Re: [PATCH 0/2] loop: Better discard for block devices

From: Gwendal Grignou <gwendal@chromium.org>
To: evgreen@chromium.org
Cc: bvanassche@acm.org, axboe@kernel.dk, asavery@chromium.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] loop: Better discard for block devices
Date: Thu, 1 Nov 2018 15:44:31 -0700	[thread overview]
Message-ID: <CAPUE2uuhfk7kstwO2kp_Rs=9QQPgGQGY+QqR76zpZpTkKp5Mig@mail.gmail.com> (raw)
In-Reply-To: <CAE=gft4xwXuJ=aHbF08SxFxtSRX_6KBzPHxCCMbA6F7RnGQ8jA@mail.gmail.com>

On Thu, Nov 1, 2018 at 11:15 AM Evan Green <evgreen@chromium.org> wrote:
>
> On Tue, Oct 30, 2018 at 4:50 PM Bart Van Assche <bvanassche@acm.org> wrote:
> >
> > On Tue, 2018-10-30 at 16:06 -0700, Evan Green wrote:
> > > This series addresses some errors seen when using the loop
> > > device directly backed by a block device. The first change plumbs
> > > out the correct error message, and the second change prevents the
> > > error from occurring in many cases.
> >
> > Hi Evan,
> >
> > Can you provide some information about the use case? Why do you think that
> > it would be useful to support backing a loop device by a block device? Why
> > to use the loop driver instead of dm-linear for this use case?
> >
>
> Hi Bart,
> In our case, the Chrome OS installer uses the loop device to map
> slices of the disk that will ultimately represent partitions [1]. I
> believe it has been doing install this way for a very long time, and
> has been working well. It actually continues to work, but on block
> devices that don't support discard operations, things are a tiny bit
> bumpy. This series is meant to smooth out those bumps. As far as I
> knew this was a supported scenario.
>
> -Evan
> [1] https://chromium.googlesource.com/chromiumos/platform/installer/+/master/chromeos-install

The code has moved to
https://chromium.googlesource.com/chromiumos/platform2/+/master/installer/chromeos-install
but the idea is the same. We create a loop device to abstract the
persistent destination. The destination can be a block device or a
file. The later case is used for creating master images to be flashed
on memory chip before soldering on the production line.
It is handy when the final device is 4K block aligned but the builder
is using 512b block aligned device, we can mount a device over a file
that will behave like the real device we will flash the image on.

Gwendal.