From: Philipp Reisner <philipp.reisner@linbit.com>
To: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Jens Axboe <axboe@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "drbd-dev@lists.linbit.com" <drbd-dev@lists.linbit.com>
Subject: Re: [Drbd-dev] [PATCH 05/30] drbd: Introduce new disk config option rs-discard-granularity
Date: Mon, 25 Apr 2016 11:21:17 -0500
Message-ID: <2101862.7IMGIqiMZK@phil-dell-xps.local>
User-Agent: KMail/4.13.3 (Linux/3.13.0-55-generic; KDE/4.13.3; x86_64; ; )
In-Reply-To: <571E393E.1010003@sandisk.com>
References: <1461586077-11581-1-git-send-email-philipp.reisner@linbit.com> <1461586077-11581-6-git-send-email-philipp.reisner@linbit.com> <571E393E.1010003@sandisk.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 7Bit
Content-Type: text/plain; charset="us-ascii"
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 957
Lines: 28

Am Montag, 25. April 2016, 08:35:26 schrieb Bart Van Assche:
> On 04/25/2016 05:10 AM, Philipp Reisner wrote:
> > As long as the value is 0 the feature is disabled. With setting
> > it to a positive value, DRBD limits and aligns its resync requests
> > to the rs-discard-granularity setting. If the sync source detects
> > all zeros in such a block, the resync target discards the range
> > on disk.
> 
> Hello Phil,
> 
> Can you explain why rs-discard-granularity is configurable instead of
> e.g. setting it to the least common multiple of the discard
> granularities of the underlying block devices at both sides?
> 
> Thanks,
> 

Hi Bart,

we had this idea as well. It seems that real world devices like larger
discards better than smaller discards. The other motivation was that
a device mapper logical volume might change it on the fly...
So we think it is best to delegate the decision on the discard chunk
size to user space.

best regards,
 Phil