linux/Documentation/block/queue-sysfs.rst
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   1=================
   2Queue sysfs files
   3=================
   4
   5This text file will detail the queue files that are located in the sysfs tree
   6for each block device. Note that stacked devices typically do not export
   7any settings, since their queue merely functions are a remapping target.
   8These files are the ones found in the /sys/block/xxx/queue/ directory.
   9
  10Files denoted with a RO postfix are readonly and the RW postfix means
  11read-write.
  12
  13add_random (RW)
  14---------------
  15This file allows to turn off the disk entropy contribution. Default
  16value of this file is '1'(on).
  17
  18chunk_sectors (RO)
  19------------------
  20This has different meaning depending on the type of the block device.
  21For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors
  22of the RAID volume stripe segment. For a zoned block device, either host-aware
  23or host-managed, chunk_sectors indicates the size in 512B sectors of the zones
  24of the device, with the eventual exception of the last zone of the device which
  25may be smaller.
  26
  27dax (RO)
  28--------
  29This file indicates whether the device supports Direct Access (DAX),
  30used by CPU-addressable storage to bypass the pagecache.  It shows '1'
  31if true, '0' if not.
  32
  33discard_granularity (RO)
  34------------------------
  35This shows the size of internal allocation of the device in bytes, if
  36reported by the device. A value of '0' means device does not support
  37the discard functionality.
  38
  39discard_max_hw_bytes (RO)
  40-------------------------
  41Devices that support discard functionality may have internal limits on
  42the number of bytes that can be trimmed or unmapped in a single operation.
  43The discard_max_bytes parameter is set by the device driver to the maximum
  44number of bytes that can be discarded in a single operation. Discard
  45requests issued to the device must not exceed this limit. A discard_max_bytes
  46value of 0 means that the device does not support discard functionality.
  47
  48discard_max_bytes (RW)
  49----------------------
  50While discard_max_hw_bytes is the hardware limit for the device, this
  51setting is the software limit. Some devices exhibit large latencies when
  52large discards are issued, setting this value lower will make Linux issue
  53smaller discards and potentially help reduce latencies induced by large
  54discard operations.
  55
  56discard_zeroes_data (RO)
  57------------------------
  58Obsolete. Always zero.
  59
  60fua (RO)
  61--------
  62Whether or not the block driver supports the FUA flag for write requests.
  63FUA stands for Force Unit Access. If the FUA flag is set that means that
  64write requests must bypass the volatile cache of the storage device.
  65
  66hw_sector_size (RO)
  67-------------------
  68This is the hardware sector size of the device, in bytes.
  69
  70io_poll (RW)
  71------------
  72When read, this file shows whether polling is enabled (1) or disabled
  73(0).  Writing '0' to this file will disable polling for this device.
  74Writing any non-zero value will enable this feature.
  75
  76io_poll_delay (RW)
  77------------------
  78If polling is enabled, this controls what kind of polling will be
  79performed. It defaults to -1, which is classic polling. In this mode,
  80the CPU will repeatedly ask for completions without giving up any time.
  81If set to 0, a hybrid polling mode is used, where the kernel will attempt
  82to make an educated guess at when the IO will complete. Based on this
  83guess, the kernel will put the process issuing IO to sleep for an amount
  84of time, before entering a classic poll loop. This mode might be a
  85little slower than pure classic polling, but it will be more efficient.
  86If set to a value larger than 0, the kernel will put the process issuing
  87IO to sleep for this amount of microseconds before entering classic
  88polling.
  89
  90io_timeout (RW)
  91---------------
  92io_timeout is the request timeout in milliseconds. If a request does not
  93complete in this time then the block driver timeout handler is invoked.
  94That timeout handler can decide to retry the request, to fail it or to start
  95a device recovery strategy.
  96
  97iostats (RW)
  98-------------
  99This file is used to control (on/off) the iostats accounting of the
 100disk.
 101
 102logical_block_size (RO)
 103-----------------------
 104This is the logical block size of the device, in bytes.
 105
 106max_discard_segments (RO)
 107-------------------------
 108The maximum number of DMA scatter/gather entries in a discard request.
 109
 110max_hw_sectors_kb (RO)
 111----------------------
 112This is the maximum number of kilobytes supported in a single data transfer.
 113
 114max_integrity_segments (RO)
 115---------------------------
 116Maximum number of elements in a DMA scatter/gather list with integrity
 117data that will be submitted by the block layer core to the associated
 118block driver.
 119
 120max_active_zones (RO)
 121---------------------
 122For zoned block devices (zoned attribute indicating "host-managed" or
 123"host-aware"), the sum of zones belonging to any of the zone states:
 124EXPLICIT OPEN, IMPLICIT OPEN or CLOSED, is limited by this value.
 125If this value is 0, there is no limit.
 126
 127If the host attempts to exceed this limit, the driver should report this error
 128with BLK_STS_ZONE_ACTIVE_RESOURCE, which user space may see as the EOVERFLOW
 129errno.
 130
 131max_open_zones (RO)
 132-------------------
 133For zoned block devices (zoned attribute indicating "host-managed" or
 134"host-aware"), the sum of zones belonging to any of the zone states:
 135EXPLICIT OPEN or IMPLICIT OPEN, is limited by this value.
 136If this value is 0, there is no limit.
 137
 138If the host attempts to exceed this limit, the driver should report this error
 139with BLK_STS_ZONE_OPEN_RESOURCE, which user space may see as the ETOOMANYREFS
 140errno.
