Ubuntu 18.04 setting null_blk, test fio

先確認/lib/modules/4.15.0-96-generic/kernel/drivers/block/ 是否有null_blk.ko 
若沒有，請先自行build出此driver. 或是使用apt-get install 安裝

** apt-get install extra driver

# uname -a
Linux sam 4.15.0-96-generic #97-Ubuntu SMP Wed Apr 1 03:25:46 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

# apt-cache search linux-modules-extra-4.15.0.96
linux-modules-extra-4.15.0-96-generic - Linux kernel extra modules for version 4.15.0 on 64 bit x86 SMP

# apt-get install linux-modules-extra-4.15.0-96-generic


** 自動掛載 null_blk driver

# vi /etc/modules
# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
null_blk

** 設定 null_blk 參數，只能以 module 形式，在開機讀取設定.
# vi /etc/modprobe.d/null_blk.conf 

寫入下行資料, nr_devices=1 建立 nullb0, queue_mode=0 初步測試沒queue能測出較高IOPS.
options null_blk nr_devices=1 queue_mode=0

#  reboot

如何確認queue_mode=0, queue 被關閉，請確認 mq 不存在，即表示queue_mode=0 設定成功.

#  ls /sys/class/block/nullb0
alignment_offset  bdi  capability  dev  discard_alignment  ext_range  hidden  holders  inflight  integrity  mq  power  queue  range  removable  ro  size  slaves  stat  subsystem  trace  uevent

# ls /sys/class/block/nullb0

alignment_offset  bdi  capability  dev  discard_alignment  ext_range  hidden  holders  inflight  integrity  power  queue  range  removable  ro  size  slaves  stat  subsystem  trace  uevent

Null block device driver

https://www.kernel.org/doc/Documentation/block/null_blk.txt

Null block device driver
================================================================================

I. Overview

The null block device (/dev/nullb*) is used for benchmarking the various
block-layer implementations. It emulates a block device of X gigabytes in size.
The following instances are possible:

  Single-queue block-layer
    - Request-based.
    - Single submission queue per device.
    - Implements IO scheduling algorithms (CFQ, Deadline, noop).
  Multi-queue block-layer
    - Request-based.
    - Configurable submission queues per device.
  No block-layer (Known as bio-based)
    - Bio-based. IO requests are submitted directly to the device driver.
    - Directly accepts bio data structure and returns them.

All of them have a completion queue for each core in the system.

II. Module parameters applicable for all instances:

queue_mode=[0-2]: Default: 2-Multi-queue
  Selects which block-layer the module should instantiate with.

  0: Bio-based.
  1: Single-queue.
  2: Multi-queue.

home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
  Selects what CPU node the data structures are allocated from.

gb=[Size in GB]: Default: 250GB
  The size of the device reported to the system.

bs=[Block size (in bytes)]: Default: 512 bytes
  The block size reported to the system.

nr_devices=[Number of devices]: Default: 1
  Number of block devices instantiated. They are instantiated as /dev/nullb0,
  etc.

irqmode=[0-2]: Default: 1-Soft-irq
  The completion mode used for completing IOs to the block-layer.

  0: None.
  1: Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
     when IOs are issued from another CPU node than the home the device is
     connected to.
  2: Timer: Waits a specific period (completion_nsec) for each IO before
     completion.

completion_nsec=[ns]: Default: 10,000ns
  Combined with irqmode=2 (timer). The time each completion event must wait.

submit_queues=[1..nr_cpus]:
  The number of submission queues attached to the device driver. If unset, it
  defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
  parameter is 1.

hw_queue_depth=[0..qdepth]: Default: 64
  The hardware queue depth of the device.

III: Multi-queue specific parameters

use_per_node_hctx=[0/1]: Default: 0
  0: The number of submit queues are set to the value of the submit_queues
     parameter.
  1: The multi-queue block layer is instantiated with a hardware dispatch
     queue for each CPU node in the system.

no_sched=[0/1]: Default: 0
  0: nullb* use default blk-mq io scheduler.
  1: nullb* doesn't use io scheduler.

blocking=[0/1]: Default: 0
  0: Register as a non-blocking blk-mq driver device.
  1: Register as a blocking blk-mq driver device, null_blk will set
     the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
     needs to block in its ->queue_rq() function.

shared_tags=[0/1]: Default: 0
  0: Tag set is not shared.
  1: Tag set shared between devices for blk-mq. Only makes sense with
     nr_devices > 1, otherwise there's no tag set to share.

zoned=[0/1]: Default: 0
  0: Block device is exposed as a random-access block device.
  1: Block device is exposed as a host-managed zoned block device. Requires
     CONFIG_BLK_DEV_ZONED.

zone_size=[MB]: Default: 256
  Per zone size when exposed as a zoned block device. Must be a power of two.

zone_nr_conv=[nr_conv]: Default: 0
  The number of conventional zones to create when block device is zoned.  If
  zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.