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: 2
Number of block devices instantiated. They are instantiated as /dev/nullb0,
etc.
irq_mode=[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 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,
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.
