null_blk: documentation

Add description of module and its parameters.
Signed-off-by: Matias Bjorling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/null_blk.txt

+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.