Documentation/ABI/stable/sysfs-block

What:		/sys/block/<disk>/alignment_offset
Date:		April 2009
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Storage devices may report a physical block size that is
		bigger than the logical block size (for instance a drive
		with 4KB physical sectors exposing 512-byte logical
		blocks to the operating system).  This parameter
		indicates how many bytes the beginning of the device is
		offset from the disk's natural alignment.


What:		/sys/block/<disk>/discard_alignment
Date:		May 2011
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Devices that support discard functionality may
		internally allocate space in units that are bigger than
		the exported logical block size. The discard_alignment
		parameter indicates how many bytes the beginning of the
		device is offset from the internal allocation unit's
		natural alignment.


What:		/sys/block/<disk>/diskseq
Date:		February 2021
Contact:	Matteo Croce <mcroce@microsoft.com>
Description:
		The /sys/block/<disk>/diskseq files reports the disk
		sequence number, which is a monotonically increasing
		number assigned to every drive.
		Some devices, like the loop device, refresh such number
		every time the backing file is changed.
		The value type is 64 bit unsigned.


What:		/sys/block/<disk>/inflight
Date:		October 2009
Contact:	Jens Axboe <axboe@kernel.dk>, Nikanth Karthikesan <knikanth@suse.de>
Description:
		Reports the number of I/O requests currently in progress
		(pending / in flight) in a device driver. This can be less
		than the number of requests queued in the block device queue.
		The report contains 2 fields: one for read requests
		and one for write requests.
		The value type is unsigned int.
		Cf. Documentation/block/stat.rst which contains a single value for
		requests in flight.
		This is related to /sys/block/<disk>/queue/nr_requests
		and for SCSI device also its queue_depth.


What:		/sys/block/<disk>/integrity/device_is_integrity_capable
Date:		July 2014
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Indicates whether a storage device is capable of storing
		integrity metadata. Set if the device is T10 PI-capable.


What:		/sys/block/<disk>/integrity/format
Date:		June 2008
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Metadata format for integrity capable block device.
		E.g. T10-DIF-TYPE1-CRC.


What:		/sys/block/<disk>/integrity/protection_interval_bytes
Date:		July 2015
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Describes the number of data bytes which are protected
		by one integrity tuple. Typically the device's logical
		block size.


What:		/sys/block/<disk>/integrity/read_verify
Date:		June 2008
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Indicates whether the block layer should verify the
		integrity of read requests serviced by devices that
		support sending integrity metadata.


What:		/sys/block/<disk>/integrity/tag_size
Date:		June 2008
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Number of bytes of integrity tag space available per
		512 bytes of data.


What:		/sys/block/<disk>/integrity/write_generate
Date:		June 2008
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Indicates whether the block layer should automatically
		generate checksums for write requests bound for
		devices that support receiving integrity metadata.


What:		/sys/block/<disk>/<partition>/alignment_offset
Date:		April 2009
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Storage devices may report a physical block size that is
		bigger than the logical block size (for instance a drive
		with 4KB physical sectors exposing 512-byte logical
		blocks to the operating system).  This parameter
		indicates how many bytes the beginning of the partition
		is offset from the disk's natural alignment.


What:		/sys/block/<disk>/<partition>/discard_alignment
Date:		May 2011
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		Devices that support discard functionality may
		internally allocate space in units that are bigger than
		the exported logical block size. The discard_alignment
		parameter indicates how many bytes the beginning of the
		partition is offset from the internal allocation unit's
		natural alignment.


What:		/sys/block/<disk>/<partition>/stat
Date:		February 2008
Contact:	Jerome Marchand <jmarchan@redhat.com>
Description:
		The /sys/block/<disk>/<partition>/stat files display the
		I/O statistics of partition <partition>. The format is the
		same as the format of /sys/block/<disk>/stat.


What:		/sys/block/<disk>/queue/add_random
Date:		June 2010
Contact:	linux-block@vger.kernel.org
Description:
		[RW] This file allows to turn off the disk entropy contribution.
		Default value of this file is '1'(on).


What:		/sys/block/<disk>/queue/chunk_sectors
Date:		September 2016
Contact:	Hannes Reinecke <hare@suse.com>
Description:
		[RO] chunk_sectors has different meaning depending on the type
		of the disk. For a RAID device (dm-raid), chunk_sectors
		indicates the size in 512B sectors of the RAID volume stripe
		segment. For a zoned block device, either host-aware or
		host-managed, chunk_sectors indicates the size in 512B sectors
		of the zones of the device, with the eventual exception of the
		last zone of the device which may be smaller.


