Understanding UNIX / Linux filesystem Superblock

Posted on in Categories File system, Linux, UNIX last updated November 8, 2005

This is second part of “Understanding UNIX/Linux file system”, part I is here. Let us take an example of 20 GB hard disk. The entire disk space subdivided into multiple file system blocks. And blocks used for what?

Unix / Linux filesystem blocks

The blocks used for two different purpose:

  1. Most blocks stores user data aka files (user data).
  2. Some blocks in every file system store the file system’s metadata. So what the hell is a metadata?

In simple words Metadata describes the structure of the file system. Most common metadata structure are superblock, inode and directories. Following paragraphs describes each of them.

Superblock

Each file system is different and they have type like ext2, ext3 etc. Further each file system has size like 5 GB, 10 GB and status such as mount status. In short each file system has a superblock, which contains information about file system such as:

  • File system type
  • Size
  • Status
  • Information about other metadata structures

If this information lost, you are in trouble (data loss) so Linux maintains multiple redundant copies of the superblock in every file system. This is very important in many emergency situation, for example you can use backup copies to restore damaged primary super block. Following command displays primary and backup superblock location on /dev/sda3:
# dumpe2fs /dev/hda3 | grep -i superblock
Output:

Primary superblock at 0, Group descriptors at 1-1
Backup superblock at 32768, Group descriptors at 32769-32769
Backup superblock at 98304, Group descriptors at 98305-98305
Backup superblock at 163840, Group descriptors at 163841-163841
Backup superblock at 229376, Group descriptors at 229377-229377
Backup superblock at 294912, Group descriptors at 294913-294913

Continue reading rest of the Understanding Linux file system series (this is part II):

  • 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?

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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  5. Hi Vivek,

    I have a doubt as how many redundant backups of superblock are created in the file system ? Is it made on every block group or only on initial few block groups ?
    In the filesystem that I created of size 128 MB with 16 block groups, when I run the dumpe2fs command on it, it gives the info on superblocks (These are not the exact wordings of the output ) :
    primary superblock is present on block group 0,
    back up superblock is present on block group 1, 3, 5, 7, 9.

    Similarly for a file system with 256 MB size and 31 block groups, primary superblock is present on block group 0 and backups are present on block groups 1,3,5,7, and 9.

    So my doubt is how does the file system how many backups of the superblock to be created ?
    And the same doubt for block group descriptors as well. Block group descriptors are not present in each block group but only in the block groups where superblock is present.

    Hoping for an earliest response.

    Thanks,
    Swapnil

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  8. To Swapnil,

    You have sparse_super mode so super block and backups are saved in groups 0,1,3^n,5^n,7^n (e.g. 0,1,3,5,7,9,25,27,…). If you take a look at mke2fs code, you’ll see these places where super blocks are saved.

    p.s. this is a very late comment but just wanted to comment..

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