aboutsummaryrefslogtreecommitdiff
path: root/Documentation/block/null_blk.rst
blob: edbbab2f12f8c7b99f6a7059acbfa3760b1ac98e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
.. SPDX-License-Identifier: GPL-2.0

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

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.
It does not execute any read/write operation, just mark them as complete in
the request queue. The following instances are possible:

  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.

Module parameters
=================

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

  =  ============
  0  Bio-based
  1  Single-queue (deprecated)
  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]: Default: 1
  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.

Multi-queue specific parameters
-------------------------------

use_per_node_hctx=[0/1]: Default: 0
  Number of hardware context queues.

  =  =====================================================================
  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
  Enable/disable the io scheduler.

  =  ======================================
  0  nullb* use default blk-mq io scheduler
  1  nullb* doesn't use io scheduler
  =  ======================================

blocking=[0/1]: Default: 0
  Blocking behavior of the request queue.

  =  ===============================================================
  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
  Sharing tags between devices.

  =  ================================================================
  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
  Device is a random-access or a zoned block device.

  =  ======================================================================
  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.