Changing filesystem resize patterns

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In a filesystem session at the 2022 Linux Storage, Filesystem, Memory-management and BPF Summit (LSFMM), Ted Ts'o brought up the subject of filesystems that get resized frequently and whether the default parameters for filesystem creation should change as a result. It stems from a conversation that he had with XFS developer Darrick Wong, who is experiencing some of the same challenges as ext4 in this area. He outlined the problem and how it comes about, then led the discussion on ways to perhaps address it.

Linux filesystems were generally designed to support being resized, but the expectation is that they start as a fairly large filesystem and then big chunks are added periodically. Filesystem data structures are sized and created based on how big the filesystem being created will be. He gave the example of RAID array using md-raid that gets a new disk, then the filesystem is resized to take advantage of it. That new disk may be a substantial fraction of the size of the existing filesystem, but the filesystem is already rather large.

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