examples/bdev: update fio bdev plugin readme file

# Introduction

This directory contains a plug-in module for fio to enable use

with SPDK. Fio is free software published under version 2 of

the GPL license.

This directory contains a plug-in module for fio to enable use with SPDK. Fio is free software

published under version 2 of the GPL license.

## Compiling fio

Clone the fio source repository from https://github.com/axboe/fio

Clone the [fio source repository](https://github.com/axboe/fio)

```bash

    git clone https://github.com/axboe/fio

## Compiling SPDK

Clone the SPDK source repository from https://github.com/spdk/spdk

Clone the [SPDK source repository](https://github.com/spdk/spdk)

```bash

    git clone https://github.com/spdk/spdk

To use the SPDK fio plugin with fio, specify the plugin binary using LD_PRELOAD when running

fio and set ioengine=spdk_bdev in the fio configuration file (see example_config.fio in the same

directory as this README).

directory as this README). Following example command assumes `fio` is in your system `$PATH` environment variable.

```bash

LD_PRELOAD=<path to spdk repo>/build/fio/spdk_bdev fio

```

The fio configuration file must contain one new parameter:

The fio configuration file must contain parameter pointing to a JSON configuration file containing SPDK bdev configuration:

```bash

spdk_json_conf=./examples/bdev/fio_plugin/bdev.json

processing consumes extra CPU cycles which will degrade performance over time with

the fio_plugin since all I/O are submitted and completed on a single CPU core.

### Step-by-step usage examples

These examples assume you have built fio and SPDK with `--with-fio` option enabled.

#### Using fio bdev plugin with local NVMe storage

- Bind local NVMe drives to userspace driver

- Run gen_nvme.sh script to create a JSON file with bdev subsystem configuration

    ```bash

    scripts/gen_nvme.sh --json-with-subsystems > /tmp/bdev.json

    cat /tmp/bdev_local.json  | jq

    {

        "subsystems": [

        {

            "subsystem": "bdev",

            "config": [

            {

                "method": "bdev_nvme_attach_controller",

                "params": {

                "trtype": "PCIe",

                "name": "Nvme0",

                "traddr": "0000:0a:00.0"

                }

            },

            {

                "method": "bdev_nvme_attach_controller",

                "params": {

                "trtype": "PCIe",

                "name": "Nvme1",

                "traddr": "0000:85:00.0"

                }

            }

            ]

        }

        ]

    }

    ```

- Prepare fio configuration file

    ```bash

    cat /tmp/fio.conf

    [global]

    ioengine=/spdk/build/fio/spdk_bdev

    spdk_json_conf=/tmp/bdev.json

    thread=1

    direct=1

    group_reporting=1

    bs=4k

    rw=randread

    rwmixread=70

    time_based=1

    runtime=10

    norandommap=1

    [filename0]

    filename=Nvme0n1

    filename=Nvme1n1

    iodepth=8

    ```

- Run fio with spdk bdev plugin

    ```bash

    /usr/src/fio/fio /tmp/fio.conf

    ```

#### Using fio bdev plugin as NVMe-oF initiator with remote storage

- Start SPDK NVMe-oF Target process and configure it with block devices and NVMe-oF subsystems

    ```bash

    build/bin/nvmf_tgt &

    sleep 3

    scripts/rpc.py bdev_malloc_create 10 512 -b Malloc0

    scripts/rpc.py nvmf_create_transport -t TCP

    scripts/rpc.py nvmf_create_subsystem nqn.2018-09.io.spdk:cnode1 -a -s S000001

    scripts/rpc.py nvmf_subsystem_add_listener nqn.2018-09.io.spdk:cnode1 -t tcp -f ipv4 -s 4420 -a 10.0.0.1

    scripts/rpc.py nvmf_subsystem_add_ns nqn.2018-09.io.spdk:cnode1 Malloc0

    ```

- Run gen_nvme.sh script to prepare a JSON file containing bdev subsystem configuration

  for initiator which will allow it to connect to target

    ```bash

    scripts/gen_nvme.sh --json-with-subsystems --mode=remote \

    --trid=tcp:10.0.0.1:4420:nqn.2018-09.io.spdk:cnode1 > /tmp/bdev.json

    cat /tmp/bdev.json | jq

    {

        "subsystems": [

        {

            "subsystem": "bdev",

            "config": [

            {

                "method": "bdev_nvme_attach_controller",

                "params": {

                "trtype": "tcp",

                "adrfam": "IPv4",

                "name": "Nvme0",

                "subnqn": "nqn.2018-09.io.spdk:cnode1",

                "traddr": "10.0.0.1",

                "trsvcid": "4420"

                }

            }

            ]

        }

        ]

    }

    ```

- Prepare fio configuration file

    ```bash

    cat /tmp/fio.conf

    [global]

    ioengine=/spdk/build/fio/spdk_bdev

    spdk_json_conf=/tmp/bdev.json

    thread=1

    direct=1

    group_reporting=1

    bs=4k

    rw=randread

    rwmixread=70

    time_based=1

    runtime=10

    norandommap=1

    [filename0]

    filename=Nvme0n1

    iodepth=8

    ```

- Run fio bdev plugin as initiator

    ```bash

    /usr/src/fio/fio /tmp/fio.conf

    ```

## Zoned Block Devices

SPDK has a zoned block device API (bdev_zone.h) which currently supports Open-channel SSDs,

state at any point in time. It is very important to not exceed this limit.

You can control how many zones fio will keep in an open state by using the

``--max_open_zones`` option.

`--max_open_zones` option.

If you use a fio version newer than 3.26, fio will automatically detect and set the proper value.

If you use an old version of fio, make sure to provide the proper --max_open_zones value yourself.

### Maximum Active Zones

Zoned block devices may also have a resource constraint on the number of zones that can be active at

any point in time. Unlike ``max_open_zones``, fio currently does not manage this constraint, and

any point in time. Unlike `max_open_zones`, fio currently does not manage this constraint, and

there is thus no option to limit it either.

Since the max active zones limit (by definition) has to be greater than or equal to the max open