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Understanding UNIX / Linux filesystem Superblockby NIXCRAFT on NOVEM BER 8 , 2 0 0 5 · 1 4 COM M ENT S · L AST UPDAT ED SEPT EM BER 2 5 , 2 0 0 7

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

Unix / Linux filesystem blocksThe 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 metadatastructure are superblock, inode and directories. Following paragraphs describes each of them.

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

File system typeSizeStatusInformation about other metadata structures

If this information lost, you are in trouble (data loss) so Linux maintains multiple redundantcopies of the superblock in every file system. This is very important in many emergencysituation, 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 1-1Backup superblock at 32768, Group descriptors at 32769-32769Backup superblock at 98304, Group descriptors at 98305-98305Backup superblock at 163840, Group descriptors at 163841-163841Backup superblock at 229376, Group descriptors at 229377-229377Backup 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 superblockPart II - Understanding Linux superblockPart III - An example of Surviving a Linux Filesystem Failures

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Part III - An example of Surviving a Linux Filesystem FailuresPart IV - Understanding filesystem InodesPart V - Understanding filesystem directoriesPart VI - Understanding UNIX/Linux symbolic (soft) and hard linksPart VII - Why isn't it possible to create hard links across file system boundaries?

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{ 14 comments… read them below or add one }

Anonymous May 31, 2006 at 5:13 pm

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R E L A T E D P O S T S

Understanding UNIX / Linux filesystemdirectories

Why isn’t it possible to create hardlinks across file system boundaries?

Understanding UNIX / Linux filesystemInodes

How to: Linux / UNIX Delete or RemoveFiles With Inode Number

Understanding UNIX / Linux FileSystem

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Anonymous

# dumpe2fs /dev/hda3grep -i superblockabove command is not running here |(pipe) is missing, anyway

I am getting different output as you mentioned[root@sachan /root]# dumpe2fs /dev/hda1 | grep -i superblockdumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09[root@sachan /root]#

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nixcraft May 31, 2006 at 9:59 pm

Opps pipe was missing, thanks for heads up.

Hmm it should give the output.

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Kristian May 5, 2007 at 4:33 pm

It should be mentioned that dumpe2fs only is for the ext2/ext3 file systems.

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bipul August 1, 2007 at 8:14 am

Hello Unix Teamhow are you

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Understanding UNIX / Linux symbolic(soft) and hard links

How To Measure Linux Filesystem I/OPerformance With iozone

Surviving a Linux Filesystem Failures

The importance of Linux partitions

Mount a Linux filesystem on a SANfrom multiple nodes at the same time

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Swapnil August 31, 2007 at 6:05 am

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 dumpe2fscommand on it, it gives the info on superblocks (These are not the exact wordings of theoutput ) :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 ispresent 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 notpresent 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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Sunil April 18, 2008 at 6:40 am

Hey this command id not working on my system…

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[sunilv@lnx202-28 ~]$ dump2fs /u/sunilv | grep -i superblockdump2fs: Command not found.

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rakesh September 13, 2011 at 7:52 am

dude.. commands is dump2efs

u used dump2fs

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vivek April 18, 2008 at 10:46 am

Use full path /sbin/dumpe2fs, you are logged in as regular user and /sbin and /usr/sbin not inPATH settings.

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c November 24, 2008 at 6:10 am

To Swapnil,

You have sparse_super mode so super block and backups are saved in groups0,1,3 n̂,5 n̂,7 n̂ (e.g. 0,1,3,5,7,9,25,27,…). If you take a look at mke2fs code, you’ll see theseplaces where super blocks are saved.

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

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Govindasamy C January 28, 2009 at 5:25 am

Its working fine for mesunil i think you did not give space after the command dumpe2fs

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sox May 24, 2011 at 1:37 am

what is meant by offset information in a filesystem?

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saikumar July 14, 2011 at 3:59 am

is it possible to install linux in personal lappies? core2duo processor( dell laptop)…is it possible even vmware workstation is installed in laptops….

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lanh May 17, 2012 at 8:06 am

Good content; I like how you explain this topic very simply, as linux is typically hard to grasp.

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Aravind October 3, 2012 at 7:00 am

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Very fine . . . . . . .u

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Tagged as: internal data structures, linux f ilesystem, metadata structure, superblock, unix f ilesystem

P R E V I O U S P O S T : Understanding UNIX / Linux File System

N E X T P O S T : Surviving a Linux Filesystem Failures

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