1Null block device driver
2================================================================================
3
4I. Overview
5
6The null block device (/dev/nullb*) is used for benchmarking the various
7block-layer implementations. It emulates a block device of X gigabytes in size.
8The following instances are possible:
9
10  Single-queue block-layer
11    - Request-based.
12    - Single submission queue per device.
13    - Implements IO scheduling algorithms (CFQ, Deadline, noop).
14  Multi-queue block-layer
15    - Request-based.
16    - Configurable submission queues per device.
17  No block-layer (Known as bio-based)
18    - Bio-based. IO requests are submitted directly to the device driver.
19    - Directly accepts bio data structure and returns them.
20
21All of them have a completion queue for each core in the system.
22
23II. Module parameters applicable for all instances:
24
25queue_mode=[0-2]: Default: 2-Multi-queue
26  Selects which block-layer the module should instantiate with.
27
28  0: Bio-based.
29  1: Single-queue.
30  2: Multi-queue.
31
32home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
33  Selects what CPU node the data structures are allocated from.
34
35gb=[Size in GB]: Default: 250GB
36  The size of the device reported to the system.
37
38bs=[Block size (in bytes)]: Default: 512 bytes
39  The block size reported to the system.
40
41nr_devices=[Number of devices]: Default: 2
42  Number of block devices instantiated. They are instantiated as /dev/nullb0,
43  etc.
44
45irqmode=[0-2]: Default: 1-Soft-irq
46  The completion mode used for completing IOs to the block-layer.
47
48  0: None.
49  1: Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
50     when IOs are issued from another CPU node than the home the device is
51     connected to.
52  2: Timer: Waits a specific period (completion_nsec) for each IO before
53     completion.
54
55completion_nsec=[ns]: Default: 10.000ns
56  Combined with irqmode=2 (timer). The time each completion event must wait.
57
58submit_queues=[0..nr_cpus]:
59  The number of submission queues attached to the device driver. If unset, it
60  defaults to 1 on single-queue and bio-based instances. For multi-queue,
61  it is ignored when use_per_node_hctx module parameter is 1.
62
63hw_queue_depth=[0..qdepth]: Default: 64
64  The hardware queue depth of the device.
65
66III: Multi-queue specific parameters
67
68use_per_node_hctx=[0/1]: Default: 0
69  0: The number of submit queues are set to the value of the submit_queues
70     parameter.
71  1: The multi-queue block layer is instantiated with a hardware dispatch
72     queue for each CPU node in the system.
73