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. 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.