implementation of Ext4

technical details about the implementation of Ext4:

Journaling:
Ext4 uses a write-ahead logging (WAL) system for journaling. This means that before any changes are made to the file system, they are first written to the journal. The journal is a circular buffer in the file system that stores information about the changes that have been made, including metadata changes, file creations, and file deletions. When the file system is mounted, any changes that were not yet written to the file system are replayed from the journal to ensure consistency.

Inode Structure:
Inodes in Ext4 are 256 bytes in size and contain information about files and directories. The first 12 block pointers in the inode are direct block pointers, which point to the first 12 data blocks of the file. For larger files, Ext4 uses single, double, and triple indirect blocks to store additional block pointers. In addition to file data, inodes also store metadata such as ownership, permissions, timestamps, and a checksum to ensure data integrity.

Block Allocation:
Ext4 uses a block group structure to manage the allocation of storage space on the file system. Each block group contains a fixed number of blocks, and includes a block bitmap that indicates which blocks are free and which are allocated. To minimize fragmentation, Ext4 uses a technique called delayed allocation, which defers the allocation of blocks until they are actually needed. This can help to reduce the amount of fragmentation on the file system and improve performance.

Extents:
Ext4 also includes support for extents, which are contiguous ranges of data blocks. This allows Ext4 to more efficiently manage large files, since a single extent can represent a large number of contiguous blocks. Extents can be stored in the inode itself, or in a separate extent tree structure that allows for more efficient allocation and management of large files.

Journal Checksumming:
Finally, Ext4 includes support for journal checksumming, which helps to ensure data integrity in the journal itself. The journal is divided into multiple blocks, each of which includes a checksum that can be used to verify the integrity of the data in the block. This can help to prevent data corruption in the journal, which can in turn help to ensure the consistency and reliability of the file system as a whole.