Linux file system superblock

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 1-1Backup 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