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

["Martin K. Petersen" <martin.petersen@oracle.com>]

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.

Anyway. That's orthogonal to this particular change...

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>

-- 
Martin K. Petersen