From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from na01-bn1-obe.outbound.protection.outlook.com (mail-bn1on0099.outbound.protection.outlook.com [157.56.110.99]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail09.linbit.com (LINBIT Mail Daemon) with ESMTPS id A69C61051881 for ; Mon, 25 Apr 2016 21:03:35 +0200 (CEST) To: Philipp Reisner 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> <2101862.7IMGIqiMZK@phil-dell-xps.local> From: Bart Van Assche Message-ID: <571E667E.4080200@sandisk.com> Date: Mon, 25 Apr 2016 11:48:30 -0700 MIME-Version: 1.0 In-Reply-To: <2101862.7IMGIqiMZK@phil-dell-xps.local> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Cc: Jens Axboe , "linux-kernel@vger.kernel.org" , "drbd-dev@lists.linbit.com" Subject: Re: [Drbd-dev] [PATCH 05/30] drbd: Introduce new disk config option rs-discard-granularity List-Id: "*Coordination* of development, patches, contributions -- *Questions* \(even to developers\) go to drbd-user, please." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 04/25/2016 09:42 AM, Philipp Reisner wrote: > 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. >> >> 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? > > 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. Hello Phil, Are you aware that for aligned discard requests the discard granularity does not affect the size of discard requests at all? Regarding LVM volumes: if the discard granularity for such volumes can change on the fly, shouldn't I/O be quiesced by the LVM kernel driver before it changes the discard granularity? I think that increasing discard granularity while I/O is in progress should be considered as a bug. Bart.