Block layer statistics in /sys/block/<dev>/stat
  ===============================================
  
  This file documents the contents of the /sys/block/<dev>/stat file.
  
  The stat file provides several statistics about the state of block
  device <dev>.
  
  Q. Why are there multiple statistics in a single file?  Doesn't sysfs
     normally contain a single value per file?
  A. By having a single file, the kernel can guarantee that the statistics
     represent a consistent snapshot of the state of the device.  If the
     statistics were exported as multiple files containing one statistic
     each, it would be impossible to guarantee that a set of readings
     represent a single point in time.
  
  The stat file consists of a single line of text containing 11 decimal
  values separated by whitespace.  The fields are summarized in the
  following table, and described in more detail below.
  
  Name            units         description
  ----            -----         -----------
  read I/Os       requests      number of read I/Os processed
  read merges     requests      number of read I/Os merged with in-queue I/O
  read sectors    sectors       number of sectors read
  read ticks      milliseconds  total wait time for read requests
  write I/Os      requests      number of write I/Os processed
  write merges    requests      number of write I/Os merged with in-queue I/O
  write sectors   sectors       number of sectors written
  write ticks     milliseconds  total wait time for write requests
  in_flight       requests      number of I/Os currently in flight
  io_ticks        milliseconds  total time this block device has been active
  time_in_queue   milliseconds  total wait time for all requests
  
  read I/Os, write I/Os
  =====================
  
  These values increment when an I/O request completes.
  
  read merges, write merges
  =========================
  
  These values increment when an I/O request is merged with an
  already-queued I/O request.
  
  read sectors, write sectors
  ===========================
  
  These values count the number of sectors read from or written to this
  block device.  The "sectors" in question are the standard UNIX 512-byte
  sectors, not any device- or filesystem-specific block size.  The
  counters are incremented when the I/O completes.
  
  read ticks, write ticks
  =======================
  
  These values count the number of milliseconds that I/O requests have
  waited on this block device.  If there are multiple I/O requests waiting,
  these values will increase at a rate greater than 1000/second; for
  example, if 60 read requests wait for an average of 30 ms, the read_ticks
  field will increase by 60*30 = 1800.
  
  in_flight
  =========
  
  This value counts the number of I/O requests that have been issued to
  the device driver but have not yet completed.  It does not include I/O
  requests that are in the queue but not yet issued to the device driver.
  
  io_ticks
  ========
  
  This value counts the number of milliseconds during which the device has
  had I/O requests queued.
  
  time_in_queue
  =============
  
  This value counts the number of milliseconds that I/O requests have waited
  on this block device.  If there are multiple I/O requests waiting, this
  value will increase as the product of the number of milliseconds times the
  number of requests waiting (see "read ticks" above for an example).