1.. SPDX-License-Identifier: GPL-2.0 2 3======================== 4Null block device driver 5======================== 6 7Overview 8======== 9 10The null block device (``/dev/nullb*``) is used for benchmarking the various 11block-layer implementations. It emulates a block device of X gigabytes in size. 12It does not execute any read/write operation, just mark them as complete in 13the request queue. The following instances are possible: 14 15 Multi-queue block-layer 16 17 - Request-based. 18 - Configurable submission queues per device. 19 20 No block-layer (Known as bio-based) 21 22 - Bio-based. IO requests are submitted directly to the device driver. 23 - Directly accepts bio data structure and returns them. 24 25All of them have a completion queue for each core in the system. 26 27Module parameters 28================= 29 30queue_mode=[0-2]: Default: 2-Multi-queue 31 Selects which block-layer the module should instantiate with. 32 33 = ============ 34 0 Bio-based 35 1 Single-queue (deprecated) 36 2 Multi-queue 37 = ============ 38 39home_node=[0--nr_nodes]: Default: NUMA_NO_NODE 40 Selects what CPU node the data structures are allocated from. 41 42gb=[Size in GB]: Default: 250GB 43 The size of the device reported to the system. 44 45bs=[Block size (in bytes)]: Default: 512 bytes 46 The block size reported to the system. 47 48nr_devices=[Number of devices]: Default: 1 49 Number of block devices instantiated. They are instantiated as /dev/nullb0, 50 etc. 51 52irqmode=[0-2]: Default: 1-Soft-irq 53 The completion mode used for completing IOs to the block-layer. 54 55 = =========================================================================== 56 0 None. 57 1 Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead 58 when IOs are issued from another CPU node than the home the device is 59 connected to. 60 2 Timer: Waits a specific period (completion_nsec) for each IO before 61 completion. 62 = =========================================================================== 63 64completion_nsec=[ns]: Default: 10,000ns 65 Combined with irqmode=2 (timer). The time each completion event must wait. 66 67submit_queues=[1..nr_cpus]: Default: 1 68 The number of submission queues attached to the device driver. If unset, it 69 defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module 70 parameter is 1. 71 72hw_queue_depth=[0..qdepth]: Default: 64 73 The hardware queue depth of the device. 74 75memory_backed=[0/1]: Default: 0 76 Whether or not to use a memory buffer to respond to IO requests 77 78 = ============================================= 79 0 Transfer no data in response to IO requests 80 1 Use a memory buffer to respond to IO requests 81 = ============================================= 82 83discard=[0/1]: Default: 0 84 Support discard operations (requires memory-backed null_blk device). 85 86 = ===================================== 87 0 Do not support discard operations 88 1 Enable support for discard operations 89 = ===================================== 90 91cache_size=[Size in MB]: Default: 0 92 Cache size in MB for memory-backed device. 93 94mbps=[Maximum bandwidth in MB/s]: Default: 0 (no limit) 95 Bandwidth limit for device performance. 96 97Multi-queue specific parameters 98------------------------------- 99 100use_per_node_hctx=[0/1]: Default: 0 101 Number of hardware context queues. 102 103 = ===================================================================== 104 0 The number of submit queues are set to the value of the submit_queues 105 parameter. 106 1 The multi-queue block layer is instantiated with a hardware dispatch 107 queue for each CPU node in the system. 108 = ===================================================================== 109 110no_sched=[0/1]: Default: 0 111 Enable/disable the io scheduler. 112 113 = ====================================== 114 0 nullb* use default blk-mq io scheduler 115 1 nullb* doesn't use io scheduler 116 = ====================================== 117 118blocking=[0/1]: Default: 0 119 Blocking behavior of the request queue. 120 121 = =============================================================== 122 0 Register as a non-blocking blk-mq driver device. 123 1 Register as a blocking blk-mq driver device, null_blk will set 124 the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always 125 needs to block in its ->queue_rq() function. 126 = =============================================================== 127 128shared_tags=[0/1]: Default: 0 129 Sharing tags between devices. 130 131 = ================================================================ 132 0 Tag set is not shared. 133 1 Tag set shared between devices for blk-mq. Only makes sense with 134 nr_devices > 1, otherwise there's no tag set to share. 135 = ================================================================ 136 137zoned=[0/1]: Default: 0 138 Device is a random-access or a zoned block device. 139 140 = ====================================================================== 141 0 Block device is exposed as a random-access block device. 142 1 Block device is exposed as a host-managed zoned block device. Requires 143 CONFIG_BLK_DEV_ZONED. 144 = ====================================================================== 145 146zone_size=[MB]: Default: 256 147 Per zone size when exposed as a zoned block device. Must be a power of two. 148 149zone_nr_conv=[nr_conv]: Default: 0 150 The number of conventional zones to create when block device is zoned. If 151 zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1. 152