Null block device driver

1. 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.

2. 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.
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.