What is the difference between ext3 and ext4 in Linux system?

A file system is a method of organizing files stored on storage devices such as disks. The Linux system can support a variety of currently popular file systems, including: EXT2, EXT3, EXT4, FAT, FAT32, etc. So what are the differences between ext3 and ext4 in Linux systems? The following is a detailed introduction.

　What is the difference between ext3 and ext4 in Linux system?

　1. Larger file system and larger files

ext3: 16TB file system and maximum 2TB file;

　ext4: 1EB file system and maximum 16TB file.

　2. Unlimited number of subdirectories

ext3:32000 subdirectory;

　ext4: unlimited number of subdirectories.

　3. Extents

　ext3: uses indirect block mapping, which is extremely inefficient when operating large files. For example, for a 100MB file, a mapping table of 25,600 data blocks needs to be established in ext3.

　ext4: The concept of Extents is introduced. Each Extent is a set of continuous data blocks. The above file is represented as the file data is stored in the next 25,600 data blocks, which improves a lot of efficiency.

4. Multi-block allocation

The data block allocator of ext3 can only allocate one 4KB block at a time. To write a 100MB file, the data block allocator must be called 25,600 times, while the multiblock allocator of ext4 multiblock
allocator supports allocating multiple blocks in one call. data block.

　5. Delayed allocation

The data block allocation strategy of ext3 is to allocate as soon as possible, while ext4 delays allocation as much as possible until the file is written in the cache before starting to allocate data blocks and write them to the disk, so that the data block allocation of the entire file can be optimized. Paired with the first two features, performance can be significantly improved.

　6. Inode related features

ext3:128byte;

　ext4:256byte.

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