.. SPDX-License-Identifier: GPL-2.0 ======================== 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. 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.