Re: [PATCH 2/2] loop: Better discard support for block devices

From: Evan Green
Date: Mon Dec 10 2018 - 12:32:10 EST

On Wed, Dec 5, 2018 at 7:15 PM Martin K. Petersen
<martin.petersen@xxxxxxxxxx> wrote:
> Evan,
> > Ah, I see. But I think it's useful to reflect max_discard_sectors,
> > max_write_zeroes_sectors, discard_granularity, and discard_alignment
> > from the block device to the loop device. With the exception of
> > discard_alignment, these parameters are visible via sysfs,
> discard_alignment is visible in sysfs, just not in the queue directory
> since alignment can be different on a per-partition basis. So there is
> one discard_alignment at the root of each /sys/block/foo directory and
> one for each partition subdirectory. This mirrors the alignment_offset
> variable which indicates a partitions alignment wrt. the underlying
> physical block size.
> That said, there are not many devices that actually report a non-zero
> discard alignment so it's not as useful as the device manufacturers
> (that were looking for an implementation shortcut) envisioned.

Ah ok, thanks.

> > I'm not totally sure about discard_alignment, that seems to be useful
> > in cases of merging blk requests. So I can stop mirroring that one if
> > it's harmful or not helpful. But unless it's a nak, I'd really love to
> > keep most of the mirroring. In which case the bool doesn't do a whole
> > lot of simplifying.
> I think it's fine to export these. The block device topology was
> explicitly designed to be stackable like this.

Yeah, it seemed to fall in pretty naturally, which is why I was hoping
it might not be so controversial. Thanks Martin.