXT2: part_size 976721360
Filesystem created: Tue Sep 18 09:20:54 2018
Linux 16 16 1 476930 62 16 976721360
ext4 blocksize=4096 Large_file Sparse_SB Backup_SB, 4000 GB / 3725 GiB
Search for partition aborted
get_geometry_from_list_part_aux head=8 nbr=2
get_geometry_from_list_part_aux head=16 nbr=1
Warning: the current number of heads per cylinder is 64 but the correct value may be 8.

Results
Linux RAID 0 8 1 476914 55 32 976721408 [fileserver:2]
md 1.x L.Endian Raid 1 - Array Slot : 1 (empty, 0), 4000 GB / 3725 GiB
Linux 16 16 1 476930 63 32 976721408
ext4 blocksize=4096 Large_file Sparse_SB, 4000 GB / 3725 GiB

Hint for advanced users: dmsetup may be used if you prefer to avoid rewriting the partition table for the moment:
echo "0 7813771264 linear /dev/sdf 2048" | dmsetup create test0
echo "0 7813771264 linear /dev/sdf 266240" | dmsetup create test1

interface_write()
1 P Linux RAID 0 8 1 476914 55 32 976721408 [fileserver:2]


dir_partition inode=2
Linux 16 16 1 476930 63 32 976721408
ext4 blocksize=4096 Large_file Sparse_SB, 4000 GB / 3725 GiB
Directory /
2 drwxr-xr-x 0 0 4096 18-Sep-2018 09:48 .
2 drwxr-xr-x 0 0 4096 18-Sep-2018 09:48 ..
11 drwx------ 0 0 16384 18-Sep-2018 09:20 lost+found
68550657 drwxr-xr-x 1000 114 4096 14-Nov-2023 18:41 shares

interface_write()
1 P Linux RAID 0 8 1 476914 55 32 976721408 [fileserver:2]
simulate write!

write_mbr_i386: starting...
write_all_log_i386: starting...
No extended partition

TestDisk exited normally.
Top
hollymcr
Posts: 2
Joined: 18 Nov 2023, 18:21

Re: Mounting a filesystem with dmsetup

#2 Post by hollymcr »

FWIW I have solved the problem a different way:

Code: Select all

    # Locate the EXT4 partition using grep
    $ sudo grep -obaU -m 1 -e $'\x53\xef' /dev/sdf
    136315960:S▒

    # Mount it
    $ sudo mkdir /mnt/sdf
    $ sudo mount -o loop,offset=$((136315960-1080)) /dev/sdf /mnt/sdf
I don't know how those numbers relate to the values I was getting from testdisk or how to make a dmsetup table from them.
Top
Post Reply

Return to “Filesystem repair”