From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ipmail01.adl2.internode.on.net ([150.101.137.133]:25487 "EHLO ipmail01.adl2.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726228AbfBQVJw (ORCPT ); Sun, 17 Feb 2019 16:09:52 -0500 Date: Mon, 18 Feb 2019 08:09:48 +1100 From: Dave Chinner Subject: Re: [LSF/MM TOPIC] More async operations for file systems - async discard? Message-ID: <20190217210948.GB14116@dastard> References: <92ab41f7-35bc-0f56-056f-ed88526b8ea4@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <92ab41f7-35bc-0f56-056f-ed88526b8ea4@gmail.com> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Ric Wheeler Cc: lsf-pc@lists.linux-foundation.org, linux-xfs , linux-fsdevel , linux-ext4 , linux-btrfs , linux-block@vger.kernel.org On Sun, Feb 17, 2019 at 03:36:10PM -0500, Ric Wheeler wrote: > One proposal for btrfs was that we should look at getting discard > out of the synchronous path in order to minimize the slowdown > associated with enabling discard at mount time. Seems like an > obvious win for "hint" like operations like discard. We already have support for that. blkdev_issue_discard() is synchornous, yes, but __blkdev_issue_discard() will only build the discard bio chain - it is up to the caller to submit and wait for it. Some callers (XFS, dm-thinp, nvmet, etc) use a bio completion to handle the discard IO completion, hence allowing async dispatch and processing of the discard chain without blocking the caller. Others (like ext4) simply call submit_bio_wait() to do wait synchronously on completion of the discard bio chain. > I do wonder where we stand now with the cost of the various discard > commands - how painful is it for modern SSD's? AIUI, it still depends on the SSD implementation, unfortunately. Cheers, Dave. -- Dave Chinner david@fromorbit.com