1 .. SPDX-License-Identifier: GPL-2.0
2
3 ========================
4 Null block device driver
5 ========================
6
7 Overview
8 ========
9
10 The null block device (``/dev/nullb*``) is use
11 block-layer implementations. It emulates a blo
12 It does not execute any read/write operation,
13 the request queue. The following instances are
14
15 Multi-queue block-layer
16
17 - Request-based.
18 - Configurable submission queues per devic
19
20 No block-layer (Known as bio-based)
21
22 - Bio-based. IO requests are submitted dir
23 - Directly accepts bio data structure and
24
25 All of them have a completion queue for each c
26
27 Module parameters
28 =================
29
30 queue_mode=[0-2]: Default: 2-Multi-queue
31 Selects which block-layer the module should
32
33 = ============
34 0 Bio-based
35 1 Single-queue (deprecated)
36 2 Multi-queue
37 = ============
38
39 home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
40 Selects what CPU node the data structures ar
41
42 gb=[Size in GB]: Default: 250GB
43 The size of the device reported to the syste
44
45 bs=[Block size (in bytes)]: Default: 512 bytes
46 The block size reported to the system.
47
48 nr_devices=[Number of devices]: Default: 1
49 Number of block devices instantiated. They a
50 etc.
51
52 irqmode=[0-2]: Default: 1-Soft-irq
53 The completion mode used for completing IOs
54
55 = =========================================
56 0 None.
57 1 Soft-irq. Uses IPI to complete IOs across
58 when IOs are issued from another CPU node
59 connected to.
60 2 Timer: Waits a specific period (completio
61 completion.
62 = =========================================
63
64 completion_nsec=[ns]: Default: 10,000ns
65 Combined with irqmode=2 (timer). The time ea
66
67 submit_queues=[1..nr_cpus]: Default: 1
68 The number of submission queues attached to
69 defaults to 1. For multi-queue, it is ignore
70 parameter is 1.
71
72 hw_queue_depth=[0..qdepth]: Default: 64
73 The hardware queue depth of the device.
74
75 memory_backed=[0/1]: Default: 0
76 Whether or not to use a memory buffer to res
77
78 = =========================================
79 0 Transfer no data in response to IO reques
80 1 Use a memory buffer to respond to IO requ
81 = =========================================
82
83 discard=[0/1]: Default: 0
84 Support discard operations (requires memory-
85
86 = =====================================
87 0 Do not support discard operations
88 1 Enable support for discard operations
89 = =====================================
90
91 cache_size=[Size in MB]: Default: 0
92 Cache size in MB for memory-backed device.
93
94 mbps=[Maximum bandwidth in MB/s]: Default: 0 (
95 Bandwidth limit for device performance.
96
97 Multi-queue specific parameters
98 -------------------------------
99
100 use_per_node_hctx=[0/1]: Default: 0
101 Number of hardware context queues.
102
103 = =========================================
104 0 The number of submit queues are set to th
105 parameter.
106 1 The multi-queue block layer is instantiat
107 queue for each CPU node in the system.
108 = =========================================
109
110 no_sched=[0/1]: Default: 0
111 Enable/disable the io scheduler.
112
113 = ======================================
114 0 nullb* use default blk-mq io scheduler
115 1 nullb* doesn't use io scheduler
116 = ======================================
117
118 blocking=[0/1]: Default: 0
119 Blocking behavior of the request queue.
120
121 = =========================================
122 0 Register as a non-blocking blk-mq driver
123 1 Register as a blocking blk-mq driver devi
124 the BLK_MQ_F_BLOCKING flag, indicating th
125 needs to block in its ->queue_rq() functi
126 = =========================================
127
128 shared_tags=[0/1]: Default: 0
129 Sharing tags between devices.
130
131 = =========================================
132 0 Tag set is not shared.
133 1 Tag set shared between devices for blk-mq
134 nr_devices > 1, otherwise there's no tag
135 = =========================================
136
137 zoned=[0/1]: Default: 0
138 Device is a random-access or a zoned block d
139
140 = =========================================
141 0 Block device is exposed as a random-acces
142 1 Block device is exposed as a host-managed
143 CONFIG_BLK_DEV_ZONED.
144 = =========================================
145
146 zone_size=[MB]: Default: 256
147 Per zone size when exposed as a zoned block
148
149 zone_nr_conv=[nr_conv]: Default: 0
150 The number of conventional zones to create w
151 zone_nr_conv >= nr_zones, it will be reduced
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