From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from imap.thunk.org (imap.thunk.org [IPv6:2600:3c02::f03c:91ff:fe96:be03]) (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 0761A211A2DA1 for ; Thu, 13 Dec 2018 20:11:23 -0800 (PST) Date: Thu, 13 Dec 2018 23:11:18 -0500 From: "Theodore Y. Ts'o" Subject: Re: [dm-devel] Snapshot target and DAX-capable devices Message-ID: <20181214041118.GE20880@thunk.org> References: <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> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20181212224321.GA2902@redhat.com> 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: Mike Snitzer 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 05:43:22PM -0500, Mike Snitzer wrote: > > 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. Well, here's one user: https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/scrub/e2scrub.in This is a spiffed up version of my original script: https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/contrib/e2croncheck I suppose we should look into enhancing e2scrub so it can deal with volumes stored on dm-thin pools..... > 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. I would think the *snapshot* would have to be the dm-thin device, not the origin volume, correct? The original LVM module would still be mounted through the original LVM device-mapper device, so it couldn't get transmogrified to be a dm-thin device, right? - Ted _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm