Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be
  nice if you could use them as cache... Hence bcache.
  
  Wiki and git repositories are at:
    http://bcache.evilpiepirate.org
    http://evilpiepirate.org/git/linux-bcache.git
    http://evilpiepirate.org/git/bcache-tools.git
  
  It's designed around the performance characteristics of SSDs - it only allocates
  in erase block sized buckets, and it uses a hybrid btree/log to track cached
  extants (which can be anywhere from a single sector to the bucket size). It's
  designed to avoid random writes at all costs; it fills up an erase block
  sequentially, then issues a discard before reusing it.
  
  Both writethrough and writeback caching are supported. Writeback defaults to
  off, but can be switched on and off arbitrarily at runtime. Bcache goes to
  great lengths to protect your data - it reliably handles unclean shutdown. (It
  doesn't even have a notion of a clean shutdown; bcache simply doesn't return
  writes as completed until they're on stable storage).
  
  Writeback caching can use most of the cache for buffering writes - writing
  dirty data to the backing device is always done sequentially, scanning from the
  start to the end of the index.
  
  Since random IO is what SSDs excel at, there generally won't be much benefit
  to caching large sequential IO. Bcache detects sequential IO and skips it;
  it also keeps a rolling average of the IO sizes per task, and as long as the
  average is above the cutoff it will skip all IO from that task - instead of
  caching the first 512k after every seek. Backups and large file copies should
  thus entirely bypass the cache.
  
  In the event of a data IO error on the flash it will try to recover by reading
  from disk or invalidating cache entries.  For unrecoverable errors (meta data
  or dirty data), caching is automatically disabled; if dirty data was present
  in the cache it first disables writeback caching and waits for all dirty data
  to be flushed.
  
  Getting started:
  You'll need make-bcache from the bcache-tools repository. Both the cache device
  and backing device must be formatted before use.
    make-bcache -B /dev/sdb
    make-bcache -C /dev/sdc
  
  make-bcache has the ability to format multiple devices at the same time - if
  you format your backing devices and cache device at the same time, you won't
  have to manually attach:
    make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
  
  bcache-tools now ships udev rules, and bcache devices are known to the kernel
  immediately.  Without udev, you can manually register devices like this:
  
    echo /dev/sdb > /sys/fs/bcache/register
    echo /dev/sdc > /sys/fs/bcache/register
  
  Registering the backing device makes the bcache device show up in /dev; you can
  now format it and use it as normal. But the first time using a new bcache
  device, it'll be running in passthrough mode until you attach it to a cache.
  See the section on attaching.
  
  The devices show up as:
  
    /dev/bcache<N>
  
  As well as (with udev):
  
    /dev/bcache/by-uuid/<uuid>
    /dev/bcache/by-label/<label>
  
  To get started:
  
    mkfs.ext4 /dev/bcache0
    mount /dev/bcache0 /mnt
  
  You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
  
  Cache devices are managed as sets; multiple caches per set isn't supported yet
  but will allow for mirroring of metadata and dirty data in the future. Your new
  cache set shows up as /sys/fs/bcache/<UUID>
  
  ATTACHING:
  
  After your cache device and backing device are registered, the backing device
  must be attached to your cache set to enable caching. Attaching a backing
  device to a cache set is done thusly, with the UUID of the cache set in
  /sys/fs/bcache:
  
    echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
  
  This only has to be done once. The next time you reboot, just reregister all
  your bcache devices. If a backing device has data in a cache somewhere, the
  /dev/bcache<N> device won't be created until the cache shows up - particularly
  important if you have writeback caching turned on.
  
  If you're booting up and your cache device is gone and never coming back, you
  can force run the backing device:
  
    echo 1 > /sys/block/sdb/bcache/running
  
  (You need to use /sys/block/sdb (or whatever your backing device is called), not
  /sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
  partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
  
  The backing device will still use that cache set if it shows up in the future,
  but all the cached data will be invalidated. If there was dirty data in the
  cache, don't expect the filesystem to be recoverable - you will have massive
  filesystem corruption, though ext4's fsck does work miracles.
  
  ERROR HANDLING:
  
  Bcache tries to transparently handle IO errors to/from the cache device without
  affecting normal operation; if it sees too many errors (the threshold is
  configurable, and defaults to 0) it shuts down the cache device and switches all
  the backing devices to passthrough mode.
  
   - For reads from the cache, if they error we just retry the read from the
     backing device.
  
   - For writethrough writes, if the write to the cache errors we just switch to
     invalidating the data at that lba in the cache (i.e. the same thing we do for
     a write that bypasses the cache)
  
   - For writeback writes, we currently pass that error back up to the
     filesystem/userspace. This could be improved - we could retry it as a write
     that skips the cache so we don't have to error the write.
  
