LXR linux/Documentation/block/null

   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: 1
  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=[1..nr_cpus]:
  59  The number of submission queues attached to the device driver. If unset, it
  60  defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
  61  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
  74no_sched=[0/1]: Default: 0
  75  0: nullb* use default blk-mq io scheduler.
  76  1: nullb* doesn't use io scheduler.
  77
  78blocking=[0/1]: Default: 0
  79  0: Register as a non-blocking blk-mq driver device.
  80  1: Register as a blocking blk-mq driver device, null_blk will set
  81     the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
  82     needs to block in its ->queue_rq() function.
  83
  84shared_tags=[0/1]: Default: 0
  85  0: Tag set is not shared.
  86  1: Tag set shared between devices for blk-mq. Only makes sense with
  87     nr_devices > 1, otherwise there's no tag set to share.
  88
  89zoned=[0/1]: Default: 0
  90  0: Block device is exposed as a random-access block device.
  91  1: Block device is exposed as a host-managed zoned block device. Requires
  92     CONFIG_BLK_DEV_ZONED.
  93
  94zone_size=[MB]: Default: 256
  95  Per zone size when exposed as a zoned block device. Must be a power of two.
  96