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Commit7e55c60acf("md/raid5: Pivot raid5_make_request()") changed the order in which requests for underlying disks are created. Since for large sequential IO adding of requests frequently races with md_raid5 thread submitting bios to underlying disks, this results in a change in IO pattern because intermediate states of new order of request creation result in more smaller discontiguous requests. For RAID5 on top of three rotational disks our performance testing revealed this results in regression in write throughput: iozone -a -s 131072000 -y 4 -q 8 -i 0 -i 1 -R before7e55c60acf: KB reclen write rewrite read reread 131072000 4 493670 525964 524575 513384 131072000 8 540467 532880 512028 513703 after7e55c60acf: KB reclen write rewrite read reread 131072000 4 421785 456184 531278 509248 131072000 8 459283 456354 528449 543834 To reduce the amount of discontiguous requests we can start generating requests with the stripe with the lowest chunk offset as that has the best chance of being adjacent to IO queued previously. This improves the performance to: KB reclen write rewrite read reread 131072000 4 497682 506317 518043 514559 131072000 8 514048 501886 506453 504319 restoring big part of the regression. Fixes:7e55c60acf("md/raid5: Pivot raid5_make_request()") Cc: stable@vger.kernel.org # v6.0+ Signed-off-by: Jan Kara <jack@suse.cz> Reviewed-by: Logan Gunthorpe <logang@deltatee.com> Signed-off-by: Song Liu <song@kernel.org> Link: https://lore.kernel.org/r/20230417171537.17899-1-jack@suse.cz
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.