Re: ext4 recovery

[James Courtier-Dutton <james.dutton@gmail.com>]

On Tue, 14 Jan 2020 at 21:25, Theodore Y. Ts'o <tytso@mit.edu> wrote:
>
> On Tue, Jan 14, 2020 at 04:03:53PM +0000, James Courtier-Dutton wrote:
> >
> > Say I started with 1 disk using LVM with an ext4 partition.
> > I then added another disk. Added it to the LVM group, expanded the
> > ext4 partition to then fill 2 disks.
> > I then added another disk. Added it to the LVM group, expanded the
> > ext4 partition to then fill 3 disks.
>
> Where you using RAID 0, or some more advanced RAID level?
>
> > One of the disk has now failed.
>
> How has it failed?  It is dead dead dead?  Or are there a large number
> of sector errors?
>
> > Are there any tools available for ext4 that could help recover this?
> > Note, I am a single user, not a company, so there is no money to give
> > to a data recovery company, so I wish to try myself.
>
> How valuable is your data?  The first thing I would recommend, if your
> data is worth it (and only you can make that decision) is to create a
> new RAID set (using larger disks if that helps reduce the price) so
> you can make an block-level image backup using the dd_rescue program.
>
> If you can, then run e2fsck on the backup copy, and then see what you
> can recover from that image set.  This will save time (how much is
> your time worth?) and perhaps increase the amount of data you can
> recover (how much is your data worth?).
>
>                                         - Ted

Hi,

Thank you all for the help.
I have made some progress.
Disk 1 is 100% OK.
Disk 2 did dd_rescue and recovered about 30% of the sectors.
Disk 3 is 100% OK.

I have made images of all of that. LVM worked out what order they went
in, and ext4 even let me mount the LVM volume.
I have managed to recover a lot of the data, so thank you for the hints.
What I would like to do now is find out which files were on Disk 2.
I have found the fiemap  IOCTL that gives me the logical sectors for
the file itself.
How do I convert the value from the IOCTL into a physical sector, or
at least an offset into the LVM volume?
I can then work at that sector 0 to A is on Disk 1,  A to B is on Disk
2, and B to C is on Disk 3.

For example, this is how it is done on btrfs. What is the equivalent on ext4?

$ sudo filefrag -v UEFI_Spec_2_8_final.pdf
Filesystem type is: 9123683e
File size of UEFI_Spec_2_8_final.pdf is 18586279 (4538 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    4537:    9798022..   9802559:   4538:             last,eof
UEFI_Spec_2_8_final.pdf: 1 extent found
$ sudo btrfs-map-logical -l $[9798022*4096] /dev/nvme0n1p7
mirror 1 logical 40132698112 physical 31475654656 device /dev/nvme0n1p7
$ sudo dd if=/dev/nvme0n1p7 bs=1
skip=31475654656 count=64 2>/dev/null | hexdump -C
00000000  25 50 44 46 2d 31 2e 36  0d 25 e2 e3 cf d3 0d 0a  |%PDF-1.6.%......|
00000010  33 30 35 35 31 39 20 30  20 6f 62 6a 0d 3c 3c 2f  |305519 0 obj.<</|
00000020  46 69 6c 74 65 72 2f 46  6c 61 74 65 44 65 63 6f  |Filter/FlateDeco|
00000030  64 65 2f 46 69 72 73 74  20 31 31 37 39 2f 4c 65  |de/First 1179/Le|
00000040


Kind Regards

James