[5.10] [Feature] fuse: introduce write alignment · Pull Request !3533 · anolis/cloud-kernel

bugzilla: https://bugzilla.openanolis.cn/show_bug.cgi?id=9568

Sometimes the file offset alignment needs to be opt-in to achieve the
optimum performance at the backend store.

For example when ErasureCode [1] is used at the backend store, the
optimum write performance is achieved when the WRITE request is aligned
with the stripe size of ErasureCode.  Otherwise a non-aligned WRITE
request needs to be split at the stripe size boundary.  It is quite
costly to handle these split partial requests, as firstly the whole
stripe to which the split partial request belongs needs to be read out,
then overwrite the read stripe buffer with the request, and finally write
the whole stripe back to the persistent storage.

Thus the backend store can suffer severe performance degradation when
WRITE requests can not fit into one stripe exactly.  The write performance
can be 10x slower when the request is 256KB in size given 4MB stripe size.
Also there can be 50% performance degradation in theory if the request
is not stripe boundary aligned.

Besides, the conveyed test indicates that, the non-alignment issue
becomes more severe when decreasing fuse's max_ratio, maybe partly
because the background writeback now is more likely to run parallelly
with the dirtier.

fuse's max_ratio	ratio of aligned WRITE requests
----------------	-------------------------------
70			99.9%
40			74%
20			45%
10			20%

With the patched version, which makes the alignment constraint opt-in
when constructing WRITE requests, the ratio of aligned WRITE requests
increases to 98% (previously 20%) when fuse's max_ratio is 10.

[1] https://lore.kernel.org/linux-fsdevel/20240124070512.52207-1-jefflexu@linux.alibaba.com/T/#m9bce469998ea6e4f911555c6f7be1e077ce3d8b4

类型	指派人员	状态
审查	hsiangkao josephhz JeffleXu	已完成 (0/0人)
测试	小龙已测试通过	已完成 (1/1人)

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