ext4 online defragmentation

ext4 greatly extends the filesystem size to 1024PB compared to 16TB in ext3, and it is capable of storing many huge files. Previous study has shown that fragmentation can cause performance degradation for such large filesystems, and it is important to allocate blocks as contiguous as possible to improve I/O performance.

In this paper, the design and implementation of an online defragmentation extension for ext4 is proposed. It is designed to solve three types of fragmentation, namely single file fragmentation, relevant file fragmentation, and free space fragmentation. This paper reports its design, implementation, and performance measurement results.

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