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Commit 12f8f4fc authored by Matias Bjorling's avatar Matias Bjorling Committed by Jens Axboe
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null_blk: documentation 



Add description of module and its parameters.

Signed-off-by: default avatarMatias Bjorling <m@bjorling.me>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 0c56010c

Documentation/block/null_blk.txt

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Null block device driver

================================================================================



I. Overview



The null block device (/dev/nullb*) is used for benchmarking the various

block-layer implementations. It emulates a block device of X gigabytes in size.

The following instances are possible:



  Single-queue block-layer

    - Request-based.

    - Single submission queue per device.

    - Implements IO scheduling algorithms (CFQ, Deadline, noop).

  Multi-queue block-layer

    - Request-based.

    - Configurable submission queues per device.

  No block-layer (Known as bio-based)

    - Bio-based. IO requests are submitted directly to the device driver.

    - Directly accepts bio data structure and returns them.



All of them has a completion queue for each core in the system.



II. Module parameters applicable for all instances:



queue_mode=[0-2]: Default: 2-Multi-queue

  Selects which block-layer the module should instantiate with.



  0: Bio-based.

  1: Single-queue.

  2: Multi-queue.



home_node=[0--nr_nodes]: Default: NUMA_NO_NODE

  Selects what socket the data structures is allocated from.



gb=[Size in GB]: Default: 250GB

  The size of the device reported to the system.



bs=[Block size (in bytes)]: Default: 512 bytes

  The block size reported to the system.



nr_devices=[Num. devices]: Default: 2

  Number of block devices instantiated. They are instantiated as /dev/nullb0,

  etc.



irq_mode=[0-2]: Default: Soft-irq

  The completion mode used for completing IOs to the block-layer.



  0: None.

  1: Soft-irq. Uses ipi to complete IOs across sockets. Simulates the overhead

     when IOs are issued from another socket than the home the device is

     connected to.

  2: Timer: Waits a specific period (completion_nsec) for each IO before

     completion.



completion_nsec=[Num. ns]: Default: 10.000ns

  Combined with irq_mode=2 (timer). The time each completion event must wait.



submit_queues=[0..nr_cpus]:

  The number of submission queues attached to the device driver. If unset, it

  defaults to 1 on single-queue and bio-based instances. For multi-queue,

  its ignored when use_per_node_hctx module parameter is 1.



hw_queue_depth=[0..qdepth]: Defaults: 64

  The hardware queue depth of the device.



III: Multi-queue specific parameters



use_per_node_hctx=[0/1]: Defaults: 1

  If 1, the multi-queue block layer is instantiated with a hardware dispatch

  queue for each CPU node in the system. If 0, it is instantiated with the

  number of queues defined in the submit_queues parameter.