From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shao Miller Subject: Re: Shared Memory Pages for Same Base Device Date: Mon, 22 Sep 2014 10:08:13 -0400 Message-ID: <54202D4D.1030901@treefrog.ca> References: <541DA23F.6000003@treefrog.ca> <20140922130803.GA5343@redhat.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20140922130803.GA5343@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: dm-devel@redhat.com List-Id: dm-devel.ids On 9/22/2014 9:08 AM, Mike Snitzer wrote: > > Unfortunately device-mapper thin provisioning doesn't offer shared > pagecache pages across snapshot volumes. This is a block layer > limitation (the block layer doesn't allow pages to be shared across > block devices, and dm-thinp snapshot volumes are each a block device). > Modifying the VM, block and DM subsystems to provide this capability is > not an easy task and as such is really not a near-term priority. I sincerely appreciate your response, Mike. I figured as much, but hopefully this thread will be useful to future searchers. > Interestingly BTRFS does _not_ offer this page sharing either. I'm told > that the only emerging solution for this is overlayfs. This is very surprising. I've just done some testing and /proc/PID/maps for the same binary from two different snapshots deriving from the same base do show the same block-device major and minor, with the same offset. Do you happen to have a reference for that detail about BTRFS, or perhaps an idea for the right venue to learn more about that? Unfortunately, I guess it's off-topic for device-mapper. Also unfortunately, it seems that overlayfs isn't mainline Linux nor out-of-the-box for some distributions. (Like CentOS 7.) I enjoy device-mapper for CoW iSCSI and AoE. Keep up the great work, folks! -- Shao Miller /Network Technician/ /905-836-4442 *ext: 112*/ www.treefrog.ca/shao-miller * * /*Treefrog Inc.*/ 905-836-4442 567 Davis Drive, Newmarket, ON www.treefrog.ca - @Treefrog