2024년 7월 29일 (월) 오후 11:47, Vincent <laivcf@xxxxxxxxx>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 >
