doc: add bdevperf doc section

                         accel_fw.md \

                         applications.md \

                         bdev.md \

                         bdevperf.md \

                         bdev_module.md \

                         bdev_pg.md \

                         blob.md \

# Using bdevperf application {#bdevperf}

## Introduction

bdevperf is an SPDK application that is used for performance testing

of block devices (bdevs) exposed by the SPDK bdev layer.  It is an

alternative to the SPDK bdev fio plugin for benchmarking SPDK bdevs.

In some cases, bdevperf can provide much lower overhead than the fio

plugin, resulting in much better performance for tests using a limited

number of CPU cores.

bdevperf exposes command line interface that allows to specify

SPDK framework options as well as testing options.

Since SPDK 20.07, bdevperf supports configuration file that is similar

to FIO. It allows user to create jobs parameterized by

filename, cpumask, blocksize, queuesize, etc.

## Config file

Bdevperf's config file is similar to FIO's config file format.

Below is an example config file that uses all available parameters:

~~~{.ini}

[global]

filename=Malloc0:Malloc1

bs=1024

iosize=256

rw=randrw

rwmixread=90

[A]

cpumask=0xff

[B]

cpumask=[0-128]

filename=Malloc1

[global]

filename=Malloc0

rw=write

[C]

bs=4096

iosize=128

offset=1000000

length=1000000

~~~

Jobs `[A]` `[B]` or `[C]`, inherit default values from `[global]`

section residing above them. So in the example, job `[A]` inherits

`filename` value and uses both `Malloc0` and `Malloc1` bdevs as targets,

job `[B]` overrides its `filename` value and uses `Malloc1` and

job `[C]` inherits value `Malloc0` for its `filename`.

Interaction with CLI arguments is not the same as in FIO however.

If bdevperf receives CLI argument, it overrides values

of corresponding parameter for all `[global]` sections of config file.

So if example config is used, specifying `-q` argument

will make jobs `[A]` and `[B]` use its value.

Below is a full list of supported parameters with descriptions.

Param     | Default           | Description

--------- | ----------------- | -----------

filename  |                   | Bdevs to use, separated by ":"

cpumask   | Maximum available | CPU mask. Format is defined at @ref cpu_mask

bs        |                   | Block size (io size)

iodepth   |                   | Queue depth

rwmixread | `50`              | Percentage of a mixed workload that should be reads

offset    | `0`               | Start I/O at the provided offset on the bdev

length    | 100% of bdev size | End I/O at `offset`+`length` on the bdev

rw        |                   | Type of I/O pattern

Available rw types:

- read

- randread

- write

- randwrite

- verify

- reset

- unmap

- write_zeroes

- flush

- rw

- randrw

- @subpage spdkcli

- @subpage nvme-cli

- @subpage bdevperf