Re: [PATCH 1/2] mkfs: Break block discard into chunks of 2 GB

From: Pavel Reichl <preichl@redhat.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Dave Chinner <david@fromorbit.com>, linux-xfs@vger.kernel.org
Subject: Re: [PATCH 1/2] mkfs: Break block discard into chunks of 2 GB
Date: Fri, 22 Nov 2019 16:59:21 +0100	[thread overview]
Message-ID: <CAJc7PzX0sra12ikpVAY4LE-zRxamJK+JiNxj69MS+MOTmP730g@mail.gmail.com> (raw)
In-Reply-To: <20191122153807.GD6219@magnolia>

On Fri, Nov 22, 2019 at 4:38 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
>
> On Fri, Nov 22, 2019 at 10:18:38AM +1100, Dave Chinner wrote:
> > On Thu, Nov 21, 2019 at 10:44:44PM +0100, Pavel Reichl wrote:
> > > Signed-off-by: Pavel Reichl <preichl@redhat.com>
> > > ---
> >
> > This is mixing an explanation about why the change is being made
> > and what was considered when making decisions about the change.
> >
> > e.g. my first questions on looking at the patch were:
> >
> >       - why do we need to break up the discards into 2GB chunks?
> >       - why 2GB?
>
> Yeah, I'm wondering that too.

OK, thank you both for the question - simple answer is that I took
what is used in e2fsprogs as default and I expected a discussion about
proper value during review process :-)
>
> >       - why not use libblkid to query the maximum discard size
> >         and use that as the step size instead?
>
> FWIW my SATA SSDs the discard-max is 2G whereas on the NVME it's 2T.  I
> guess firmwares have gotten 1000x better in the past few years, possibly
> because of the hundred or so 10x programmers that they've all been hiring.
>
> >       - is there any performance impact from breaking up large
> >         discards that might be optimised by the kernel into many
> >         overlapping async operations into small, synchronous
> >         discards?
>
> Also:
> What is the end goal that you have in mind?  Is the progress reporting
> the ultimate goal?  Or is it to break up the BLKDISCARD calls so that
> someone can ^C a mkfs operation and not have it just sit there
> continuing to run?

The goal is mainly the progress reporting but the possibility to do ^C
is also convenient. It seems that some users are not happy about the
BLKDISCARD taking too long and at the same time not being informed
about that - so they think that the command actually hung.

>
> --D
>
> > i.e. the reviewer can read what the patch does, but that deosn't
> > explain why the patch does this. Hence it's a good idea to explain
> > the problem being solved or the feature requirements that have lead
> > to the changes in the patch....
> >
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@fromorbit.com
>