changelog: fill out major 16.12 features

Changelog

=========

Upcoming Release

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v16.12: NVMe over Fabrics host, hotplug, and multi-process

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### NVMe library

The NVMe library has been changed to create its own request memory pool rather than

requiring the user to initialize the global `request_mempool` variable.  Apps can be

rather than physical addresses.  Callers of `spdk_nvme_ns_cmd_readv()` and

`spdk_nvme_ns_cmd_writev()` must update their `next_sge_fn` callbacks to match.

Libpciaccess has been removed as a dependency and DPDK PCI enumeration is

The NVMe library now supports NVMe over Fabrics devices in addition to the existing

support for local PCIe-attached NVMe devices.  For an example of how to enable

NVMe over Fabrics support in an application, see `examples/nvme/identify` and

`examples/nvme/perf`.

Hot insert/remove support for NVMe devices has been added.  To enable NVMe hotplug

support, an application should call the `spdk_nvme_probe()` function on a regular

basis to probe for new devices (reported via the existing `probe_cb` callback) and

removed devices (reported via a new `remove_cb` callback).  Hotplug is currently

only supported on Linux with the `uio_pci_generic` driver, and newly-added NVMe

devices must be bound to `uio_pci_generic` by an external script or tool.

### NVMe over Fabrics target (`nvmf_tgt`)

The `nvmf_tgt` configuration file format has been updated significantly to enable

new features.  See the example configuration file `etc/spdk/nvmf.conf.in` for

more details on the new and changed options.

The NVMe over Fabrics target now supports virtual mode subsystems, which allow the

user to export devices from the SPDK block device abstraction layer as NVMe over

Fabrics subsystems.  Direct mode (raw NVMe device access) is also still supported,

and a single `nvmf_tgt` may export both types of subsystems simultaneously.

### General changes

`libpciaccess` has been removed as a dependency and DPDK PCI enumeration is

used instead. Prior to DPDK 16.07 enumeration by class code was not supported,

so for earlier DPDK versions only Intel SSDs will be discovered. Starting with

DPDK 16.07 all devices will be discovered correctly by class code.

so for earlier DPDK versions, only Intel SSD DC P3x00 devices will be discovered

by the NVMe library.

The `env` environment abstraction library has been introduced, and a default

DPDK-based implementation is provided as part of SPDK.  The goal of the `env`

layer is to enable use of alternate user-mode memory allocation and PCI access

libraries.  See `PORTING.md` for more details.

The build process has been modified to produce all of the library files in the

`build/lib` directory.  This is intended to simplify the use of SPDK from external

to the library path via `-L` and linking the SPDK libraries by name (for example,

`-lspdk_nvme -lspdk_log -lspdk_util`).

`nvmf_tgt` and `iscsi_tgt` now have a JSON-RPC interface, which allows the user

to query and modify the configuration at runtime.  The RPC service is disabled by

default, since it currently does not provide any authentication or security

mechanisms; it should only be enabled on systems with controlled user access

behind a firewall. An example RPC client implemented in Python is provided in

`scripts/rpc.py`.

v16.08: iSCSI target, NVMe over Fabrics maturity

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