Dm-thin for local storage

The Storage Manager (SM) currently supports 2 kinds of local storage:

 1. .vhd files on an ext3 filesystem on an LVM LV on a local disk
 2. vhd-format data written directly to LVM LVs on a local disk

We can also directly import and export .vhd-format data using HTTP PUT and GET operations, see Disk import/export.

In all cases the data path uses "blktap" (the kernel module) and "tapdisk" (the user-space process). This means that:

 1. constant maintenance is required because blktap is an out-of-tree kernel module
 2. every I/O request incurs extra latency due to kernelspace/userspace transitions, a big problem on fast flash devices (PCIe)
 3. we only support vhd, and not vmdk or qcow2 (and in future direct access to object stores?)

Analysis

We currently use the vhd format and blktap/tapdisk implementation for 2 distinct purposes:

 1. as a convenient, reasonably efficient, standard format for sharing images such as templates
 2. as a means of implementing thin provisioning on the data path: where blocks are allocated on demand, and storage is over provisioned

If instead of using vhd format and blktap/tapdisk everywhere we

 1. use a tool (e.g. qemu-img) which reads and writes vhd, qcow2, vmdk and which can be mounted as a block device on an unmodified kernel (e.g. via NBD)
 2. use device-mapper modules to provide thin provisioning and low-latency access to the data

then we

 1. avoid the blktap kernel module maintenance
 2. reduce the common-case I/O request latency by keeping it all in-kernel
 3. extend the number of formats we support, and make it easier to support direct object store access in future.