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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 have 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 CPU node the data structures are 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=[Number of devices]: Default: 1
- Number of block devices instantiated. They are instantiated as /dev/nullb0,
- etc.
-
-irqmode=[0-2]: Default: 1-Soft-irq
- The completion mode used for completing IOs to the block-layer.
-
- 0: None.
- 1: Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
- when IOs are issued from another CPU node than the home the device is
- connected to.
- 2: Timer: Waits a specific period (completion_nsec) for each IO before
- completion.
-
-completion_nsec=[ns]: Default: 10,000ns
- Combined with irqmode=2 (timer). The time each completion event must wait.
-
-submit_queues=[1..nr_cpus]:
- The number of submission queues attached to the device driver. If unset, it
- defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
- parameter is 1.
-
-hw_queue_depth=[0..qdepth]: Default: 64
- The hardware queue depth of the device.
-
-III: Multi-queue specific parameters
-
-use_per_node_hctx=[0/1]: Default: 0
- 0: The number of submit queues are set to the value of the submit_queues
- parameter.
- 1: The multi-queue block layer is instantiated with a hardware dispatch
- queue for each CPU node in the system.
-
-no_sched=[0/1]: Default: 0
- 0: nullb* use default blk-mq io scheduler.
- 1: nullb* doesn't use io scheduler.
-
-blocking=[0/1]: Default: 0
- 0: Register as a non-blocking blk-mq driver device.
- 1: Register as a blocking blk-mq driver device, null_blk will set
- the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
- needs to block in its ->queue_rq() function.
-
-shared_tags=[0/1]: Default: 0
- 0: Tag set is not shared.
- 1: Tag set shared between devices for blk-mq. Only makes sense with
- nr_devices > 1, otherwise there's no tag set to share.
-
-zoned=[0/1]: Default: 0
- 0: Block device is exposed as a random-access block device.
- 1: Block device is exposed as a host-managed zoned block device. Requires
- CONFIG_BLK_DEV_ZONED.
-
-zone_size=[MB]: Default: 256
- Per zone size when exposed as a zoned block device. Must be a power of two.
-
-zone_nr_conv=[nr_conv]: Default: 0
- The number of conventional zones to create when block device is zoned. If
- zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.