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For a device that does not advertize an optimal I/O size, the function blk_apply_bdi_limits() defaults to an initial setting of the ra_pages field of struct backing_dev_info to VM_READAHEAD_PAGES, that is, 128 KB. This low I/O size value is far from being optimal for hard-disk devices: when reading files from multiple contexts using buffered I/Os, the seek overhead between the small read commands generated to read-ahead multiple files will significantly limit the performance that can be achieved. This fact applies to all ATA devices as ATA does not define an optimal I/O size and the SCSI SAT specification does not define a default value to expose to the host. Modify blk_apply_bdi_limits() to use a device max_sectors limit to calculate the ra_pages field of struct backing_dev_info, when the device is a rotational one (BLK_FEAT_ROTATIONAL feature is set). For a SCSI disk, this defaults to 2560 KB, which significantly improve performance for buffered reads. Using XFS and sequentially reading randomly selected (large) files stored on a SATA HDD, the maximum throughput achieved with 8 readers reading files with 1MB buffered I/Os increases from 122 MB/s to 167 MB/s (+36%). The improvement is even larger when reading files using 128 KB buffered I/Os, with a throughput increasing from 57 MB/s to 165 MB/s (+189%). Signed-off-by: Damien Le Moal <dlemoal@kernel.org> Reviewed-by: John Garry <john.g.garry@oracle.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com> Link: https://lore.kernel.org/r/20250616062856.1629897-1-dlemoal@kernel.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
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 reStructuredText 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.