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
  12hw_sector_size (RO)
  13-------------------
  14This is the hardware sector size of the device, in bytes.
  15
  16max_hw_sectors_kb (RO)
  17----------------------
  18This is the maximum number of kilobytes supported in a single data transfer.
  19
  20max_sectors_kb (RW)
  21-------------------
  22This is the maximum number of kilobytes that the block layer will allow
  23for a filesystem request. Must be smaller than or equal to the maximum
  24size allowed by the hardware.
  25
  26nomerges (RW)
  27-------------
  28This enables the user to disable the lookup logic involved with IO merging
  29requests in the block layer. Merging may still occur through a direct
  301-hit cache, since that comes for (almost) free. The IO scheduler will not
  31waste cycles doing tree/hash lookups for merges if nomerges is 1. Defaults
  32to 0, enabling all merges.
  33
  34nr_requests (RW)
  35----------------
  36This controls how many requests may be allocated in the block layer for
  37read or write requests. Note that the total allocated number may be twice
  38this amount, since it applies only to reads or writes (not the accumulated
  39sum).
  40
  41read_ahead_kb (RW)
  42------------------
  43Maximum number of kilobytes to read-ahead for filesystems on this block
  44device.
  45
  46rq_affinity (RW)
  47----------------
  48If this option is enabled, the block layer will migrate request completions
  49to the CPU that originally submitted the request. For some workloads
  50this provides a significant reduction in CPU cycles due to caching effects.
  51
  52scheduler (RW)
  53--------------
  54When read, this file will display the current and available IO schedulers
  55for this block device. The currently active IO scheduler will be enclosed
  56in [] brackets. Writing an IO scheduler name to this file will switch
  57control of this block device to that new IO scheduler. Note that writing
  58an IO scheduler name to this file will attempt to load that IO scheduler
  59module, if it isn't already present in the system.
  60
  61
  62
  63Jens Axboe <jens.axboe@oracle.com>, February 2009
  64