Re: exfat: slow write performance

[Namjae Jeon <linkinjeon@kernel.org>]

2024년 7월 29일 (월) 오후 11:47, Vincent <laivcf@gmail.com>님이 작성:
>
> I've found that write performance of exFAT is a lot lower than other
> filesystems on our setup with a fast NVMe SSD:
>
> - Kernel: 6.10.0-364.vanilla.fc40.x86_64
> - Test cmd: fio -name=fioWr -ioengine=libaio -directory=<mount point>
> -blocksize=1M -readwrite=write -filesize=10G -end_fsync=1 -numjobs=8
> -direct=1 -group_reporting
> - Benchmarks:
>     - exFAT: Direct I/O: ~1370 MiB/s (Buffered: ~1250 MiB/s)
> (mkfs.exfat -c 1M -b 1M <device>)
>     - ext4: Direct I/O: ~2230 MiB/s (Buffered: ~2150 MiB/s)
>     - xfs: Direct I/O: ~2220 MiB/s (Buffered: ~2200 MiB/s)
>
> I found that direct I/O is disabled for most of these writes in exFAT,
> code responsible is "exfat_direct_IO()" in "fs/exfat/inode.c". As the
> written file is being expanded it is falling back to normal buffered
> write. The relevant code also has the following FIXME comment
> (inherited from the fat driver):
>
> /*
> * FIXME: blockdev_direct_IO() doesn't use ->write_begin(),
> * so we need to update the ->i_size_aligned to block boundary.
> *
> * But we must fill the remaining area or hole by nul for
> * updating ->i_size_aligned
> *
> * Return 0, and fallback to normal buffered write.
> */
>
> I have tried working around this problem by preallocating the file. I
> see good write speeds in the direct I/O case (matching ext4 ~2200
> MiB/s, Buffered: ~1700 MiB/s), the problem is that preallocation is
> slow since native fallocate is not supported.
>
> What would the maintainers recommend as the best course of action for
> us here? I have noticed [1] that FALLOC_FL_KEEP_SIZE was implemented
> in fat, so perhaps that would be simple to work on first?
It is difficult to implement due to exfat on-disk layout constraints.
>
> Does anyone with more experience in the exFAT driver know the full
> reasons behind the direct I/O disabling, and what it would take to
> support direct I/O during file extension? Perhaps recent changes may
> have made fixing this simpler?
I suggest you try direct IO write after extending the file size you
want with ftruncate.
>
> Thanks.
>
> 1: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/fat?id=b13bb33eacb7266d66a3adf03adaa0886d091789
>