[PATCH 3/7] docs: sysfs-block: fill in missing documentation from queue-sysfs.rst

Eric Biggers
2021-12-01T08:47:20Z

sysfs documentation is supposed to go in Documentation/ABI/.
However, /sys/block/<disk>/queue/* are documented in
Documentation/block/queue-sysfs.rst, and sometimes redundantly in
Documentation/ABI/testing/sysfs-block too.

Let's consolidate this documentation into Documentation/ABI/.

Therefore, copy the relevant docs from queue-sysfs.rst into sysfs-block.

This primarily means adding the 25 missing files that were documented in
queue-sysfs.rst only, as well as mentioning the RO/RW status of files.

Documentation/ABI/ requires "Date" and "Contact" fields. For the Date
fields, I used the date of the commit which added support for each file.
For the "Contact" fields, I used linux-block.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
Documentation/ABI/testing/sysfs-block | 481 ++++++++++++++++++++------
1 file changed, 380 insertions(+), 101 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index c70fce6b76c17..94711edc6529d 100644
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -46,7 +46,7 @@ Description:
The value type is unsigned int.
Cf. Documentation/block/stat.rst which contains a single value for
requests in flight.
- This is related to nr_requests in Documentation/block/queue-sysfs.rst
+ This is related to /sys/block/<disk>/queue/nr_requests
and for SCSI device also its queue_depth.

@@ -134,207 +134,486 @@ Description:
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:
- chunk_sectors has different meaning depending on the type
+ [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.
+ 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:
- 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.
+ [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:
- Devices that support discard functionality may have
- internal limits on the number of bytes that can be
- trimmed or unmapped in a single operation. Some storage
- protocols also have inherent limits on the number of
- blocks that can be described in a single command. The
- discard_max_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_bytes
- value of 0 means that the device does not support
- discard functionality.
+ [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:
- Will always return 0. Don't rely on any specific behavior
+ [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:
- 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.
+ [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:
- This is the smallest unit the storage device can
- address. It is typically 512 bytes.
+ [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:
- For zoned block devices (zoned attribute indicating
+ [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:
- For zoned block devices (zoned attribute indicating
+ [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.
+ 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:
- 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.
+ [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:
- 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.
+ [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:
- 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.
+ [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:
- 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.
+ [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:
- 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.
+ [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/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/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:
- Some devices support a write same operation in which a
+ [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.
+ 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:
- 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.
+ [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:
- 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".
+ [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
--
2.34.1