bdev/compress: add docs and changelog entry

separate buffer. To check support for separatate metadata, use spdk_bdev_is_md_separate().

All bdevs now have a UUID. For devices whose backing hardware does not provide a UUID,

one is automatically generated.

Across runs of SPDK, bdevs whose UUID is automatically generated may change.

one is automatically generated. Across runs of SPDK, bdevs whose UUID is automatically

generated may change.

A new virtual bdev module `compress` has been added to provide compression services on top of

a thinly provisioned logical volume.  See documentation for complete details.

### nvme

`rpc.py delete_rbd_bdev Rbd0`

# Compression Virtual Bdev Module {#bdev_config_compress}

The compression bdev module can be configured to provide compression/decompression

services for an underlying thinly provisioned logical volume. Although the underlying

module can be anything (i.e. NVME bdev) the overall compression benefits will not be realized

unless the data stored on disk is placed appropriately. The compression vbdev module

relies on an internal SPDK library called `reduce` to accomplish this, see @ref reduce

for detailed information.

The vbdev module relies on the DPDK CompressDev Framework to provide all compression

functionality. The framework provides support for many different software only

compression modules as well as hardware assisted support for Intel QAT. At this

time the vbdev module supports the DPDK drivers for ISAL and QAT.

Persistent memory is used to store metadata associated with the layout of the data on the

backing device. SPDK relies on [PMDK](http://pmem.io/pmdk/) to interface persistent memory so any hardware

supported by PMDK should work. If the directory for PMEM supplied upon vbdev creation does

not point to persistent memory (i.e. a regular filesystem) performance will be severely

impacted.  The vbdev module and reduce libraries were designed to use persistent memory for

any production use.

Example command

`rpc.py construct_compress_bdev -p /pmem_files -b myLvol`

In this example, a compression vbdev is created using persistent memory that is mapped to

the directory `pmem_files` on top of the existing thinly provisioned logical volume `myLvol`.

The resulting compression bdev will be named `COMP_LVS/myLvol` where LVS is the name of the

logical volume store that `myLvol` resides on.

The logical volume is referred to as the backing device and once the compression vbdev is

created it cannot be separated from the persistent memory file that will be created in

the specified directory.  If the persistent memory file is not available, the compression

vbdev will also not be available.

By default the vbdev module will choose the QAT driver if the hardware and drivers are

available and loaded.  If not, it will revert to the software-only ISAL driver. By using

the following command, the driver may be specified however this is not persistent so it

must be done either upon creation or before the underlying logical volume is loaded to

be honored. In the example below, `0` is telling the vbdev module to use QAT if available

otherwise use ISAL, this is the default and if sufficient the command is not required. Passing

a value of 1 tells the driver to use QAT and if not available then the creation or loading

the vbdev should fail to create or load.  A value of '2' as shown below tells the module

to use ISAL and if for some reason it is not available, the vbdev should fail to create or load.

`rpc.py set_compress_pmd -p 2`

To remove a compression vbdev, use the following command which will also delete the PMEM

file.  If the logical volume is deleted the PMEM file will not be removed and the

compression vbdev will not be available.

`rpc.py delete_compress_bdev COMP_LVS/myLvol`

# Crypto Virtual Bdev Module {#bdev_config_crypto}

The crypto virtual bdev module can be configured to provide at rest data encryption