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Linux-File-System

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1 Core Concepts

  • Inode: Data structures that store metadata about files, such as file size, ownership, permissions, and timestamps, but not the file name or data
  • Superblock: A metadata structure that contains information about the filesystem as a whole, including its size, the size of the blocks, the number of inodes, and pointers to other metadata structures
  • Mounting: The process of attaching a filesystem to the directory tree at a specified mount point, allowing access to the files and directories in that filesystem
  • Permissions and Ownership: Each file and directory has associated permissions (read, write, execute) and ownership (user, group, others), controlling access and modifications
  • File Types: Regular files, directories, symbolic links, special files (character and block devices), sockets, and named pipes (FIFOs)
  • Hard Links: Multiple directory entries that reference the same inode
  • Symbolic Links (Symlinks): Special files that reference another file or directory by path
  • Filesystem Types: Various types of filesystems supported by Linux, such as ext4, XFS, Btrfs, and others, each with its own features and use cases
  • Virtual File System (VFS): An abstraction layer that provides a common interface for different filesystem types, allowing uniform access to different storage devices and filesystems
  • Block Devices and Block Size: Storage devices (like hard drives and SSDs) managed in blocks, which are the smallest unit of data transfer
  • Journal：A feature in journaling filesystems (e.g., ext3, ext4)
  • Filesystem Checks and Repair: Utilities like fsck to check and repair filesystem integrity

1.1 Ext Filesystem

  • Superblock:
    • Stores critical data structures of the file system, including its size, block size, number of free blocks, and inodes
    • Read into memory when the file system is mounted
  • Block Group:
    • The file system is divided into several block groups, each containing a fixed number of blocks
    • Each block group includes data blocks, an inode table, a block bitmap, and an inode bitmap
  • Data Block:
    • Blocks used to store the actual file data
    • Block sizes are typically 1KB, 2KB, 4KB, or 8KB
  • Inode (Index Node):
    • Stores metadata about files and directories, including file type, permissions, owner, size, timestamps, and pointers to data blocks
    • Each file and directory has a unique inode
  • Block Bitmap:
    • Records which data blocks in a block group are used and which are free
  • Inode Bitmap:
    • Records which inodes in a block group are used and which are free
  • Inode Table:
    • An array containing all the inodes in the file system, with each inode representing a file or directory’s metadata
  • Directory Entry:
    • Links file names in a directory to their corresponding inode numbers
  • Journal:
    • Available in Ext3 and Ext4, it logs metadata operations to enhance file system reliability and recovery
  • Extended Attributes:
    • Allow users and applications to add extra metadata to files
  • Disk Quotas:
    • Control the amount of disk space and number of inodes that users and groups can use in the file system
  • Reserved Blocks:
    • Typically reserve some blocks for the system administrator to prevent the file system from being completely filled by regular users, which could affect system operations

2 Trouble-shooting

2.1 inode

Observations:

  • The more the number of inodes, the lower the reading performance will be, and read performance can be significantly affected when number of inode reaches million level

Tools:

  • df -ih
  • ls -i

2.2 superblock

Tools:

  • sudo dumpe2fs /dev/sda | grep -i superblock

3 Reference