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@xxxxxxxxxx> Thanks. -- Damien Le Moal Western Digital Research
