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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 10:05:51 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 10:05:51 +0000
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Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+.. SPDX-License-Identifier: GPL-2.0
+
+========================
+Null block device driver
+========================
+
+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.
+It does not execute any read/write operation, just mark them as complete in
+the request queue. The following instances are possible:
+
+ 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.
+
+Module parameters
+=================
+
+queue_mode=[0-2]: Default: 2-Multi-queue
+ Selects which block-layer the module should instantiate with.
+
+ = ============
+ 0 Bio-based
+ 1 Single-queue (deprecated)
+ 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]: Default: 1
+ 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.
+
+Multi-queue specific parameters
+-------------------------------
+
+use_per_node_hctx=[0/1]: Default: 0
+ Number of hardware context queues.
+
+ = =====================================================================
+ 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
+ Enable/disable the io scheduler.
+
+ = ======================================
+ 0 nullb* use default blk-mq io scheduler
+ 1 nullb* doesn't use io scheduler
+ = ======================================
+
+blocking=[0/1]: Default: 0
+ Blocking behavior of the request queue.
+
+ = ===============================================================
+ 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
+ Sharing tags between devices.
+
+ = ================================================================
+ 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.
+ = ================================================================
+
+zoned=[0/1]: Default: 0
+ Device is a random-access or a zoned block device.
+
+ = ======================================================================
+ 0 Block device is exposed as a random-access block device.
+ 1 Block device is exposed as a host-managed zoned block device. Requires
+ CONFIG_BLK_DEV_ZONED.
+ = ======================================================================
+
+zone_size=[MB]: Default: 256
+ Per zone size when exposed as a zoned block device. Must be a power of two.
+
+zone_nr_conv=[nr_conv]: Default: 0
+ The number of conventional zones to create when block device is zoned. If
+ zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.