Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 Null block device driver 2 ================================================================================ 3 4 I. Overview 5 6 The null block device (/dev/nullb*) is used for benchmarking the various 7 block-layer implementations. It emulates a block device of X gigabytes in size. 8 The following instances are possible: 9 10 Single-queue block-layer 11 - Request-based. 12 - Single submission queue per device. 13 - Implements IO scheduling algorithms (CFQ, Deadline, noop). 14 Multi-queue block-layer 15 - Request-based. 16 - Configurable submission queues per device. 17 No block-layer (Known as bio-based) 18 - Bio-based. IO requests are submitted directly to the device driver. 19 - Directly accepts bio data structure and returns them. 20 21 All of them have a completion queue for each core in the system. 22 23 II. Module parameters applicable for all instances: 24 25 queue_mode=[0-2]: Default: 2-Multi-queue 26 Selects which block-layer the module should instantiate with. 27 28 0: Bio-based. 29 1: Single-queue. 30 2: Multi-queue. 31 32 home_node=[0--nr_nodes]: Default: NUMA_NO_NODE 33 Selects what CPU node the data structures are allocated from. 34 35 gb=[Size in GB]: Default: 250GB 36 The size of the device reported to the system. 37 38 bs=[Block size (in bytes)]: Default: 512 bytes 39 The block size reported to the system. 40 41 nr_devices=[Number of devices]: Default: 1 42 Number of block devices instantiated. They are instantiated as /dev/nullb0, 43 etc. 44 45 irqmode=[0-2]: Default: 1-Soft-irq 46 The completion mode used for completing IOs to the block-layer. 47 48 0: None. 49 1: Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead 50 when IOs are issued from another CPU node than the home the device is 51 connected to. 52 2: Timer: Waits a specific period (completion_nsec) for each IO before 53 completion. 54 55 completion_nsec=[ns]: Default: 10,000ns 56 Combined with irqmode=2 (timer). The time each completion event must wait. 57 58 submit_queues=[1..nr_cpus]: 59 The number of submission queues attached to the device driver. If unset, it 60 defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module 61 parameter is 1. 62 63 hw_queue_depth=[0..qdepth]: Default: 64 64 The hardware queue depth of the device. 65 66 III: Multi-queue specific parameters 67 68 use_per_node_hctx=[0/1]: Default: 0 69 0: The number of submit queues are set to the value of the submit_queues 70 parameter. 71 1: The multi-queue block layer is instantiated with a hardware dispatch 72 queue for each CPU node in the system. 73 74 use_lightnvm=[0/1]: Default: 0 75 Register device with LightNVM. Requires blk-mq and CONFIG_NVM to be enabled. 76 77 no_sched=[0/1]: Default: 0 78 0: nullb* use default blk-mq io scheduler. 79 1: nullb* doesn't use io scheduler. 80 81 shared_tags=[0/1]: Default: 0 82 0: Tag set is not shared. 83 1: Tag set shared between devices for blk-mq. Only makes sense with 84 nr_devices > 1, otherwise there's no tag set to share.