1 ========================
2 Null block device driver
3 ========================
8 The null block device (/dev/nullb*) is used for benchmarking the various
9 block-layer implementations. It emulates a block device of X gigabytes in size.
10 The following instances are possible:
12 Single-queue block-layer
15 - Single submission queue per device.
16 - Implements IO scheduling algorithms (CFQ, Deadline, noop).
18 Multi-queue block-layer
21 - Configurable submission queues per device.
23 No block-layer (Known as bio-based)
25 - Bio-based. IO requests are submitted directly to the device driver.
26 - Directly accepts bio data structure and returns them.
28 All of them have a completion queue for each core in the system.
30 2. Module parameters applicable for all instances
31 =================================================
33 queue_mode=[0-2]: Default: 2-Multi-queue
34 Selects which block-layer the module should instantiate with.
42 home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
43 Selects what CPU node the data structures are allocated from.
45 gb=[Size in GB]: Default: 250GB
46 The size of the device reported to the system.
48 bs=[Block size (in bytes)]: Default: 512 bytes
49 The block size reported to the system.
51 nr_devices=[Number of devices]: Default: 1
52 Number of block devices instantiated. They are instantiated as /dev/nullb0,
55 irqmode=[0-2]: Default: 1-Soft-irq
56 The completion mode used for completing IOs to the block-layer.
58 = ===========================================================================
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
63 2 Timer: Waits a specific period (completion_nsec) for each IO before
65 = ===========================================================================
67 completion_nsec=[ns]: Default: 10,000ns
68 Combined with irqmode=2 (timer). The time each completion event must wait.
70 submit_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
75 hw_queue_depth=[0..qdepth]: Default: 64
76 The hardware queue depth of the device.
78 III: Multi-queue specific parameters
80 use_per_node_hctx=[0/1]: Default: 0
82 = =====================================================================
83 0 The number of submit queues are set to the value of the submit_queues
85 1 The multi-queue block layer is instantiated with a hardware dispatch
86 queue for each CPU node in the system.
87 = =====================================================================
89 no_sched=[0/1]: Default: 0
91 = ======================================
92 0 nullb* use default blk-mq io scheduler
93 1 nullb* doesn't use io scheduler
94 = ======================================
96 blocking=[0/1]: Default: 0
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 = ===============================================================
105 shared_tags=[0/1]: Default: 0
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 = ================================================================
113 zoned=[0/1]: Default: 0
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 = ======================================================================
121 zone_size=[MB]: Default: 256
122 Per zone size when exposed as a zoned block device. Must be a power of two.
124 zone_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.