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

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Message-ID: <CAE=gft4xwXuJ=aHbF08SxFxtSRX_6KBzPHxCCMbA6F7RnGQ8jA@mail.gmail.com>
Date:   Thu, 1 Nov 2018 11:15:15 -0700
From:   Evan Green <evgreen@...omium.org>
To:     bvanassche@....org
Cc:     axboe@...nel.dk, Gwendal Grignou <gwendal@...omium.org>,
        asavery@...omium.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] loop: Better discard for block devices

On Tue, Oct 30, 2018 at 4:50 PM Bart Van Assche <bvanassche@....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