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block: Improve read ahead size for rotational devices

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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>
This commit is contained in:
Damien Le Moal
2025-06-16 15:28:56 +09:00
committed by Jens Axboe
parent 86aa721820
commit 459779d04a
1 changed files with 10 additions and 2 deletions
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12
block/blk-settings.c
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@@ -62,16 +62,24 @@ EXPORT_SYMBOL(blk_set_stacking_limits);
void blk_apply_bdi_limits(struct backing_dev_info *bdi,
struct queue_limits *lim)
{
u64 io_opt = lim->io_opt;
/*
* For read-ahead of large files to be effective, we need to read ahead
* at least twice the optimal I/O size.
* at least twice the optimal I/O size. For rotational devices that do
* not report an optimal I/O size (e.g. ATA HDDs), use the maximum I/O
* size to avoid falling back to the (rather inefficient) small default
* read-ahead size.
*
* There is no hardware limitation for the read-ahead size and the user
* might have increased the read-ahead size through sysfs, so don't ever
* decrease it.
*/
if (!io_opt && (lim->features & BLK_FEAT_ROTATIONAL))
io_opt = (u64)lim->max_sectors << SECTOR_SHIFT;
bdi->ra_pages = max3(bdi->ra_pages,
lim->io_opt * 2 / PAGE_SIZE,
io_opt * 2 >> PAGE_SHIFT,
VM_READAHEAD_PAGES);
bdi->io_pages = lim->max_sectors >> PAGE_SECTORS_SHIFT;
}