From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail1.bemta23.messagelabs.com ([67.219.246.208]:48142 "EHLO mail1.bemta23.messagelabs.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726533AbeLNIZE (ORCPT ); Fri, 14 Dec 2018 03:25:04 -0500 From: Huaisheng HS1 Ye To: Mike Snitzer CC: Huaisheng Ye , Jan Kara , "linux-nvdimm@lists.01.org" , NingTing Cheng , Dave Chinner , colyli , Christoph Hellwig , "dm-devel@redhat.com" , Mikulas Patocka , "linux-fsdevel@vger.kernel.org" , "Theodore Y. Ts'o" Subject: RE: [External] Re: Snapshot target and DAX-capable devices Date: Fri, 14 Dec 2018 08:24:49 +0000 Message-ID: References: <20180830093028.GC1767@quack2.suse.cz> <20180830184907.GA14867@redhat.com> <20180830233809.GH1572@dastard> <20180831094255.GB11622@quack2.suse.cz> <167a3303a01.11a848ab768799.5161498967766415143@zoho.com> <20181212161254.GA20790@infradead.org> <20181212175047.GA24962@redhat.com> <20181212211547.GA24926@thunk.org> <20181212224321.GA2902@redhat.com> In-Reply-To: <20181212224321.GA2902@redhat.com> Content-Language: zh-CN Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org List-ID: From: Mike Snitzer Sent: Thursday, December 13, 2018 6:43 AM > On Wed, Dec 12 2018 at 4:15pm -0500, > Theodore Y. Ts'o wrote: >=20 > > On Wed, Dec 12, 2018 at 12:50:47PM -0500, Mike Snitzer wrote: > > > On Wed, Dec 12 2018 at 11:12am -0500, > > > Christoph Hellwig wrote: > > > > > > > Does it really make sense to enhance dm-snapshot? I thought all se= rious > > > > users of snapshots had moved on to dm-thinp? > > > > > > There are cases where dm-snapshot is still useful for people. But th= ose > > > are very niche users. I'm not opposed to others proposing enhancemen= ts > > > for dm-snapshot in general but it is definitely not a priority (Googl= e's > > > dm-bow is an example of a case where dm-snapshot may get extended to > > > fulfill google's needs). > > > > I would expect that dm-snapshot will be used quite a lot for > > short-lived snapshots (that only live during a database backup or an > > fsck run). I would hardly call that a "niche use case". >=20 > dm-snapshot is only ~60% performant for 1 snapshot. Try to do > additional snapshots and performance crawls to a stop (though I haven't > reassessed performance in a while). >=20 > dm-snapshot has been in Linux since before 2005, I don't know of all the > users of it -- maybe there are a ton of users who only take a single > temporary snapshot and we're all oblivious. >=20 > Definitely not seeing many bugs against it (but it has been around > forever). I do know that there are relatively few people showing > interest in it. But for 4.21 I did stage a couple useful performance > fixes: > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.gi= t/commit/?h=3Ddm-4. > 21&id=3D61d594bb7e1cf86dca49cbc9524eb80169d9fca6 > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.gi= t/commit/?h=3Ddm-4. > 21&id=3Dd1f7898c7a1b24aa9ae670f9cc21b65e730827eb Hi Mike, Could these two patches be applied to current code of LVM? Although there is a difficult problem as mmap for dm-snapshot with DAX-capable, the two patches can be used for other complex DM targets when trying to implement DAX. [RFC PATCH v2 2/3] dm: expand hc_map in mapped_device for lack of map https://lkml.org/lkml/2018/11/21/273 [RFC PATCH v2 3/3] dm: expand valid types for dm-ioctl https://lkml.org/lkml/2018/11/21/276 Cheers, Huaisheng Ye