   - When we detach, we first try to flush any dirty data (if we were running in
     writeback mode). It currently doesn't do anything intelligent if it fails to
     read some of the dirty data, though.
  
  TROUBLESHOOTING PERFORMANCE:
  
  Bcache has a bunch of config options and tunables. The defaults are intended to
  be reasonable for typical desktop and server workloads, but they're not what you
  want for getting the best possible numbers when benchmarking.
  
   - Bad write performance
  
     If write performance is not what you expected, you probably wanted to be
     running in writeback mode, which isn't the default (not due to a lack of
     maturity, but simply because in writeback mode you'll lose data if something
     happens to your SSD)
  
     # echo writeback > /sys/block/bcache0/cache_mode
  
   - Bad performance, or traffic not going to the SSD that you'd expect
  
     By default, bcache doesn't cache everything. It tries to skip sequential IO -
     because you really want to be caching the random IO, and if you copy a 10
     gigabyte file you probably don't want that pushing 10 gigabytes of randomly
     accessed data out of your cache.
  
     But if you want to benchmark reads from cache, and you start out with fio
     writing an 8 gigabyte test file - so you want to disable that.
  
     # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
  
     To set it back to the default (4 mb), do
  
     # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
  
   - Traffic's still going to the spindle/still getting cache misses
  
     In the real world, SSDs don't always keep up with disks - particularly with
     slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
     you want to avoid being bottlenecked by the SSD and having it slow everything
     down.
  
     To avoid that bcache tracks latency to the cache device, and gradually
     throttles traffic if the latency exceeds a threshold (it does this by
     cranking down the sequential bypass).
  
     You can disable this if you need to by setting the thresholds to 0:
  
     # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
     # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
  
     The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
  
   - Still getting cache misses, of the same data
  
     One last issue that sometimes trips people up is actually an old bug, due to
     the way cache coherency is handled for cache misses. If a btree node is full,
     a cache miss won't be able to insert a key for the new data and the data
     won't be written to the cache.
  
     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 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.
  
     Solution: warm the cache by doing writes, or use the testing branch (there's
     a fix for the issue there).
  
  SYSFS - BACKING DEVICE:
  
  Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
  (if attached) /sys/fs/bcache/<cset-uuid>/bdev*
  
  attach
    Echo the UUID of a cache set to this file to enable caching.
  
  cache_mode
    Can be one of either writethrough, writeback, writearound or none.
  
  clear_stats
    Writing to this file resets the running total stats (not the day/hour/5 minute
    decaying versions).
  
  detach
    Write to this file to detach from a cache set. If there is dirty data in the
    cache, it will be flushed first.
  
  dirty_data
    Amount of dirty data for this backing device in the cache. Continuously
    updated unlike the cache set's version, but may be slightly off.
  
  label
    Name of underlying device.
  
  readahead
    Size of readahead that should be performed.  Defaults to 0.  If set to e.g.
    1M, it will round cache miss reads up to that size, but without overlapping
    existing cache entries.
  
  running
    1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
    it's in passthrough mode or caching).
  
  sequential_cutoff
    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.
  
  sequential_merge
    If non zero, bcache keeps a list of the last 128 requests submitted to compare
    against all new requests to determine which new requests are sequential
    continuations of previous requests for the purpose of determining sequential
    cutoff. This is necessary if the sequential cutoff value is greater than the
    maximum acceptable sequential size for any single request. 
  
  state
    The backing device can be in one of four different states:
  
    no cache: Has never been attached to a cache set.
  
    clean: Part of a cache set, and there is no cached dirty data.
  
    dirty: Part of a cache set, and there is cached dirty data.
  
    inconsistent: The backing device was forcibly run by the user when there was
    dirty data cached but the cache set was unavailable; whatever data was on the
    backing device has likely been corrupted.
  
  stop
    Write to this file to shut down the bcache device and close the backing
    device.
  
  writeback_delay
    When dirty data is written to the cache and it previously did not contain
    any, waits some number of seconds before initiating writeback. Defaults to
    30.
  
  writeback_percent
    If nonzero, bcache tries to keep around this percentage of the cache dirty by
    throttling background writeback and using a PD controller to smoothly adjust
    the rate.
  
  writeback_rate
    Rate in sectors per second - if writeback_percent is nonzero, background
    writeback is throttled to this rate. Continuously adjusted by bcache but may
    also be set by the user.
  
  writeback_running
    If off, writeback of dirty data will not take place at all. Dirty data will
    still be added to the cache until it is mostly full; only meant for
    benchmarking. Defaults to on.
  
