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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.