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.
zone_size=[MB]: Default: 256
Per zone size when exposed as a zoned block device. Must be a power of two.
