| 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. |