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UnderstandingUNIX/LinuxfilesystemSuperblock

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 space subdivided into multiple file system blocks. And blocks used for what?

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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/sda1 | grep -i superblock
Output:

Primary superblock at 0, Group descriptors at 1-7
 Backup superblock at 32768, Group descriptors at 32769-32775
 Backup superblock at 98304, Group descriptors at 98305-98311
 Backup superblock at 163840, Group descriptors at 163841-163847
 Backup superblock at 229376, Group descriptors at 229377-229383
 Backup superblock at 294912, Group descriptors at 294913-294919
 Backup superblock at 819200, Group descriptors at 819201-819207
 Backup superblock at 884736, Group descriptors at 884737-884743
 Backup superblock at 1605632, Group descriptors at 1605633-1605639
 Backup superblock at 2654208, Group descriptors at 2654209-2654215
 Backup superblock at 4096000, Group descriptors at 4096001-4096007
 Backup superblock at 7962624, Group descriptors at 7962625-7962631
 Backup superblock at 11239424, Group descriptors at 11239425-11239431
 Backup superblock at 20480000, Group descriptors at 20480001-20480007
 Backup superblock at 23887872, Group descriptors at 23887873-23887879

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