152 lines
5.2 KiB
ReStructuredText
152 lines
5.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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========================
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Null block device driver
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========================
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Overview
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========
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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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It does not execute any read/write operation, just mark them as complete in
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the request queue. The following instances are possible:
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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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Module parameters
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=================
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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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= ============
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0 Bio-based
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1 Single-queue (deprecated)
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2 Multi-queue
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= ============
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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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= ===========================================================================
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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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= ===========================================================================
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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]: Default: 1
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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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memory_backed=[0/1]: Default: 0
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Whether or not to use a memory buffer to respond to IO requests
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= =============================================
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0 Transfer no data in response to IO requests
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1 Use a memory buffer to respond to IO requests
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= =============================================
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discard=[0/1]: Default: 0
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Support discard operations (requires memory-backed null_blk device).
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= =====================================
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0 Do not support discard operations
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1 Enable support for discard operations
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= =====================================
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cache_size=[Size in MB]: Default: 0
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Cache size in MB for memory-backed device.
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mbps=[Maximum bandwidth in MB/s]: Default: 0 (no limit)
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Bandwidth limit for device performance.
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Multi-queue specific parameters
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-------------------------------
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use_per_node_hctx=[0/1]: Default: 0
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Number of hardware context queues.
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= =====================================================================
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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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= =====================================================================
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no_sched=[0/1]: Default: 0
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Enable/disable the io scheduler.
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= ======================================
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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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= ======================================
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blocking=[0/1]: Default: 0
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Blocking behavior of the request queue.
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= ===============================================================
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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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= ===============================================================
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shared_tags=[0/1]: Default: 0
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Sharing tags between devices.
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= ================================================================
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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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= ================================================================
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zoned=[0/1]: Default: 0
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Device is a random-access or a zoned block device.
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= ======================================================================
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0 Block device is exposed as a random-access block device.
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1 Block device is exposed as a host-managed zoned block device. Requires
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CONFIG_BLK_DEV_ZONED.
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= ======================================================================
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zone_size=[MB]: Default: 256
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Per zone size when exposed as a zoned block device. Must be a power of two.
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zone_nr_conv=[nr_conv]: Default: 0
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The number of conventional zones to create when block device is zoned. If
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zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.
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