Greetings, I would like to submit a proposal to discuss the design of extsize and forcealign and various open questions around it. ** Background ** Modern NVMe/SCSI disks with atomic write capabilities can allow writes to a multi-KB range on disk to go atomically. This feature has a wide variety of use cases especially for databases like mysql and postgres that can leverage atomic writes to gain significant performance. However, in order to enable atomic writes on Linux, the underlying disk may have some size and alignment constraints that the upper layers like filesystems should follow. extsize with forcealign is one of the ways filesystems can make sure the IO submitted to the disk adheres to the atomic writes constraints. extsize is a hint to the FS to allocate extents at a certian logical alignment and size. forcealign builds on this by forcing the allocator to enforce the alignment guarantees for physical blocks as well, which is essential for atomic writes. ** Points of discussion ** Extsize hints feature is already supported by XFS [1] with forcealign still under development and discussion [2]. After taking a look at ext4's multi-block allocator design, supporting extsize with forcealign can be done in ext4 as well. There is a RFC proposed which adds support for extsize hints feature in ext4 [3]. However there are some caveats and deviations from XFS design. With these in mind, I would like to propose LSFMM topic on: * exact semantics of extsize w/ forcealign which can bring a consistent interface among ext4 and xfs and possibly any other FS that plans to implement them in the future. * Documenting how forcealign with extsize should behave with various FS operations like fallocate, truncate, punch hole, insert/collapse range etc * Implementing extsize with delayed allocation and the challenges there. * Discussing tooling support of forcealign like how are we planning to maintain block alignment gurantees during fsck, resize and other times where we might need to move blocks around? * Documenting any areas where FSes might differ in their implementations of the same. Example, ext4 doesn't plan to support non power of 2 extsizes whereas XFS has support for that. Hopefully this discussion will be relevant in defining consistent semantics for extsize hints and forcealign which might as well come useful for other FS developers too. Thoughts and suggestions are welcome. References: [1] https://man7.org/linux/man-pages/man2/ioctl_xfs_fsgetxattr.2.html [2] https://lore.kernel.org/linux-xfs/20240813163638.3751939-1-john.g.garry@xxxxxxxxxx/ [3] https://lore.kernel.org/linux-ext4/cover.1733901374.git.ojaswin@xxxxxxxxxxxxx/ Regards, ojaswin
