[Parted-maintainers] Bug#923561: parted: Incorrect optimal alignment for USB device

[Phillip Susi]

On 3/5/2019 10:58 AM, Kevin Locke wrote:
> Sounds great.  How do you propose that the kernel determine the
> optimal alignment?

md does it using the stripe size.  Not sure if anything other the md or
dm would make sense to populate the value.  Well, I guess hardware raid
drivers.

> I disagree that what you quoted says that that the md driver uses
> optimal_io_size for anything, much less unconditionally.  Also, since

Now that I read it again, it just says raid arrays, not md specifically,
so I guess that means md plus hardware raid drivers.  I know from
experience that md does use it that way.

> the disk on which I am running parted is not a RAID array, I don't
> think the documentation above says that it is anything more than
> "preferred unit for sustained I/O".

Yes, the first part says that, but then it goes on to say that normal
disks generally leave it zero, and raid disks set it to the stripe width.

>> Wait, how can optimal_io_size NOT be a multiple of the block size?
> 
> For my disk, the logical block size is 512 bytes, the physical block
> size is 4,096, opt_xfer_blocks is 65,535, so optimal_io_size is
> 65,535*512=33,553,920 which is not a multiple of 4,096.  I considered
> advocating that the kernel check this, but decided against it.

Oh, that is weird.  I guess such a sanity check would fix the issue for
your USB stick, but what about others?

> SCSI devices can report any value (measured in logical blocks) for VPD
> Optimal Transfer Length.  It is not restricted to multiples of the
> physical block size.  For my disk, it is not, which is the cause of
> the current issue.

So for 512e disks basically, the optimal transfer length can be not a
multiple of physical block size and foolish drives try to specify the
maximum possible value in logical 512 byte sectors, and that ends up
being 1 logical sector too small to align to 4k.  For 512n 4kn disks,
the optimal size can never not be a multiple of the sector size, so the
sanity check would pass and still give you a massive alignment you don't
want.