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docs: block: null_blk: enhance document style
Use proper ReST syntax for chapters. Add more information to enhance
standardization in the file and to make the rendering more homogeneous.
Add a SPDX identifier. Mark single-queue mode as deprecated.
Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
authored by
André Almeida
and committed by
Jens Axboe
04c56957
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Documentation/block/null_blk.rst
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Documentation/block/null_blk.rst
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.. SPDX-License-Identifier: GPL-2.0
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========================
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Null block device driver
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========================
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1. Overview
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===========
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Overview
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========
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The null block device (/dev/nullb*) is used for benchmarking the various
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The null block device (``/dev/nullb*``) is used for benchmarking the various
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block-layer implementations. It emulates a block device of X gigabytes in size.
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The following instances are possible:
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Single-queue block-layer
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- Request-based.
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- Single submission queue per device.
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- Implements IO scheduling algorithms (CFQ, Deadline, noop).
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It does not execute any read/write operation, just mark them as complete in
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the request queue. The following instances are possible:
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Multi-queue block-layer
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···
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All of them have a completion queue for each core in the system.
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2. Module parameters applicable for all instances
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=================================================
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Module parameters
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=================
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queue_mode=[0-2]: Default: 2-Multi-queue
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Selects which block-layer the module should instantiate with.
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= ============
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0 Bio-based
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1 Single-queue
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1 Single-queue (deprecated)
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2 Multi-queue
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= ============
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···
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completion_nsec=[ns]: Default: 10,000ns
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Combined with irqmode=2 (timer). The time each completion event must wait.
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submit_queues=[1..nr_cpus]:
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submit_queues=[1..nr_cpus]: Default: 1
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The number of submission queues attached to the device driver. If unset, it
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defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
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parameter is 1.
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hw_queue_depth=[0..qdepth]: Default: 64
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The hardware queue depth of the device.
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III: Multi-queue specific parameters
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Multi-queue specific parameters
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-------------------------------
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use_per_node_hctx=[0/1]: Default: 0
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Number of hardware context queues.
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= =====================================================================
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0 The number of submit queues are set to the value of the submit_queues
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= =====================================================================
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no_sched=[0/1]: Default: 0
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Enable/disable the io scheduler.
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= ======================================
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0 nullb* use default blk-mq io scheduler
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= ======================================
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blocking=[0/1]: Default: 0
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Blocking behavior of the request queue.
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= ===============================================================
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0 Register as a non-blocking blk-mq driver device.
···
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= ===============================================================
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shared_tags=[0/1]: Default: 0
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Sharing tags between devices.
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= ================================================================
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0 Tag set is not shared.
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= ================================================================
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zoned=[0/1]: Default: 0
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Device is a random-access or a zoned block device.
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= ======================================================================
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0 Block device is exposed as a random-access block device.