Documentation/block/null_blk.rst

1 .. SPDX-License-Identifier: GPL-2.0
4 Null block device driver
10 The null block device (``/dev/nullb*``) is used for benchmarking the various
11 block-layer implementations. It emulates a block device of X gigabytes in size.
15   Multi-queue block-layer
17     - Request-based.
18     - Configurable submission queues per device.
20   No block-layer (Known as bio-based)
22     - Bio-based. IO requests are submitted directly to the device driver.
23     - Directly accepts bio data structure and returns them.
30 queue_mode=[0-2]: Default: 2-Multi-queue
31   Selects which block-layer the module should instantiate with.
34   0  Bio-based
35   1  Single-queue (deprecated)
36   2  Multi-queue
39 home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
45 bs=[Block size (in bytes)]: Default: 512 bytes
46   The block size reported to the system.
49   Number of block devices instantiated. They are instantiated as /dev/nullb0,
52 irqmode=[0-2]: Default: 1-Soft-irq
53   The completion mode used for completing IOs to the block-layer.
57   1  Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
69   defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
84   Support discard operations (requires memory-backed null_blk device).
92   Cache size in MB for memory-backed device.
97 Multi-queue specific parameters
98 -------------------------------
106   1  The multi-queue block layer is instantiated with a hardware dispatch
114   0  nullb* use default blk-mq io scheduler
122   0  Register as a non-blocking blk-mq driver device.
123   1  Register as a blocking blk-mq driver device, null_blk will set
125      needs to block in its ->queue_rq() function.
133   1  Tag set shared between devices for blk-mq. Only makes sense with
138   Device is a random-access or a zoned block device.
141   0  Block device is exposed as a random-access block device.
142   1  Block device is exposed as a host-managed zoned block device. Requires
147   Per zone size when exposed as a zoned block device. Must be a power of two.
150   The number of conventional zones to create when block device is zoned.  If
151   zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.