What:		/sys/block/<disk>/queue/dax
Date:		June 2016
Contact:	linux-block@vger.kernel.org
Description:
		[RO] This file indicates whether the device supports Direct
		Access (DAX), used by CPU-addressable storage to bypass the
		pagecache.  It shows '1' if true, '0' if not.


What:		/sys/block/<disk>/queue/discard_granularity
Date:		May 2011
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RO] Devices that support discard functionality may internally
		allocate space using units that are bigger than the logical
		block size. The discard_granularity parameter indicates the size
		of the internal allocation unit in bytes if reported by the
		device. Otherwise the discard_granularity will be set to match
		the device's physical block size. A discard_granularity of 0
		means that the device does not support discard functionality.


What:		/sys/block/<disk>/queue/discard_max_bytes
Date:		May 2011
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RW] While discard_max_hw_bytes is the hardware limit for the
		device, this setting is the software limit. Some devices exhibit
		large latencies when large discards are issued, setting this
		value lower will make Linux issue smaller discards and
		potentially help reduce latencies induced by large discard
		operations.


What:		/sys/block/<disk>/queue/discard_max_hw_bytes
Date:		July 2015
Contact:	linux-block@vger.kernel.org
Description:
		[RO] Devices that support discard functionality may have
		internal limits on the number of bytes that can be trimmed or
		unmapped in a single operation.  The `discard_max_hw_bytes`
		parameter is set by the device driver to the maximum number of
		bytes that can be discarded in a single operation.  Discard
		requests issued to the device must not exceed this limit.  A
		`discard_max_hw_bytes` value of 0 means that the device does not
		support discard functionality.


What:		/sys/block/<disk>/queue/discard_zeroes_data
Date:		May 2011
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RO] Will always return 0.  Don't rely on any specific behavior
		for discards, and don't read this file.


What:		/sys/block/<disk>/queue/fua
Date:		May 2018
Contact:	linux-block@vger.kernel.org
Description:
		[RO] Whether or not the block driver supports the FUA flag for
		write requests.  FUA stands for Force Unit Access. If the FUA
		flag is set that means that write requests must bypass the
		volatile cache of the storage device.


What:		/sys/block/<disk>/queue/hw_sector_size
Date:		January 2008
Contact:	linux-block@vger.kernel.org
Description:
		[RO] This is the hardware sector size of the device, in bytes.


What:		/sys/block/<disk>/queue/independent_access_ranges/
Date:		October 2021
Contact:	linux-block@vger.kernel.org
Description:
		[RO] The presence of this sub-directory of the
		/sys/block/xxx/queue/ directory indicates that the device is
		capable of executing requests targeting different sector ranges
		in parallel. For instance, single LUN multi-actuator hard-disks
		will have an independent_access_ranges directory if the device
		correctly advertizes the sector ranges of its actuators.

		The independent_access_ranges directory contains one directory
		per access range, with each range described using the sector
		(RO) attribute file to indicate the first sector of the range
		and the nr_sectors (RO) attribute file to indicate the total
		number of sectors in the range starting from the first sector of
		the range.  For example, a dual-actuator hard-disk will have the
		following independent_access_ranges entries.::

			$ tree /sys/block/<disk>/queue/independent_access_ranges/
			/sys/block/<disk>/queue/independent_access_ranges/
			|-- 0
			|   |-- nr_sectors
			|   `-- sector
			`-- 1
			    |-- nr_sectors
			    `-- sector

		The sector and nr_sectors attributes use 512B sector unit,
		regardless of the actual block size of the device. Independent
		access ranges do not overlap and include all sectors within the
		device capacity. The access ranges are numbered in increasing
		order of the range start sector, that is, the sector attribute
		of range 0 always has the value 0.


What:		/sys/block/<disk>/queue/io_poll
Date:		November 2015
Contact:	linux-block@vger.kernel.org
Description:
		[RW] When read, this file shows whether polling is enabled (1)
		or disabled (0).  Writing '0' to this file will disable polling
		for this device.  Writing any non-zero value will enable this
		feature.


What:		/sys/block/<disk>/queue/io_poll_delay
Date:		November 2016
Contact:	linux-block@vger.kernel.org
Description:
		[RW] If polling is enabled, this controls what kind of polling
		will be performed. It defaults to -1, which is classic polling.
		In this mode, the CPU will repeatedly ask for completions
		without giving up any time.  If set to 0, a hybrid polling mode
		is used, where the kernel will attempt to make an educated guess
		at when the IO will complete. Based on this guess, the kernel
		will put the process issuing IO to sleep for an amount of time,
		before entering a classic poll loop. This mode might be a little
		slower than pure classic polling, but it will be more efficient.
		If set to a value larger than 0, the kernel will put the process
		issuing IO to sleep for this amount of microseconds before
		entering classic polling.


