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From:   Evan Green <evgreen@chromium.org>
Date:   Mon, 10 Dec 2018 09:31:27 -0800
Message-ID: <CAE=gft7ymeVu0chvCYWn8tw9ss_pZgyUHt=5sSdxfRenDD46Xw@mail.gmail.com>
Subject: Re: [PATCH 2/2] loop: Better discard support for block devices
To:     martin.petersen@oracle.com
Cc:     ming.lei@redhat.com, axboe@kernel.dk,
        Gwendal Grignou <gwendal@chromium.org>, asavery@chromium.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
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On Wed, Dec 5, 2018 at 7:15 PM Martin K. Petersen
<martin.petersen@oracle.com> 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.
-Evan