  SYSFS - BACKING DEVICE STATS:
  
  There are directories with these numbers for a running total, as well as
  versions that decay over the past day, hour and 5 minutes; they're also
  aggregated in the cache set directory as well.
  
  bypassed
    Amount of IO (both reads and writes) that has bypassed the cache
  
  cache_hits
  cache_misses
  cache_hit_ratio
    Hits and misses are counted per individual IO as bcache sees them; a
    partial hit is counted as a miss.
  
  cache_bypass_hits
  cache_bypass_misses
    Hits and misses for IO that is intended to skip the cache are still counted,
    but broken out here.
  
  cache_miss_collisions
    Counts instances where data was going to be inserted into the cache from a
    cache miss, but raced with a write and data was already present (usually 0
    since the synchronization for cache misses was rewritten)
  
  cache_readaheads
    Count of times readahead occurred.
  
  SYSFS - CACHE SET:
  
  Available at /sys/fs/bcache/<cset-uuid>
  
  average_key_size
    Average data per key in the btree.
  
  bdev<0..n>
    Symlink to each of the attached backing devices.
  
  block_size
    Block size of the cache devices.
  
  btree_cache_size
    Amount of memory currently used by the btree cache
  
  bucket_size
    Size of buckets
  
  cache<0..n>
    Symlink to each of the cache devices comprising this cache set. 
  
  cache_available_percent
    Percentage of cache device which doesn't contain dirty data, and could
    potentially be used for writeback.  This doesn't mean this space isn't used
    for clean cached data; the unused statistic (in priority_stats) is typically
    much lower.
  
  clear_stats
    Clears the statistics associated with this cache
  
  dirty_data
    Amount of dirty data is in the cache (updated when garbage collection runs).
  
  flash_vol_create
    Echoing a size to this file (in human readable units, k/M/G) creates a thinly
    provisioned volume backed by the cache set.
  
  io_error_halflife
  io_error_limit
    These determines how many errors we accept before disabling the cache.
    Each error is decayed by the half life (in # ios).  If the decaying count
    reaches io_error_limit dirty data is written out and the cache is disabled.
  
  journal_delay_ms
    Journal writes will delay for up to this many milliseconds, unless a cache
    flush happens sooner. Defaults to 100.
  
  root_usage_percent
    Percentage of the root btree node in use.  If this gets too high the node
    will split, increasing the tree depth.
  
  stop
    Write to this file to shut down the cache set - waits until all attached
    backing devices have been shut down.
  
  tree_depth
    Depth of the btree (A single node btree has depth 0).
  
  unregister
    Detaches all backing devices and closes the cache devices; if dirty data is
    present it will disable writeback caching and wait for it to be flushed.
  
  SYSFS - CACHE SET INTERNAL:
  
  This directory also exposes timings for a number of internal operations, with
  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
    Number of journal entries that are newer than the index.
  
  btree_nodes
    Total nodes in the btree.
  
  btree_used_percent
    Average fraction of btree in use.
  
  bset_tree_stats
    Statistics about the auxiliary search trees
  
  btree_cache_max_chain
    Longest chain in the btree node cache's hash table
  
  cache_read_races
    Counts instances where while data was being read from the cache, the bucket
    was reused and invalidated - i.e. where the pointer was stale after the read
    completed. When this occurs the data is reread from the backing device.
  
  trigger_gc
    Writing to this file forces garbage collection to run.
  
  SYSFS - CACHE DEVICE:
  
  Available at /sys/block/<cdev>/bcache
  
  block_size
    Minimum granularity of writes - should match hardware sector size.
  
  btree_written
    Sum of all btree writes, in (kilo/mega/giga) bytes
  
  bucket_size
    Size of buckets
  
  cache_replacement_policy
    One of either lru, fifo or random.
  
  discard
    Boolean; if on a discard/TRIM will be issued to each bucket before it is
    reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
    slow).
  
  freelist_percent
    Size of the freelist as a percentage of nbuckets. Can be written to to
    increase the number of buckets kept on the freelist, which lets you
    artificially reduce the size of the cache at runtime. Mostly for testing
    purposes (i.e. testing how different size caches affect your hit rate), but
    since buckets are discarded when they move on to the freelist will also make
    the SSD's garbage collection easier by effectively giving it more reserved
    space.
  
  io_errors
    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).
  
  nbuckets
    Total buckets in this cache
  
  priority_stats
    Statistics about how recently data in the cache has been accessed.
    This can reveal your working set size.  Unused is the percentage of
    the cache that doesn't contain any data.  Metadata is bcache's
    metadata overhead.  Average is the average priority of cache buckets.
    Next is a list of quantiles with the priority threshold of each.
  
  written
    Sum of all data that has been written to the cache; comparison with
    btree_written gives the amount of write inflation in bcache.