From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from aserp1040.oracle.com ([141.146.126.69]:46752 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757264Ab2KIAoM (ORCPT ); Thu, 8 Nov 2012 19:44:12 -0500 Date: Fri, 9 Nov 2012 08:44:01 +0800 From: Liu Bo To: Stefan Behrens Cc: linux-btrfs@vger.kernel.org Subject: Re: [PATCH 15/26] Btrfs: add a new source file with device replace code Message-ID: <20121109004400.GB5172@gmail.com> Reply-To: bo.li.liu@oracle.com References: <4520a248e2bcf0493b35673ea4cac2d4e0757e08.1352217243.git.sbehrens@giantdisaster.de> <20121108145038.GB3034@gmail.com> <509BEAD4.8070309@giantdisaster.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <509BEAD4.8070309@giantdisaster.de> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Thu, Nov 08, 2012 at 06:24:36PM +0100, Stefan Behrens wrote: > On Thu, 8 Nov 2012 22:50:47 +0800, Liu Bo wrote: > > On Tue, Nov 06, 2012 at 05:38:33PM +0100, Stefan Behrens wrote: [...] > >> + btrfs_dev_replace_unlock(dev_replace); > >> + > >> + btrfs_wait_ordered_extents(root, 0); > >> + > >> + /* force writing the updated state information to disk */ > >> + trans = btrfs_start_transaction(root, 0); > > > > why a start_transaction here? Any reasons? > > (same question also for some other places) > > > > Without this transaction, there is outstanding I/O which is not flushed. > Pending writes that go only to the old disk need to be flushed before > the mode is switched to write all live data to the source disk and to > the target disk as well. The copy operation that is part of the scrub > code works on the commit root for performance reasons. Every write > request that is performed after the commit root is established needs to > go to both disks. Those requests that already have the bdev assigned > (i.e., btrfs_map_bio() was already called) cannot be duplicated anymore > to write to the new disk as well. > > btrfs_dev_replace_finishing() looks similar and goes through a > transaction commit between the steps where the bdev in the mapping tree > is swapped and the step when the old bdev is freed. Otherwise the bdev > would be accessed after being freed. > I see, if you're only about to flush metadata, why not join a transaction? thanks, liubo