doc: Fix typo in documentation/bcache.txt

   In practice this isn't an issue because as soon as a write comes along it'll

   cause the btree node to be split, and you need almost no write traffic for

   this to not show up enough to be noticable (especially since bcache's btree

   this to not show up enough to be noticeable (especially since bcache's btree

   nodes are huge and index large regions of the device). But when you're

   benchmarking, if you're trying to warm the cache by reading a bunch of data

   and there's no other traffic - that can be a problem.

  it's in passthrough mode or caching).

sequential_cutoff

  A sequential IO will bypass the cache once it passes this threshhold; the

  A sequential IO will bypass the cache once it passes this threshold; the

  most recent 128 IOs are tracked so sequential IO can be detected even when

  it isn't all done at once.

  since the synchronization for cache misses was rewritten)

cache_readaheads

  Count of times readahead occured.

  Count of times readahead occurred.

SYSFS - CACHE SET:

SYSFS - CACHE SET INTERNAL:

This directory also exposes timings for a number of internal operations, with

separate files for average duration, average frequency, last occurence and max

separate files for average duration, average frequency, last occurrence and max

duration: garbage collection, btree read, btree node sorts and btree splits.

active_journal_entries

  space.

io_errors

  Number of errors that have occured, decayed by io_error_halflife.

  Number of errors that have occurred, decayed by io_error_halflife.

metadata_written

  Sum of all non data writes (btree writes and all other metadata).