From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 9FF1C2119C8B0 for ; Wed, 12 Dec 2018 14:43:27 -0800 (PST) Date: Wed, 12 Dec 2018 17:43:22 -0500 From: Mike Snitzer Subject: Re: Snapshot target and DAX-capable devices Message-ID: <20181212224321.GA2902@redhat.com> 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> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20181212211547.GA24926@thunk.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-nvdimm-bounces@lists.01.org Sender: "Linux-nvdimm" To: "Theodore Y. Ts'o" Cc: Jan Kara , "linux-nvdimm@lists.01.org" , chengnt , Dave Chinner , colyli , Christoph Hellwig , "dm-devel@redhat.com" , Mikulas Patocka , "linux-fsdevel@vger.kernel.org" List-ID: On Wed, Dec 12 2018 at 4:15pm -0500, Theodore Y. Ts'o wrote: > 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 serious > > > users of snapshots had moved on to dm-thinp? > > > > There are cases where dm-snapshot is still useful for people. But those > > are very niche users. I'm not opposed to others proposing enhancements > > for dm-snapshot in general but it is definitely not a priority (Google'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". 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). 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. 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.git/commit/?h=dm-4.21&id=61d594bb7e1cf86dca49cbc9524eb80169d9fca6 https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.21&id=d1f7898c7a1b24aa9ae670f9cc21b65e730827eb > One other major advantage that dm-snapshot has is that you can take a > snapshot for any LVM volume. For dm-thinp you have migrate your > storage to a thinp pool, and that adds a fair amount of friction to > users migrating to dm-thinp. dm-thinp has the concept of an "external origin". Changes to origin LVM volume get copied out to the thin-pool (same copy cost as old dm-snapshot). But IIRC from that point on your LVM volume is a dm-thin device. Mike _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm