88 lines
3.0 KiB
Plaintext
88 lines
3.0 KiB
Plaintext
Null block device driver
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================================================================================
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I. Overview
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The null block device (/dev/nullb*) is used for benchmarking the various
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block-layer implementations. It emulates a block device of X gigabytes in size.
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The following instances are possible:
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Single-queue block-layer
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- Request-based.
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- Single submission queue per device.
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- Implements IO scheduling algorithms (CFQ, Deadline, noop).
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Multi-queue block-layer
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- Request-based.
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- Configurable submission queues per device.
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No block-layer (Known as bio-based)
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- Bio-based. IO requests are submitted directly to the device driver.
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- Directly accepts bio data structure and returns them.
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All of them have a completion queue for each core in the system.
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II. Module parameters applicable for all instances:
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queue_mode=[0-2]: Default: 2-Multi-queue
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Selects which block-layer the module should instantiate with.
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0: Bio-based.
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1: Single-queue.
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2: Multi-queue.
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home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
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Selects what CPU node the data structures are allocated from.
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gb=[Size in GB]: Default: 250GB
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The size of the device reported to the system.
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bs=[Block size (in bytes)]: Default: 512 bytes
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The block size reported to the system.
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nr_devices=[Number of devices]: Default: 1
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Number of block devices instantiated. They are instantiated as /dev/nullb0,
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etc.
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irqmode=[0-2]: Default: 1-Soft-irq
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The completion mode used for completing IOs to the block-layer.
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0: None.
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1: Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
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when IOs are issued from another CPU node than the home the device is
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connected to.
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2: Timer: Waits a specific period (completion_nsec) for each IO before
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completion.
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completion_nsec=[ns]: Default: 10,000ns
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Combined with irqmode=2 (timer). The time each completion event must wait.
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submit_queues=[1..nr_cpus]:
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The number of submission queues attached to the device driver. If unset, it
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defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
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parameter is 1.
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hw_queue_depth=[0..qdepth]: Default: 64
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The hardware queue depth of the device.
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III: Multi-queue specific parameters
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use_per_node_hctx=[0/1]: Default: 0
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0: The number of submit queues are set to the value of the submit_queues
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parameter.
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1: The multi-queue block layer is instantiated with a hardware dispatch
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queue for each CPU node in the system.
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no_sched=[0/1]: Default: 0
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0: nullb* use default blk-mq io scheduler.
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1: nullb* doesn't use io scheduler.
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blocking=[0/1]: Default: 0
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0: Register as a non-blocking blk-mq driver device.
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1: Register as a blocking blk-mq driver device, null_blk will set
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the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
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needs to block in its ->queue_rq() function.
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shared_tags=[0/1]: Default: 0
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0: Tag set is not shared.
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1: Tag set shared between devices for blk-mq. Only makes sense with
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nr_devices > 1, otherwise there's no tag set to share.
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