Re: [PATCH] block: Improve read ahead size for rotational devices

[Damien Le Moal <dlemoal@kernel.org>]

On 6/16/25 15:28, Damien Le Moal wrote:
> 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>

Jens,

Ping ? This is reviewed. Did it got lost ? Or will it be part of a different PR ?

-- 
Damien Le Moal
Western Digital Research