From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([59.151.112.132]:59787 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1755459AbcKOGdt (ORCPT ); Tue, 15 Nov 2016 01:33:49 -0500 From: Qu Wenruo Subject: About difference in extent sharing in btrfs and xfs Message-ID: <1de48152-0ec5-1bf0-1ea9-b5ccbe4a3867@cn.fujitsu.com> Date: Tue, 15 Nov 2016 14:33:31 +0800 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: linux-xfs@vger.kernel.org, fstests@vger.kernel.org, btrfs Cc: "Darrick J. Wong" Hi, xfs guys and btrfs guys. Although the test case generic/372 exists for some time, I noticed that btrfs always fails the test case, due to the difference in how btrfs and xfs handle shared extents. The difference is, btrfs can handle shared extents which points to a subset of a larger extent, so it doesn't need to split reflink source. In case of the test case. On Disk Extent A: Bytenr X |<-----------Data=0x61, Length=320K---------------------------| File1: File Extent 0 -> Extent A, offset=0, referred len=320K File2: File Extent 0 -> Hole File Extent 64K -> Extent A, offset=192k, refferred len=64k File Extent 128K -> Hole File Extent 192K -> Extent A, offset=64k, refferred len=64k Unlike Xfs, Btrfs don't split the source extent, as its file extent has more fields which can handle offset/length inside the large extent. The btrfs way to handle shared extent has its pros and cons. For example, it's very flex, but it wastes more space for COW since the whole extent can only be freed after all referencer is freed. But since the test case is generic test case, I think it doesn't take such btrfs behavior into consideration. So it always fails on btrfs. How about moving it to xfs specific tests? Thanks, Qu