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

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

On 6/17/25 6:24 AM, Martin K. Petersen wrote:
> 
> Hi Damien!
> 
>> 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).
> 
> I much prefer doing it here. I don't think overriding io_opt in SCSI is
> appropriate. Applications and filesystems need to be able to determine
> whether a SCSI device reports an optimal I/O size or not. Overloading
> the queue limit with readahead semantics does not belong in SCSI.
> 
>> For a SCSI disk, this defaults to 2560 KB, which significantly improve
>> performance for buffered reads.
> 
> I believe this number came from a common RAID stripe configuration at
> the time. However, it's really not a great default and has caused
> problems with many devices that expect a power of two. Personally, I'd
> like this default to be something like 2MB or 4MB. MD, DM, and most
> hardware RAID devices report their stripe width correctly so the
> existing "RAID-friendly" default really shouldn't be needed.

That sounds good. Recently, I have been doing a lot of performance benchmarks
with large IOs on HDDs (2, 4 8 and 16 MB IOs). And with the improved memory
allocation these days (transparent huge pages), even a simple malloc() IO
buffer can have far less memory segments that the HBA maximum a majority of the
time. So doing such large I/Os is fairly easy and really improves HDD
performance. And in that context, the current default 1280 value for
max_sectors_kb is really a limiting factor. So I am all for increasing the
default to something like 4MB. I will send a patch.

> Anyway. That's orthogonal to this particular change...
> 
> Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>

Thanks.


-- 
Damien Le Moal
Western Digital Research