Null block device driver¶
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.
It does not execute any read/write operation, just mark them as complete in
the request queue. The following instances are possible:
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.
Module parameters¶
- queue_mode=[0-2]: Default: 2-Multi-queue
Selects which block-layer the module should instantiate with.
0
Bio-based
1
Single-queue (deprecated)
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]: Default: 1
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.
- memory_backed=[0/1]: Default: 0
Whether or not to use a memory buffer to respond to IO requests
0
Transfer no data in response to IO requests
1
Use a memory buffer to respond to IO requests
- discard=[0/1]: Default: 0
Support discard operations (requires memory-backed null_blk device).
0
Do not support discard operations
1
Enable support for discard operations
- cache_size=[Size in MB]: Default: 0
Cache size in MB for memory-backed device.
- mbps=[Maximum bandwidth in MB/s]: Default: 0 (no limit)
Bandwidth limit for device performance.
Multi-queue specific parameters¶
- use_per_node_hctx=[0/1]: Default: 0
Number of hardware context queues.
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
Enable/disable the io scheduler.
0
nullb* use default blk-mq io scheduler
1
nullb* doesn’t use io scheduler
- blocking=[0/1]: Default: 0
Blocking behavior of the request queue.
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
Sharing tags between devices.
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
Device is a random-access or a zoned block device.
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.