Surviving a Linux Filesystem Failures

When you use term filesystem failure, you mean corrupted filesystem data structures (or objects such as inode, directories, superblock etc. This can be caused by any one of the following reason:

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* Mistakes by Linux/UNIX Sys admin
* Buggy device driver or utilities (especially third party utilities)
* Power outage (very rarer on production system) due to UPS failure
* Kernel bugs (that is why you don’t run latest kernel on production Linux/UNIX system, most of time you need to use stable kernel release)

Due to filesystem failure:

  • File system will refuse to mount
  • Entire system get hangs
  • Even if filesystem mount operation result into success, users may notice strange behavior when mounted such as system reboot, gibberish characters in directory listings etc

So how the hell you are gonna Surviving a Filesystem Failures? Most of time fsck (front end to ext2/ext3 utility) can fix the problem, first simply run e2fsck – to check a Linux ext2/ext3 file system (assuming /home [/dev/sda3 partition] filesystem for demo purpose), first unmount /dev/sda3 then type following command :
# e2fsck -f /dev/sda3
Where,

  • -f : Force checking even if the file system seems clean.

Please note that If the superblock is not found, e2fsck will terminate with a fatal error. However Linux maintains multiple redundant copies of the superblock in every file system, so you can use -b {alternative-superblock} option to get rid of this problem. The location of the backup superblock is dependent on the filesystem’s blocksize:

  • For filesystems with 1k blocksizes, a backup superblock can be found at block 8193
  • For filesystems with 2k blocksizes, at block 16384
  • For 4k blocksizes, at block 32768.

Tip you can also try any one of the following command(s) to determine alternative-superblock locations:
# mke2fs -n /dev/sda3
OR
# dumpe2fs /dev/sda3|grep -i superblock
To repair file system by alternative-superblock use command as follows:
# e2fsck -f -b 8193 /dev/sda3

However it is highly recommended that you make backup before you run fsck command on system, use dd command to create a backup (provided that you have spare space under /disk2)
# dd if=/dev/sda2 of=/disk2/backup-sda2.img

If you are using Sun Solaris UNIX, see howto: Restoring a Bad Superblock.

Please note that things started to get complicated if hard disk participates in software RAID array. Take a look at Software-RAID HOWTO – Error Recovery. This article/tip is part of Understanding UNIX/Linux file system series, Continue reading rest of the Understanding Linux file system series (this is part III):

  • Part I – Understanding Linux superblock
  • Part II – Understanding Linux superblock
  • Part III – An example of Surviving a Linux Filesystem Failures
  • Part IV – Understanding filesystem Inodes
  • Part V – Understanding filesystem directories
  • Part VI – Understanding UNIX/Linux symbolic (soft) and hard links
  • Part VII – Why isn’t it possible to create hard links across file system boundaries?

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I'm Vivek Gite, and I write about Linux, macOS, Unix, IT, programming, infosec, and open source. Subscribe to my RSS feed or email newsletter for updates.

33 comments… add one
  • Anonymous Feb 6, 2006 @ 5:10

    I think you meant your dd command to have dd if=… of=…. (the of is missing). cheers, DrD

  • πŸ›‘οΈ Vivek Gite (Author and Admin) nixcraft Feb 6, 2006 @ 13:04

    Typo is corrected; Thanks for the heads up!

  • Thomas Scott Jan 29, 2007 @ 3:56

    Its easy to trash the old fs and
    create a new one , because data
    is never “hooked” to a particular
    fs !
    LiLo , LoadLin et al will still
    boot linux fine .
    Its easy cause the IDE HDD is
    much easier to figure , there
    is no need for partitions , nor
    any other “dependencies” .
    And drivers are much smaller .

    The reason we dont improve ,
    is someones paycheck is in it ,
    to keep it complicated .
    Its not unlike what Bill Gates
    did to USB …

  • Dan Oct 25, 2007 @ 13:27

    That works great IF your system will give you a terminal window. If you don’t get that far, you are SOL.

