Re: [RFC] bdev: use bdev_io_min() for statx DIO min IO

[Dave Chinner <david@fromorbit.com>]

On Sun, Jun 30, 2024 at 01:42:45PM -0700, Luis Chamberlain wrote:
> On Sat, Jun 29, 2024 at 10:54:19PM -0700, Christoph Hellwig wrote:
> > On Sat, Jun 29, 2024 at 08:24:00PM -0700, Luis Chamberlain wrote:
> > > > The minimum_io_size clearly is the minimum I/O size, not the minimal
> > > > nice to have one. 
> > > 
> > > I may have misread the below documentation then, because it seems to
> > > suggest this is a performance parameter, not a real minimum. Do we need
> > > to update it?
> > 
> > queue_limits.min_io is corretly described and a performance hint.
> 
> OK, great!
> 
> > The statx dio_offset_align is actual minimum I/O size and alignment and
> > not in any way related to the performance hint in minimum_io_size.
> 
> Oh, darn, I just read again 825cf206ed510 ("statx: add direct I/O
> alignment information") and the block layer change through commit
> 2d985f8c6b91b ("vfs: support STATX_DIOALIGN on block devices") and
> no where do I see any mention of it being  a min. Should we clarify
> that?

dio_offset_align is an _alignment_ parameter, not a "size"
parameter. In no way does it define either the absolute or "best
performance" minimum IO size - it just defines the minimum valid
alignment for the file offset that the filesystem/device supports.

It is implied that an IO of dio_offset_align bytes in length will be
supported, because that is the minimum length IO that meets the
offset alignment requirements defined by dio_offset_align. 

> And should we add a respective value for performance?

We could, but we already have:

	__u32 stx_blksize;     /* Block size for filesystem I/O */

which is defined as:

	stx_blksize
	      The "preferred" block size for efficient filesystem
	      I/O.  (Writing to a file in smaller chunks may cause
	      an inefficient read-modify-rewrite.)

> I suspect
> userspace will want to work with optimal values, not ones which
> could for instance incur read-modify-write.

Yup, that's the very definition of what stx_blksize should contain.

-Dave.
-- 
Dave Chinner
david@fromorbit.com