What:		/sys/block/<disk>/queue/io_timeout
Date:		November 2018
Contact:	Weiping Zhang <zhangweiping@didiglobal.com>
Description:
		[RW] io_timeout is the request timeout in milliseconds. If a
		request does not complete in this time then the block driver
		timeout handler is invoked. That timeout handler can decide to
		retry the request, to fail it or to start a device recovery
		strategy.


What:		/sys/block/<disk>/queue/iostats
Date:		January 2009
Contact:	linux-block@vger.kernel.org
Description:
		[RW] This file is used to control (on/off) the iostats
		accounting of the disk.


What:		/sys/block/<disk>/queue/logical_block_size
Date:		May 2009
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RO] This is the smallest unit the storage device can address.
		It is typically 512 bytes.


What:		/sys/block/<disk>/queue/max_active_zones
Date:		July 2020
Contact:	Niklas Cassel <niklas.cassel@wdc.com>
Description:
		[RO] For zoned block devices (zoned attribute indicating
		"host-managed" or "host-aware"), the sum of zones belonging to
		any of the zone states: EXPLICIT OPEN, IMPLICIT OPEN or CLOSED,
		is limited by this value. If this value is 0, there is no limit.

		If the host attempts to exceed this limit, the driver should
		report this error with BLK_STS_ZONE_ACTIVE_RESOURCE, which user
		space may see as the EOVERFLOW errno.


What:		/sys/block/<disk>/queue/max_discard_segments
Date:		February 2017
Contact:	linux-block@vger.kernel.org
Description:
		[RO] The maximum number of DMA scatter/gather entries in a
		discard request.


What:		/sys/block/<disk>/queue/max_hw_sectors_kb
Date:		September 2004
Contact:	linux-block@vger.kernel.org
Description:
		[RO] This is the maximum number of kilobytes supported in a
		single data transfer.


What:		/sys/block/<disk>/queue/max_integrity_segments
Date:		September 2010
Contact:	linux-block@vger.kernel.org
Description:
		[RO] Maximum number of elements in a DMA scatter/gather list
		with integrity data that will be submitted by the block layer
		core to the associated block driver.


What:		/sys/block/<disk>/queue/max_open_zones
Date:		July 2020
Contact:	Niklas Cassel <niklas.cassel@wdc.com>
Description:
		[RO] For zoned block devices (zoned attribute indicating
		"host-managed" or "host-aware"), the sum of zones belonging to
		any of the zone states: EXPLICIT OPEN or IMPLICIT OPEN, is
		limited by this value. If this value is 0, there is no limit.


What:		/sys/block/<disk>/queue/max_sectors_kb
Date:		September 2004
Contact:	linux-block@vger.kernel.org
Description:
		[RW] This is the maximum number of kilobytes that the block
		layer will allow for a filesystem request. Must be smaller than
		or equal to the maximum size allowed by the hardware.


What:		/sys/block/<disk>/queue/max_segment_size
Date:		March 2010
Contact:	linux-block@vger.kernel.org
Description:
		[RO] Maximum size in bytes of a single element in a DMA
		scatter/gather list.


What:		/sys/block/<disk>/queue/max_segments
Date:		March 2010
Contact:	linux-block@vger.kernel.org
Description:
		[RO] Maximum number of elements in a DMA scatter/gather list
		that is submitted to the associated block driver.


What:		/sys/block/<disk>/queue/minimum_io_size
Date:		April 2009
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RO] Storage devices may report a granularity or preferred
		minimum I/O size which is the smallest request the device can
		perform without incurring a performance penalty.  For disk
		drives this is often the physical block size.  For RAID arrays
		it is often the stripe chunk size.  A properly aligned multiple
		of minimum_io_size is the preferred request size for workloads
		where a high number of I/O operations is desired.


What:		/sys/block/<disk>/queue/nomerges
Date:		January 2010
Contact:	linux-block@vger.kernel.org
Description:
		[RW] Standard I/O elevator operations include attempts to merge
		contiguous I/Os. For known random I/O loads these attempts will
		always fail and result in extra cycles being spent in the
		kernel. This allows one to turn off this behavior on one of two
		ways: When set to 1, complex merge checks are disabled, but the
		simple one-shot merges with the previous I/O request are
		enabled. When set to 2, all merge tries are disabled. The
		default value is 0 - which enables all types of merge tries.


