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
75Multi-queue specific parameters
76-------------------------------
77
78use_per_node_hctx=[0/1]: Default: 0
79  Number of hardware context queues.
80
81  =  =====================================================================
82  0  The number of submit queues are set to the value of the submit_queues
83     parameter.
84  1  The multi-queue block layer is instantiated with a hardware dispatch
85     queue for each CPU node in the system.
86  =  =====================================================================
87
88no_sched=[0/1]: Default: 0
89  Enable/disable the io scheduler.
90
91  =  ======================================
92  0  nullb* use default blk-mq io scheduler
93  1  nullb* doesn't use io scheduler
94  =  ======================================
95
96blocking=[0/1]: Default: 0
97  Blocking behavior of the request queue.
98
99  =  ===============================================================
100  0  Register as a non-blocking blk-mq driver device.
101  1  Register as a blocking blk-mq driver device, null_blk will set
102     the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
103     needs to block in its ->queue_rq() function.
104  =  ===============================================================
105
106shared_tags=[0/1]: Default: 0
107  Sharing tags between devices.
108
109  =  ================================================================
110  0  Tag set is not shared.
111  1  Tag set shared between devices for blk-mq. Only makes sense with
112     nr_devices > 1, otherwise there's no tag set to share.
113  =  ================================================================
114
115zoned=[0/1]: Default: 0
116  Device is a random-access or a zoned block device.
117
118  =  ======================================================================
119  0  Block device is exposed as a random-access block device.
120  1  Block device is exposed as a host-managed zoned block device. Requires
121     CONFIG_BLK_DEV_ZONED.
122  =  ======================================================================
123
124zone_size=[MB]: Default: 256
125  Per zone size when exposed as a zoned block device. Must be a power of two.
126
127zone_nr_conv=[nr_conv]: Default: 0
128  The number of conventional zones to create when block device is zoned.  If
129  zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.
130