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

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Pavel Reichl <preichl@redhat.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 07:38:07 -0800	[thread overview]
Message-ID: <20191122153807.GD6219@magnolia> (raw)
In-Reply-To: <20191121231838.GH4614@dread.disaster.area>

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.

> 	- 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?

--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