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RE: recovering corrupt file system



Thanks for the pointer.  Turning to my other bad file system, I could use some help interpreting e2fsck.  I have the source and have been looking at various web resources, so  I suppose I could figure this out eventually.

Actually, maybe I should ask a simpler question: should I just run e2fck, accepting its recommendations, and live with the results?  No matter what I do I don't think I can recover any more information.

Here's a little diagram:
media01/root   # LVM logical volume on which the ext4 filesystem resides
VM's sda, sdb, various partitions   # physical volumes making up the media01VG
------ virtual machine above here ---
--- physical machine/ host below here -------------------
media01b.vdi                    # host file backing virtual disk sdb
# note I have made a spare copy of media01b.vdi.
# The file backing virtual sda had no hardware problems.
## various more layers here
physical disk

The physical disk at the bottom is failing. I used (g)ddrescue to copy as much of the media01b.vdi file as I could; the file is about 700G, and there were 2 chunks of 0x1000 bytes that could not be recovered and are now 0 filled.

The basic structure of the virtual disks appears intact: the partition tables are still there and the logical volumes can still be assembled.

If it's worth getting into the details, here's what e2fsck, run inside another VM that has the problems disks temporarily inserted says.  What do the individual block bitmap differences mean?  I'm guessing + and minus indicate whether the block was found in the scan only or in the file system tables on disk one, but I don't know which.  And what do the numbers mean?  Offsets in bytes? sectors? relative to ??

root wheezy02:~# e2fsck -vn /dev/media01-vg/root
e2fsck 1.42.12 (29-Aug-2014)
One or more block group descriptor checksums are invalid.  Fix? no

Group descriptor 465 checksum is 0x5e7a, should be 0xa22b.  IGNORED.
Group descriptor 482 checksum is 0x69eb, should be 0x73a5.  IGNORED.
Group descriptor 485 checksum is 0xbd9b, should be 0x21c9.  IGNORED.
Group descriptor 496 checksum is 0xe550, should be 0x9a62.  IGNORED.
Group descriptor 508 checksum is 0xf4d0, should be 0x2466.  IGNORED.
/dev/media01-vg/root contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Block bitmap differences:  +15243264 +(15511418--15511423) +(15511488--15511551) +(15523264--15523327) +(15812608--15813174) -(15813349--15813503) -(15813632--158\
14054) +(15814656--15815176) +(15815680--15816191) -(15816505--15816703) -(15823872--15824895) -(15850505--15851519) -(15852544--15853567) -(15896583--15898623) +\
(16029735--16029759) +(16029786--16029823) +(16029852--16029855) +(16029884--16031743) -(16261152--16261954) -(16263740--16263743) -16459791 +(16459795--16459799)\
 -(16459808--16459814) -(16459825--16459839) -(16460000--16460026) -(16460288--16460799) +(16668689--16670719) +(17210175--17211391) +17498112 -(17498624--1749913\
5) -(17499262--17500159) +17534976 -(17536000--17537023) -(17953505--17954815) +(18026714--18028543) +(18031327--18032639) -(18032653--18034687) +(18062252--18063\
359) +(18655232--18657173) -(19314688--19316735) -(19331072--19333119) -19406880 -25174048 -(41954376--41955015) -(42999849--42999850) -(42999852--42999859) -(429\
99861--42999862) -(43524128--43546654) -(45621280--45625870) -(45637632--45641520) -46669856 -(48242720--48246756) -(67436544--67446783)
Fix? no

Free blocks count wrong for group #473 (0, counted=134).
Fix? no

## quite a few more Free blocks wrong messages

Free blocks count wrong (48434540, counted=48384585).
Fix? no

Inode bitmap differences:  -4849670
Fix? no

Free inodes count wrong for group #592 (8187, counted=8186).
Fix? no

Free inodes count wrong (16811016, counted=16811015).
Fix? no

Padding at end of block bitmap is not set. Fix? no


/dev/media01-vg/root: ********** WARNING: Filesystem still has errors **********


       56312 inodes used (0.33%, out of 16867328)
          91 non-contiguous files (0.2%)
          53 non-contiguous directories (0.1%)
             # of inodes with ind/dind/tind blocks: 0/0/0
             Extent depth histogram: 51385/43
    19012244 blocks used (28.19%, out of 67446784)
           0 bad blocks
          14 large files

       46167 regular files
        5101 directories
          12 character device files
          25 block device files
           0 fifos


From: darkonc gmail com [darkonc gmail com] on behalf of Stephen Samuel [samuel bcgreen com]
Sent: Thursday, November 19, 2015 8:00 AM
To: Boylan, Ross
Cc: Ext3-users redhat com
Subject: Re: recovering corrupt file system

well, the next place to go, if fsck isn't enough would be to to try debugfs(1)
man debugfs.

On Wed, Nov 18, 2015 at 8:39 PM, Boylan, Ross <Ross Boylan ucsf edu> wrote:
I guess some of the trouble was that the virtual disk was mounted read-only at the VM level.  When I mounted read/write I was able to do fsck, which gave messages about replaying the logs and a couple messages about changing the inode counts (sorry, don't have the exact words).  Then I ran fsck -f, which didn't report any problems.  Then I mounted it, and everything seems OK.

I'm still interested in the general question about how to diagnose and recover from file system errors, since I have another virtual machine that was backed by a failing real disk.
________________________________________
From: Boylan, Ross
Sent: Wednesday, November 18, 2015 4:35 PM
To: Ext3-users redhat com
Subject: recovering corrupt file system

Any recommendations for tools to diagnose and recover problems on an ext4 file system?

In particular:
root jessie01:~# mount -o ro /dev/markov02/root /mnt/markov02
mount: wrong fs type, bad option, bad superblock on /dev/mapper/markov02-root,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.
and e2fsck says
root jessie01:~# e2fsck /dev/markov02/root
e2fsck 1.42.12 (29-Aug-2014)
/dev/markov02/root: recovering journal
Superblock needs_recovery flag is clear, but journal has data.

markov02/root is an LVM volume, built on partitions from 2 disks in a virtual machine.  The initial symptom was that the VM the disks were in originally would only get as far as busybox when it started.  However, I think the filesystem was OK even after that, since it was visible in busybox and in another VM.  I think virt-manager might have overwritten on of the disks because I left "allocate entire disk now" checked when I moved one of the disks between machines.

I'm making copies of the virtual disks now.
Ross Boylan

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--
Stephen Samuel http://www.bcgreen.com  Software, like love,
778-861-7641                              grows when you give it away

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