What:		/sys/block/<disk>/queue/nr_requests
Date:		July 2003
Contact:	linux-block@vger.kernel.org
Description:
		[RW] This controls how many requests may be allocated in the
		block layer for read or write requests. Note that the total
		allocated number may be twice this amount, since it applies only
		to reads or writes (not the accumulated sum).

		To avoid priority inversion through request starvation, a
		request queue maintains a separate request pool per each cgroup
		when CONFIG_BLK_CGROUP is enabled, and this parameter applies to
		each such per-block-cgroup request pool.  IOW, if there are N
		block cgroups, each request queue may have up to N request
		pools, each independently regulated by nr_requests.


What:		/sys/block/<disk>/queue/nr_zones
Date:		November 2018
Contact:	Damien Le Moal <damien.lemoal@wdc.com>
Description:
		[RO] nr_zones indicates the total number of zones of a zoned
		block device ("host-aware" or "host-managed" zone model). For
		regular block devices, the value is always 0.


What:		/sys/block/<disk>/queue/optimal_io_size
Date:		April 2009
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RO] Storage devices may report an optimal I/O size, which is
		the device's preferred unit for sustained I/O.  This is rarely
		reported for disk drives.  For RAID arrays it is usually the
		stripe width or the internal track size.  A properly aligned
		multiple of optimal_io_size is the preferred request size for
		workloads where sustained throughput is desired.  If no optimal
		I/O size is reported this file contains 0.


What:		/sys/block/<disk>/queue/physical_block_size
Date:		May 2009
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RO] This is the smallest unit a physical storage device can
		write atomically.  It is usually the same as the logical block
		size but may be bigger.  One example is SATA drives with 4KB
		sectors that expose a 512-byte logical block size to the
		operating system.  For stacked block devices the
		physical_block_size variable contains the maximum
		physical_block_size of the component devices.


What:		/sys/block/<disk>/queue/read_ahead_kb
Date:		May 2004
Contact:	linux-block@vger.kernel.org
Description:
		[RW] Maximum number of kilobytes to read-ahead for filesystems
		on this block device.


What:		/sys/block/<disk>/queue/rotational
Date:		January 2009
Contact:	linux-block@vger.kernel.org
Description:
		[RW] This file is used to stat if the device is of rotational
		type or non-rotational type.


What:		/sys/block/<disk>/queue/rq_affinity
Date:		September 2008
Contact:	linux-block@vger.kernel.org
Description:
		[RW] If this option is '1', the block layer will migrate request
		completions to the cpu "group" that originally submitted the
		request. For some workloads this provides a significant
		reduction in CPU cycles due to caching effects.

		For storage configurations that need to maximize distribution of
		completion processing setting this option to '2' forces the
		completion to run on the requesting cpu (bypassing the "group"
		aggregation logic).


What:		/sys/block/<disk>/queue/scheduler
Date:		October 2004
Contact:	linux-block@vger.kernel.org
Description:
		[RW] When read, this file will display the current and available
		IO schedulers for this block device. The currently active IO
		scheduler will be enclosed in [] brackets. Writing an IO
		scheduler name to this file will switch control of this block
		device to that new IO scheduler. Note that writing an IO
		scheduler name to this file will attempt to load that IO
		scheduler module, if it isn't already present in the system.


What:		/sys/block/<disk>/queue/stable_writes
Date:		September 2020
Contact:	linux-block@vger.kernel.org
Description:
		[RW] This file will contain '1' if memory must not be modified
		while it is being used in a write request to this device.  When
		this is the case and the kernel is performing writeback of a
		page, the kernel will wait for writeback to complete before
		allowing the page to be modified again, rather than allowing
		immediate modification as is normally the case.  This
		restriction arises when the device accesses the memory multiple
		times where the same data must be seen every time -- for
		example, once to calculate a checksum and once to actually write
		the data.  If no such restriction exists, this file will contain
		'0'.  This file is writable for testing purposes.


What:		/sys/block/<disk>/queue/throttle_sample_time
Date:		March 2017
Contact:	linux-block@vger.kernel.org
Description:
		[RW] This is the time window that blk-throttle samples data, in
		millisecond.  blk-throttle makes decision based on the
		samplings. Lower time means cgroups have more smooth throughput,
		but higher CPU overhead. This exists only when
		CONFIG_BLK_DEV_THROTTLING_LOW is enabled.


