Date: Tue, 14 Jul 2015 16:44:20 -0400
From: Mike Snitzer <snitzer@redhat.com>
To: Jens Axboe <axboe@fb.com>
Cc: linux-kernel@vger.kernel.org, hch@lst.de
Subject: Re: [PATCH 3/3] block: by default, limit maximum discard size to 64MB
Message-ID: <20150714204419.GA7915@redhat.com>
References: <1436899703-31966-1-git-send-email-axboe@fb.com>
 <1436899703-31966-4-git-send-email-axboe@fb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1436899703-31966-4-git-send-email-axboe@fb.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1104
Lines: 24

On Tue, Jul 14 2015 at  2:48pm -0400,
Jens Axboe <axboe@fb.com> wrote:

> Lots of devices exhibit very high latencies for big discards, hurting
> reads and writes. By default, limit the max discard we will build to
> 64MB. This value has shown good results across a number of devices.
> 
> This will potentially hurt discard throughput, from a provisioning
> point of view (when the user does mkfs.xfs, for instance, and mkfs
> issues a full device discard). If that becomes an issue, we could
> have different behavior for provisioning vs runtime discards.
> 
> Signed-off-by: Jens Axboe <axboe@fb.com>

Christoph suggested you impose this default for the specific
drivers/devices that benefit.  I'm not following why imposing a 64MB
default is the right thing to do for all devices.

If this goes in I know for sure I'll need to work around it in DM-thinp.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/