Documentation/block/null_blk.txt

   1 Null block device driver
   2 ================================================================================
   3
   4 I. Overview
   5
   6 The null block device (/dev/nullb*) is used for benchmarking the various
   7 block-layer implementations. It emulates a block device of X gigabytes in size.
   8 The 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
  21 All of them have a completion queue for each core in the system.
  22
  23 II. Module parameters applicable for all instances:
  24
  25 queue_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
  32 home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
  33   Selects what CPU node the data structures are allocated from.
  34
  35 gb=[Size in GB]: Default: 250GB
  36   The size of the device reported to the system.
  37
  38 bs=[Block size (in bytes)]: Default: 512 bytes
  39   The block size reported to the system.
  40
  41 nr_devices=[Number of devices]: Default: 1
  42   Number of block devices instantiated. They are instantiated as /dev/nullb0,
  43   etc.
  44
  45 irqmode=[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
  55 completion_nsec=[ns]: Default: 10,000ns
  56   Combined with irqmode=2 (timer). The time each completion event must wait.
  57
  58 submit_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
  63 hw_queue_depth=[0..qdepth]: Default: 64
  64   The hardware queue depth of the device.
  65
  66 III: Multi-queue specific parameters
  67
  68 use_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
  74 use_lightnvm=[0/1]: Default: 0
  75   Register device with LightNVM. Requires blk-mq and CONFIG_NVM to be enabled.
  76
  77 no_sched=[0/1]: Default: 0
  78   0: nullb* use default blk-mq io scheduler.
  79   1: nullb* doesn't use io scheduler.
  80
  81 shared_tags=[0/1]: Default: 0
  82   0: Tag set is not shared.
  83   1: Tag set shared between devices for blk-mq. Only makes sense with
  84      nr_devices > 1, otherwise there's no tag set to share.