 141
 142max_sectors_kb (RW)
 143-------------------
 144This is the maximum number of kilobytes that the block layer will allow
 145for a filesystem request. Must be smaller than or equal to the maximum
 146size allowed by the hardware.
 147
 148max_segments (RO)
 149-----------------
 150Maximum number of elements in a DMA scatter/gather list that is submitted
 151to the associated block driver.
 152
 153max_segment_size (RO)
 154---------------------
 155Maximum size in bytes of a single element in a DMA scatter/gather list.
 156
 157minimum_io_size (RO)
 158--------------------
 159This is the smallest preferred IO size reported by the device.
 160
 161nomerges (RW)
 162-------------
 163This enables the user to disable the lookup logic involved with IO
 164merging requests in the block layer. By default (0) all merges are
 165enabled. When set to 1 only simple one-hit merges will be tried. When
 166set to 2 no merge algorithms will be tried (including one-hit or more
 167complex tree/hash lookups).
 168
 169nr_requests (RW)
 170----------------
 171This controls how many requests may be allocated in the block layer for
 172read or write requests. Note that the total allocated number may be twice
 173this amount, since it applies only to reads or writes (not the accumulated
 174sum).
 175
 176To avoid priority inversion through request starvation, a request
 177queue maintains a separate request pool per each cgroup when
 178CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
 179per-block-cgroup request pool.  IOW, if there are N block cgroups,
 180each request queue may have up to N request pools, each independently
 181regulated by nr_requests.
 182
 183nr_zones (RO)
 184-------------
 185For zoned block devices (zoned attribute indicating "host-managed" or
 186"host-aware"), this indicates the total number of zones of the device.
 187This is always 0 for regular block devices.
 188
 189optimal_io_size (RO)
 190--------------------
 191This is the optimal IO size reported by the device.
 192
 193physical_block_size (RO)
 194------------------------
 195This is the physical block size of device, in bytes.
 196
 197read_ahead_kb (RW)
 198------------------
 199Maximum number of kilobytes to read-ahead for filesystems on this block
 200device.
 201
 202rotational (RW)
 203---------------
 204This file is used to stat if the device is of rotational type or
 205non-rotational type.
 206
 207rq_affinity (RW)
 208----------------
 209If this option is '1', the block layer will migrate request completions to the
 210cpu "group" that originally submitted the request. For some workloads this
 211provides a significant reduction in CPU cycles due to caching effects.
 212
 213For storage configurations that need to maximize distribution of completion
 214processing setting this option to '2' forces the completion to run on the
 215requesting cpu (bypassing the "group" aggregation logic).
 216
 217scheduler (RW)
 218--------------
 219When read, this file will display the current and available IO schedulers
 220for this block device. The currently active IO scheduler will be enclosed
 221in [] brackets. Writing an IO scheduler name to this file will switch
 222control of this block device to that new IO scheduler. Note that writing
 223an IO scheduler name to this file will attempt to load that IO scheduler
 224module, if it isn't already present in the system.
 225
 226write_cache (RW)
 227----------------
 228When read, this file will display whether the device has write back
 229caching enabled or not. It will return "write back" for the former
 230case, and "write through" for the latter. Writing to this file can
 231change the kernels view of the device, but it doesn't alter the
 232device state. This means that it might not be safe to toggle the
 233setting from "write back" to "write through", since that will also
 234eliminate cache flushes issued by the kernel.
 235
 236write_same_max_bytes (RO)
 237-------------------------
 238This is the number of bytes the device can write in a single write-same
 239command.  A value of '0' means write-same is not supported by this
 240device.
 241
 242wbt_lat_usec (RW)
 243-----------------
 244If the device is registered for writeback throttling, then this file shows
 245the target minimum read latency. If this latency is exceeded in a given
 246window of time (see wb_window_usec), then the writeback throttling will start
 247scaling back writes. Writing a value of '0' to this file disables the
 248feature. Writing a value of '-1' to this file resets the value to the
 249default setting.
 250
 251throttle_sample_time (RW)
 252-------------------------
 253This is the time window that blk-throttle samples data, in millisecond.
 254blk-throttle makes decision based on the samplings. Lower time means cgroups
 255have more smooth throughput, but higher CPU overhead. This exists only when
 256CONFIG_BLK_DEV_THROTTLING_LOW is enabled.
 257
 258write_zeroes_max_bytes (RO)
 259---------------------------
 260For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of
 261bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES
 262is not supported.
 263
 264zone_append_max_bytes (RO)
 265--------------------------
 266This is the maximum number of bytes that can be written to a sequential
 267zone of a zoned block device using a zone append write operation
 268(REQ_OP_ZONE_APPEND). This value is always 0 for regular block devices.
 269
 270zoned (RO)
 271----------
 272This indicates if the device is a zoned block device and the zone model of the
 273device if it is indeed zoned. The possible values indicated by zoned are
 274"none" for regular block devices and "host-aware" or "host-managed" for zoned
 275block devices. The characteristics of host-aware and host-managed zoned block
 276devices are described in the ZBC (Zoned Block Commands) and ZAC
 277(Zoned Device ATA Command Set) standards. These standards also define the
 278"drive-managed" zone model. However, since drive-managed zoned block devices
 279do not support zone commands, they will be treated as regular block devices
 280and zoned will report "none".
 281
 282zone_write_granularity (RO)
 283---------------------------
 284This indicates the alignment constraint, in bytes, for write operations in
 285sequential zones of zoned block devices (devices with a zoned attributed
 286that reports "host-managed" or "host-aware"). This value is always 0 for
 287regular block devices.
 288
 289Jens Axboe <jens.axboe@oracle.com>, February 2009
 290