What:		/sys/block/<disk>/queue/virt_boundary_mask
Date:		April 2021
Contact:	linux-block@vger.kernel.org
Description:
		[RO] This file shows the I/O segment memory alignment mask for
		the block device.  I/O requests to this device will be split
		between segments wherever either the memory address of the end
		of the previous segment or the memory address of the beginning
		of the current segment is not aligned to virt_boundary_mask + 1
		bytes.


What:		/sys/block/<disk>/queue/wbt_lat_usec
Date:		November 2016
Contact:	linux-block@vger.kernel.org
Description:
		[RW] If the device is registered for writeback throttling, then
		this file shows the target minimum read latency. If this latency
		is exceeded in a given window of time (see wb_window_usec), then
		the writeback throttling will start scaling back writes. Writing
		a value of '0' to this file disables the feature. Writing a
		value of '-1' to this file resets the value to the default
		setting.


What:		/sys/block/<disk>/queue/write_cache
Date:		April 2016
Contact:	linux-block@vger.kernel.org
Description:
		[RW] When read, this file will display whether the device has
		write back caching enabled or not. It will return "write back"
		for the former case, and "write through" for the latter. Writing
		to this file can change the kernels view of the device, but it
		doesn't alter the device state. This means that it might not be
		safe to toggle the setting from "write back" to "write through",
		since that will also eliminate cache flushes issued by the
		kernel.


What:		/sys/block/<disk>/queue/write_same_max_bytes
Date:		January 2012
Contact:	Martin K. Petersen <martin.petersen@oracle.com>
Description:
		[RO] Some devices support a write same operation in which a
		single data block can be written to a range of several
		contiguous blocks on storage. This can be used to wipe areas on
		disk or to initialize drives in a RAID configuration.
		write_same_max_bytes indicates how many bytes can be written in
		a single write same command. If write_same_max_bytes is 0, write
		same is not supported by the device.


What:		/sys/block/<disk>/queue/write_zeroes_max_bytes
Date:		November 2016
Contact:	Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Description:
		[RO] Devices that support write zeroes operation in which a
		single request can be issued to zero out the range of contiguous
		blocks on storage without having any payload in the request.
		This can be used to optimize writing zeroes to the devices.
		write_zeroes_max_bytes indicates how many bytes can be written
		in a single write zeroes command. If write_zeroes_max_bytes is
		0, write zeroes is not supported by the device.


What:		/sys/block/<disk>/queue/zone_append_max_bytes
Date:		May 2020
Contact:	linux-block@vger.kernel.org
Description:
		[RO] This is the maximum number of bytes that can be written to
		a sequential zone of a zoned block device using a zone append
		write operation (REQ_OP_ZONE_APPEND). This value is always 0 for
		regular block devices.


What:		/sys/block/<disk>/queue/zone_write_granularity
Date:		January 2021
Contact:	linux-block@vger.kernel.org
Description:
		[RO] This indicates the alignment constraint, in bytes, for
		write operations in sequential zones of zoned block devices
		(devices with a zoned attributed that reports "host-managed" or
		"host-aware"). This value is always 0 for regular block devices.


What:		/sys/block/<disk>/queue/zoned
Date:		September 2016
Contact:	Damien Le Moal <damien.lemoal@wdc.com>
Description:
		[RO] zoned indicates if the device is a zoned block device and
		the zone model of the device if it is indeed zoned.  The
		possible values indicated by zoned are "none" for regular block
		devices and "host-aware" or "host-managed" for zoned block
		devices. The characteristics of host-aware and host-managed
		zoned block devices are described in the ZBC (Zoned Block
		Commands) and ZAC (Zoned Device ATA Command Set) standards.
		These standards also define the "drive-managed" zone model.
		However, since drive-managed zoned block devices do not support
		zone commands, they will be treated as regular block devices and
		zoned will report "none".


What:		/sys/block/<disk>/stat
Date:		February 2008
Contact:	Jerome Marchand <jmarchan@redhat.com>
Description:
		The /sys/block/<disk>/stat files displays the I/O
		statistics of disk <disk>. They contain 11 fields:

		==  ==============================================
		 1  reads completed successfully
		 2  reads merged
		 3  sectors read
		 4  time spent reading (ms)
		 5  writes completed
		 6  writes merged
		 7  sectors written
		 8  time spent writing (ms)
		 9  I/Os currently in progress
		10  time spent doing I/Os (ms)
		11  weighted time spent doing I/Os (ms)
		12  discards completed
		13  discards merged
		14  sectors discarded
		15  time spent discarding (ms)
		16  flush requests completed
		17  time spent flushing (ms)
		==  ==============================================

		For more details refer Documentation/admin-guide/iostats.rst