The ext4 or fourth extended filesystem is a journaling file system for Linux, developed as the successor to ext3.Kernel 2.6.28, containing the ext4 filesystem, was finally released on 25 December 2008.

Features

• The ext4 filesystem can support volumes with sizes up to 1 exabyte and files with sizes up to 16 terabytes.
• Extents are introduced to replace the traditional block mapping scheme used by ext2/3 filesystems. A
• The ext4 filesystem is backward compatible with ext3 and ext2, making it possible to mount ext3 and ext2 filesystems as ext4.
• Subdirectory limit hasbeen raised to 64,000.
• ext4 provides timestamps measured in nanoseconds.

Extents

An extent is basically a bunch of contiguous physical blocks. It basically says "The data is in the next n blocks". For example, a 100 MB file can be allocated into a single extent of that size, instead of needing to create the indirect mapping for 25600 blocks (4 KB per block). Huge files are split in several extents. Extents improve the performance and also help to reduce the fragmentation, since an extent encourages continuous layouts on the disk.

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