On Sat, Sep 20 2014 at 11:50am -0400, Shao Miller <shao.miller@xxxxxxxxxxx> wrote: > Good day to all. > > If I've some block device "base" and I've two derived device-mapper > devices "derived1" and "derived2" that are copy-on-write layers over > top of that base, and I mount the filesystems on those derived block > devices and run the same program "foo" from both, does the "foo" > running from "derived1" share any [read and execute] memory pages > with the "foo" running from "derived2", since the underlying sectors > are both from the same position on "base"? > > The more general question would be about mmap, but I hope this > example is clear. "Docker"[1] uses device-mapper in scenarios like > the above example and I'm curious if they benefit from shared pages. > > [1] https://www.docker.com/ 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. Interestingly BTRFS does _not_ offer this page sharing either. I'm told that the only emerging solution for this is overlayfs. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
