linux/Documentation/block/queue-sysfs.txt
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   1Queue sysfs files
   2=================
   3
   4This text file will detail the queue files that are located in the sysfs tree
   5for each block device. Note that stacked devices typically do not export
   6any settings, since their queue merely functions are a remapping target.
   7These files are the ones found in the /sys/block/xxx/queue/ directory.
   8
   9Files denoted with a RO postfix are readonly and the RW postfix means
  10read-write.
  11
  12add_random (RW)
  13----------------
  14This file allows to trun off the disk entropy contribution. Default
  15value of this file is '1'(on).
  16
  17discard_granularity (RO)
  18-----------------------
  19This shows the size of internal allocation of the device in bytes, if
  20reported by the device. A value of '0' means device does not support
  21the discard functionality.
  22
  23discard_max_bytes (RO)
  24----------------------
  25Devices that support discard functionality may have internal limits on
  26the number of bytes that can be trimmed or unmapped in a single operation.
  27The discard_max_bytes parameter is set by the device driver to the maximum
  28number of bytes that can be discarded in a single operation. Discard
  29requests issued to the device must not exceed this limit. A discard_max_bytes
  30value of 0 means that the device does not support discard functionality.
  31
  32discard_zeroes_data (RO)
  33------------------------
  34When read, this file will show if the discarded block are zeroed by the
  35device or not. If its value is '1' the blocks are zeroed otherwise not.
  36
  37hw_sector_size (RO)
  38-------------------
  39This is the hardware sector size of the device, in bytes.
  40
  41iostats (RW)
  42-------------
  43This file is used to control (on/off) the iostats accounting of the
  44disk.
  45
  46logical_block_size (RO)
  47-----------------------
  48This is the logcal block size of the device, in bytes.
  49
  50max_hw_sectors_kb (RO)
  51----------------------
  52This is the maximum number of kilobytes supported in a single data transfer.
  53
  54max_integrity_segments (RO)
  55---------------------------
  56When read, this file shows the max limit of integrity segments as
  57set by block layer which a hardware controller can handle.
  58
  59max_sectors_kb (RW)
  60-------------------
  61This is the maximum number of kilobytes that the block layer will allow
  62for a filesystem request. Must be smaller than or equal to the maximum
  63size allowed by the hardware.
  64
  65max_segments (RO)
  66-----------------
  67Maximum number of segments of the device.
  68
  69max_segment_size (RO)
  70---------------------
  71Maximum segment size of the device.
  72
  73minimum_io_size (RO)
  74--------------------
  75This is the smallest preferred io size reported by the device.
  76
  77nomerges (RW)
  78-------------
  79This enables the user to disable the lookup logic involved with IO
  80merging requests in the block layer. By default (0) all merges are
  81enabled. When set to 1 only simple one-hit merges will be tried. When
  82set to 2 no merge algorithms will be tried (including one-hit or more
  83complex tree/hash lookups).
  84
  85nr_requests (RW)
  86----------------
  87This controls how many requests may be allocated in the block layer for
  88read or write requests. Note that the total allocated number may be twice
  89this amount, since it applies only to reads or writes (not the accumulated
  90sum).
  91
  92To avoid priority inversion through request starvation, a request
  93queue maintains a separate request pool per each cgroup when
  94CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
  95per-block-cgroup request pool.  IOW, if there are N block cgroups,
  96each request queue may have upto N request pools, each independently
  97regulated by nr_requests.
  98
  99optimal_io_size (RO)
 100--------------------
 101This is the optimal io size reported by the device.
 102
 103physical_block_size (RO)
 104------------------------
 105This is the physical block size of device, in bytes.
 106
 107read_ahead_kb (RW)
 108------------------
 109Maximum number of kilobytes to read-ahead for filesystems on this block
 110device.
 111
 112rotational (RW)
 113---------------
 114This file is used to stat if the device is of rotational type or
 115non-rotational type.
 116
 117rq_affinity (RW)
 118----------------
 119If this option is '1', the block layer will migrate request completions to the
 120cpu "group" that originally submitted the request. For some workloads this
 121provides a significant reduction in CPU cycles due to caching effects.
 122
 123For storage configurations that need to maximize distribution of completion
 124processing setting this option to '2' forces the completion to run on the
 125requesting cpu (bypassing the "group" aggregation logic).
 126
 127scheduler (RW)
 128--------------
 129When read, this file will display the current and available IO schedulers
 130for this block device. The currently active IO scheduler will be enclosed
 131in [] brackets. Writing an IO scheduler name to this file will switch
 132control of this block device to that new IO scheduler. Note that writing
 133an IO scheduler name to this file will attempt to load that IO scheduler
 134module, if it isn't already present in the system.
 135
 136
 137
 138Jens Axboe <jens.axboe@oracle.com>, February 2009
 139
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