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

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Message-ID: <yq1va47pcqp.fsf@oracle.com>
Date:   Wed, 05 Dec 2018 22:15:10 -0500
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     Evan Green <evgreen@...omium.org>
Cc:     ming.lei@...hat.com, axboe@...nel.dk,
        Gwendal Grignou <gwendal@...omium.org>, asavery@...omium.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] loop: Better discard support for block devices

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.

> 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.

-- 
Martin K. Petersen	Oracle Linux Engineering