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.