Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754108AbbFVQp1 (ORCPT ); Mon, 22 Jun 2015 12:45:27 -0400 Received: from verein.lst.de ([213.95.11.211]:43014 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752777AbbFVQpR (ORCPT ); Mon, 22 Jun 2015 12:45:17 -0400 Date: Mon, 22 Jun 2015 18:45:15 +0200 From: Christoph Hellwig To: Dan Williams Cc: Jens Axboe , "linux-nvdimm@lists.01.org" , Boaz Harrosh , "Kani, Toshimitsu" , "linux-kernel@vger.kernel.org" , Linux ACPI , linux-fsdevel , Ingo Molnar Subject: Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices Message-ID: <20150622164515.GA9281@lst.de> References: <20150621101346.GF5915@lst.de> <20150621135406.GA9572@lst.de> <20150622063028.GA30434@lst.de> <20150622072844.GA31263@lst.de> <20150622154056.GB7952@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1694 Lines: 31 On Mon, Jun 22, 2015 at 09:36:50AM -0700, Dan Williams wrote: > In that case "don't stack" is too coarse of a hammer. I see this as a > request to hide the subordinate ULD which is a new capability that DM > and MD might benefit from as well. We already have the case in MD > where it internally holds a reference to bdev that has been hot > removed, it seems not much of a stretch to have stacking drivers be > able to hide device nodes for bdevs that they are holding. I don't see why you're comparing with MD and DM here. MD and DM sit cleanly ontop of any block device. If btt was independent of libnvdimm and just used ->rw_bytes we could see it as this. But it's all a giant entangled mess, where btt for example is probed by libnvdimm. At the same time pmem.c isn't really a true block driver, it's really just a trivial shim between the block API and pmem-style memcpy. Especially with the proper pmem API btt would become cleaner just calling that directly. > Yes, if they want to use DAX they should do it consciously and audit > their application to be sure it is safe to abandon atomic sector > guarantees. With the current flexibility to do BTT on a partition > they can do this conversion piecemeal and, for example, keep metadata > on BTT and data on DAX. By that logic you'd want to attach BTT by default and allow opt-out at some level. This could be a libnvmdimm-level partitioning scheme, which would also allow storing the bit if BTT is used or not persistently. Or it could be on fine grained boundaries which might be more useful. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/