1========================
2Null block device driver
3========================
4
51. Overview
6===========
7
8The null block device (/dev/nullb*) is used for benchmarking the various
9block-layer implementations. It emulates a block device of X gigabytes in size.
10The following instances are possible:
11
12  Single-queue block-layer
13
14    - Request-based.
15    - Single submission queue per device.
16    - Implements IO scheduling algorithms (CFQ, Deadline, noop).
17
18  Multi-queue block-layer
19
20    - Request-based.
21    - Configurable submission queues per device.
22
23  No block-layer (Known as bio-based)
24
25    - Bio-based. IO requests are submitted directly to the device driver.
26    - Directly accepts bio data structure and returns them.
27
28All of them have a completion queue for each core in the system.
29
302. Module parameters applicable for all instances
31=================================================
32
33queue_mode=[0-2]: Default: 2-Multi-queue
34  Selects which block-layer the module should instantiate with.
35
36  =  ============
37  0  Bio-based
38  1  Single-queue
39  2  Multi-queue
40  =  ============
41
42home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
43  Selects what CPU node the data structures are allocated from.
44
45gb=[Size in GB]: Default: 250GB
46  The size of the device reported to the system.
47
48bs=[Block size (in bytes)]: Default: 512 bytes
49  The block size reported to the system.
50
51nr_devices=[Number of devices]: Default: 1
52  Number of block devices instantiated. They are instantiated as /dev/nullb0,
53  etc.
54
55irqmode=[0-2]: Default: 1-Soft-irq
56  The completion mode used for completing IOs to the block-layer.
57
58  =  ===========================================================================
59  0  None.
60  1  Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
61     when IOs are issued from another CPU node than the home the device is
62     connected to.
63  2  Timer: Waits a specific period (completion_nsec) for each IO before
64     completion.
65  =  ===========================================================================
66
67completion_nsec=[ns]: Default: 10,000ns
68  Combined with irqmode=2 (timer). The time each completion event must wait.
69
70submit_queues=[1..nr_cpus]:
71  The number of submission queues attached to the device driver. If unset, it
72  defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
73  parameter is 1.
74
75hw_queue_depth=[0..qdepth]: Default: 64
76  The hardware queue depth of the device.
77
78III: Multi-queue specific parameters
79
80use_per_node_hctx=[0/1]: Default: 0
81
82  =  =====================================================================
83  0  The number of submit queues are set to the value of the submit_queues
84     parameter.
85  1  The multi-queue block layer is instantiated with a hardware dispatch
86     queue for each CPU node in the system.
87  =  =====================================================================
88
89no_sched=[0/1]: Default: 0
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
98  =  ===============================================================
99  0  Register as a non-blocking blk-mq driver device.
100  1  Register as a blocking blk-mq driver device, null_blk will set
101     the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
102     needs to block in its ->queue_rq() function.
103  =  ===============================================================
104
105shared_tags=[0/1]: Default: 0
106
107  =  ================================================================
108  0  Tag set is not shared.
109  1  Tag set shared between devices for blk-mq. Only makes sense with
110     nr_devices > 1, otherwise there's no tag set to share.
111  =  ================================================================
112
113zoned=[0/1]: Default: 0
114
115  =  ======================================================================
116  0  Block device is exposed as a random-access block device.
117  1  Block device is exposed as a host-managed zoned block device. Requires
118     CONFIG_BLK_DEV_ZONED.
119  =  ======================================================================
120
121zone_size=[MB]: Default: 256
122  Per zone size when exposed as a zoned block device. Must be a power of two.
123
124zone_nr_conv=[nr_conv]: Default: 0
125  The number of conventional zones to create when block device is zoned.  If
126  zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.
127