310ee0902b
We've had reports of significant performance regression of sub-block (unaligned) direct writes due to the added exclusivity restrictions in ext4. The purpose of the exclusivity requirement for unaligned direct writes is to avoid data corruption caused by unserialized partial block zeroing in the iomap dio layer across overlapping writes. XFS has similar requirements for the same underlying reasons, yet doesn't suffer the extreme performance regression that ext4 does. The reason for this is that XFS utilizes IOMAP_DIO_OVERWRITE_ONLY mode, which allows for optimistic submission of concurrent unaligned I/O and kicks back writes that require partial block zeroing such that they can be submitted in a safe, exclusive context. Since ext4 already performs most of these checks pre-submission, it can support something similar without necessarily relying on the iomap flag and associated retry mechanism. Update the dio write submission path to allow concurrent submission of unaligned direct writes that are purely overwrite and so will not require block zeroing. To improve readability of the various related checks, move the unaligned I/O handling down into ext4_dio_write_checks(), where the dio draining and force wait logic can immediately follow the locking requirement checks. Finally, the IOMAP_DIO_OVERWRITE_ONLY flag is set to enable a warning check as a precaution should the ext4 overwrite logic ever become inconsistent with the zeroing expectations of iomap dio. The performance improvement of sub-block direct write I/O is shown in the following fio test on a 64xcpu guest vm: Test: fio --name=test --ioengine=libaio --direct=1 --group_reporting --overwrite=1 --thread --size=10G --filename=/mnt/fio --readwrite=write --ramp_time=10s --runtime=60s --numjobs=8 --blocksize=2k --iodepth=256 --allow_file_create=0 v6.2: write: IOPS=4328, BW=8724KiB/s v6.2 (patched): write: IOPS=801k, BW=1565MiB/s Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com> Reviewed-by: Jan Kara <jack@suse.cz> Link: https://lore.kernel.org/r/20230314130759.642710-1-bfoster@redhat.com Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.