  • Igor Pavkovic Dec 13, 2008 @ 9:55

    If I understood correctly there are typos within the following block:
    || The location of the backup superblock is dependent on the filesystem’s blocksize:
    ||
    || * For filesystems with 1k blocksizes, a backup superblock can be found at block 8193
    || * For filesystems with 2k blocksizes, at block 16384
    || * For 4k blocksizes, at block 32768.
    If first block is moved by one (8192 +1) then others must be moved by one also (16384 +1 and 32768 +1) , therefore backup superblock for 2k and 4k block sizes must be at 16385 and 32769.
    Is that right?

  • maxcomx Aug 30, 2009 @ 14:09

    can’t read superblock

    max@max-laptop:~$ sudo fsck -f /dev/sdd1
    fsck 1.41.4 (27-Jan-2009)
    dosfsck 3.0.1, 23 Nov 2008, FAT32, LFN
    There are differences between boot sector and its backup.
    Differences: (offset:original/backup)
    65:01/00
    1) Copy original to backup
    2) Copy backup to original
    3) No action
    ? 3
    Both FATs appear to be corrupt. Giving up.
    max@max-laptop:~$

    Hummm… WTF… what can i do.

  • Rod Nov 28, 2009 @ 17:09

    “That works great IF your system will give you a terminal window. If you donÒ€ℒt get that far, you are SOL.”

    That’s what Live CDs are for… πŸ˜‰

    Thanx for this page-I was able to completely recover a borked system. A great resource!

  • snevi Dec 3, 2009 @ 9:12

    Thank u very much for this tips!! it helps a lot to recover a main disc in my server!!!
    thanks.

  • justin Jan 25, 2010 @ 10:04

    Thanks man! Recoverd Files from my HDD which i thought was broken!
    i owe you a beer.

  • j01z Apr 18, 2011 @ 13:43

    Thanks a lot! just recovered my baby’s pictures, you’re now her fairy godfather ;p

  • victor elorza May 16, 2011 @ 2:18

    hello!
    every command i run results in this message:
    e2fsck: attempt to read block form filesystem resulted in short read while trying to open /dev/sda1. Could this be a zero-length partition?

    i can determine alternative superblock locations but the command e2fsck -f -b 32768 /dev/sda1 + with other superblock locations doesn’t work.
    please help thanks !!

  • Miguel May 24, 2011 @ 7:21

    Hi,

    I tried some data recovery programs and they all found the files on /dev/sdc3 BUT

    ANY attempt to “fsck -f -b -y /dev/sdc3” results in

    /: Attempt to read block from filesystem resulted in short read while reading block 525

    /: Attempt to read block from filesystem resulted in short read reading journal superblock

    fsck.ext2: Attempt to read block from filesystem resulted in short read while checking ext3 journal for /

    any Idea before I jump out of the window ?

    many thanks

  • Miguel May 24, 2011 @ 7:22

    oops…

    fsck -f -b “any superblock location” -y /dev/sdc3

  • Fabio Souza Aug 28, 2011 @ 16:54

    Thanks man, you save my life… God bless you!

  • Alec Oct 24, 2011 @ 1:12

    I have nothing to add except a “thank you”. Easily-comprehensible tutorials are hard to come by.

  • Ben Oct 31, 2011 @ 0:37

    This post saves the world

  • Randy Kramer Mar 6, 2012 @ 14:31

    Several comments:
    * I was most interested in learning more about a superblock, and that part of the series was very helpful to me.
    * I haven’t started digging into it, but I’d like to learn more about how a corrupted filesystem is fixed, my specific question is, are there circumstances in which a filesystem is fixed by deleting all or part of a file? (I know this was done in early versions of Dos and Windows, but I haven’t done much with either in the last 12 years, maybe that is no longer the case.) But I want to find out if that is, or ever was, the case in Linux.
    * Your tutorials are very well written, but you have some typos / grammos. If you’re interested, I can send you (sooner or later) copies of the content with the errors I see fixed. (Just as one example, the title of this page has a problem–you’re mixing singular and plural stuff, the title should either be: “Surviving Linux Filesystem Failures” (all plural) or “Surviving a Linux Filesystem Failure” (all singular).

  • pramod a g May 3, 2012 @ 14:57

    A million thanks man i was able to recover my corrupted Ubuntu.

  • slobodan_hb May 16, 2012 @ 5:45

    Thanx

  • bb Oct 25, 2012 @ 17:03

    Thanks for this tutorial.

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