1.. SPDX-License-Identifier: GPL-2.0
2
3========================
4Null block device driver
5========================
6
7Overview
8========
9
10The null block device (``/dev/nullb*``) is used for benchmarking the various
11block-layer implementations. It emulates a block device of X gigabytes in size.
12It does not execute any read/write operation, just mark them as complete in
13the request queue. The following instances are possible:
14
15  Multi-queue block-layer
16
17    - Request-based.
18    - Configurable submission queues per device.
19
20  No block-layer (Known as bio-based)
21
22    - Bio-based. IO requests are submitted directly to the device driver.
23    - Directly accepts bio data structure and returns them.
24
25All of them have a completion queue for each core in the system.
26
27Module parameters
28=================
29
30queue_mode=[0-2]: Default: 2-Multi-queue
31  Selects which block-layer the module should instantiate with.
32
33  =  ============
34  0  Bio-based
35  1  Single-queue (deprecated)
36  2  Multi-queue
37  =  ============
38
39home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
40  Selects what CPU node the data structures are allocated from.
41
42gb=[Size in GB]: Default: 250GB
43  The size of the device reported to the system.
44
45bs=[Block size (in bytes)]: Default: 512 bytes
46  The block size reported to the system.
47
48nr_devices=[Number of devices]: Default: 1
49  Number of block devices instantiated. They are instantiated as /dev/nullb0,
50  etc.
51
52irqmode=[0-2]: Default: 1-Soft-irq
53  The completion mode used for completing IOs to the block-layer.
54
55  =  ===========================================================================
56  0  None.
57  1  Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
58     when IOs are issued from another CPU node than the home the device is
59     connected to.
60  2  Timer: Waits a specific period (completion_nsec) for each IO before
61     completion.
62  =  ===========================================================================
63
64completion_nsec=[ns]: Default: 10,000ns
65  Combined with irqmode=2 (timer). The time each completion event must wait.
66
67submit_queues=[1..nr_cpus]: Default: 1
68  The number of submission queues attached to the device driver. If unset, it
69  defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
70  parameter is 1.
71
72hw_queue_depth=[0..qdepth]: Default: 64
73  The hardware queue depth of the device.
74
75memory_backed=[0/1]: Default: 0
76  Whether or not to use a memory buffer to respond to IO requests
77
78  =  =============================================
79  0  Transfer no data in response to IO requests
80  1  Use a memory buffer to respond to IO requests
81  =  =============================================
82
83discard=[0/1]: Default: 0
84  Support discard operations (requires memory-backed null_blk device).
85
86  =  =====================================
87  0  Do not support discard operations
88  1  Enable support for discard operations
89  =  =====================================
90
91cache_size=[Size in MB]: Default: 0
92  Cache size in MB for memory-backed device.
93
94mbps=[Maximum bandwidth in MB/s]: Default: 0 (no limit)
95  Bandwidth limit for device performance.
96
97Multi-queue specific parameters
98-------------------------------
99
100use_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 the value of the submit_queues
105     parameter.
106  1  The multi-queue block layer is instantiated with a hardware dispatch
107     queue for each CPU node in the system.
108  =  =====================================================================
109
110no_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
118blocking=[0/1]: Default: 0
119  Blocking behavior of the request queue.
120
121  =  ===============================================================
122  0  Register as a non-blocking blk-mq driver device.
123  1  Register as a blocking blk-mq driver device, null_blk will set
124     the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
125     needs to block in its ->queue_rq() function.
126  =  ===============================================================
127
128shared_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. Only makes sense with
134     nr_devices > 1, otherwise there's no tag set to share.
135  =  ================================================================
136
137zoned=[0/1]: Default: 0
138  Device is a random-access or a zoned block device.
139
140  =  ======================================================================
141  0  Block device is exposed as a random-access block device.
142  1  Block device is exposed as a host-managed zoned block device. Requires
143     CONFIG_BLK_DEV_ZONED.
144  =  ======================================================================
145
146zone_size=[MB]: Default: 256
147  Per zone size when exposed as a zoned block device. Must be a power of two.
148
149zone_nr_conv=[nr_conv]: Default: 0
150  The number of conventional zones to create when block device is zoned.  If
151  zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.
152