doc: Expand the intro section of the bdev documentation.

# Block Device Abstraction Layer {#bdev}

# Block Device Layer {#bdev}

# SPDK bdev Getting Started Guide {#bdev_getting_started}

# Introduction {#bdev_getting_started}

Block storage in SPDK applications is provided by the SPDK bdev layer.  SPDK bdev consists of:

The SPDK block device layer, often simply called *bdev*, is a C library

intended to be equivalent to the operating system block storage layer that

often sits immediately above the device drivers in a traditional kernel

storage stack. Specifically, this library provides the following

functionality:

* a driver module API for implementing bdev drivers

* an application API for enumerating and claiming SPDK block devices and performance operations

(read, write, unmap, etc.) on those devices

* bdev drivers for NVMe, malloc (ramdisk), Linux AIO and Ceph RBD

* configuration via SPDK configuration files or JSON RPC

* A pluggable module API for implementing block devices that interface with different types of block storage devices.

* Driver modules for NVMe, malloc (ramdisk), Linux AIO, virtio-scsi, Ceph RBD, and more.

* An application API for enumerating and claiming SPDK block devices and then performing operations (read, write, unmap, etc.) on those devices.

* Facilities to stack block devices to create complex I/O pipelines, including logical volume management (lvol) and partition support (GPT).

* Configuration of block devices via JSON-RPC and a configuration file.

* Request queueing, timeout, and reset handling.

* Multiple, lockless queues for sending I/O to block devices.

# Configuring block devices {#bdev_config}

SPDK block devices are typically configured via an SPDK configuration file.  These block devices

can then be associated with higher level abstractions such as iSCSI target nodes, NVMe-oF namespaces

or vhost-scsi controllers.  This section will describe how to configure block devices for the

SPDK bdev drivers included with SPDK.

The SPDK configuration file is typically passed to your SPDK-based application via the command line.

Refer to the help facility of your application for more details.

The block device layer is a C library with a single public header file named

bdev.h. Upon initialization, the library will read in a configuration file that

defines the block devices it will expose. The configuration file is a text

format containing sections denominated by square brackets followed by keys with

optional values. It is often passed as a command line argument to the

application. Refer to the help facility of your application for more details.

## NVMe {#bdev_config_nvme}