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 ============================
 A block layer cache (bcache)
 ============================
 
 Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be
 nice if you could use them as cache... Hence bcache.
 
 The bcache wiki can be found at:
   https://bcache.evilpiepirate.org
 
 This is the git repository of bcache-tools:
   https://git.kernel.org/pub/scm/linux/kernel/git/colyli/bcache-tools.git/
 
 The latest bcache kernel code can be found from mainline Linux kernel:
   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.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
 extents (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 bcache util from the bcache-tools repository. Both the cache device
 and backing device must be formatted before use::
 
   bcache make -B /dev/sdb
   bcache make -C /dev/sdc
 
 `bcache make` 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::
 
   bcache make -B /dev/sda /dev/sdb -C /dev/sdc
 
 If your bcache-tools is not updated to latest version and does not have the
 unified `bcache` utility, you may use the legacy `make-bcache` utility to format
 bcache device with same -B and -C parameters.
 
 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.
 If you are thinking about using bcache later, it is recommended to setup all your
 slow devices as bcache backing devices without a cache, and you can choose to add
 a caching device later.
 See 'ATTACHING' section below.
 
 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 .
 You can also control them through /sys/fs//bcache/<cset-uuid>/ .
 
 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.
 
 
 Howto/cookbook
 --------------
 
 A) Starting a bcache with a missing caching device
 
 If registering the backing device doesn't help, it's already there, you just need
 to force it to run without the cache::
 
         host:~# echo /dev/sdb1 > /sys/fs/bcache/register
         [  119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
 
 Next, you try to register your caching device if it's present. However
 if it's absent, or registration fails for some reason, you can still
 start your bcache without its cache, like so::
 
         host:/sys/block/sdb/sdb1/bcache# echo 1 > running
 
 Note that this may cause data loss if you were running in writeback mode.
 
 
 B) Bcache does not find its cache::
 
         host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
         [ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
         [ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
         [ 1933.478179] : cache set not found
 
 In this case, the caching device was simply not registered at boot
 or disappeared and came back, and needs to be (re-)registered::
 
         host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
 
 
 C) Corrupt bcache crashes the kernel at device registration time:
 
 This should never happen.  If it does happen, then you have found a bug!
 Please report it to the bcache development list: linux-bcache@vger.kernel.org
 
 Be sure to provide as much information that you can including kernel dmesg
 output if available so that we may assist.
 
 
 D) Recovering data without bcache:
 
 If bcache is not available in the kernel, a filesystem on the backing
 device is still available at an 8KiB offset. So either via a loopdev
 of the backing device created with --offset 8K, or any value defined by
 --data-offset when you originally formatted bcache with `bcache make`.
 
 For example::
 
         losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
 
 This should present your unmodified backing device data in /dev/loop0
 
 If your cache is in writethrough mode, then you can safely discard the
 cache device without loosing data.
 
 
 E) Wiping a cache device
 
 ::
 
         host:~# wipefs -a /dev/sdh2
         16 bytes were erased at offset 0x1018 (bcache)
         they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
 
 After you boot back with bcache enabled, you recreate the cache and attach it::
 
         host:~# bcache make -C /dev/sdh2
         UUID:                   7be7e175-8f4c-4f99-94b2-9c904d227045
         Set UUID:               5bc072a8-ab17-446d-9744-e247949913c1
         version:                0
         nbuckets:               106874
         block_size:             1
         bucket_size:            1024
         nr_in_set:              1
         nr_this_dev:            0
         first_bucket:           1
         [  650.511912] bcache: run_cache_set() invalidating existing data
         [  650.549228] bcache: register_cache() registered cache device sdh2
 
 start backing device with missing cache::
 
         host:/sys/block/md5/bcache# echo 1 > running
 
 attach new cache::
 
         host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach
         [  865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1
 
 
 F) Remove or replace a caching device::
 
         host:/sys/block/sda/sda7/bcache# echo 1 > detach
         [  695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
 
         host:~# wipefs -a /dev/nvme0n1p4
         wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
         Ooops, it's disabled, but not unregistered, so it's still protected
 
 We need to go and unregister it::
 
         host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
         lrwxrwxrwx 1 root root 0 Feb 25 18:33 cache0 -> ../../../devices/pci0000:00/0000:00:1d.0/0000:70:00.0/nvme/nvme0/nvme0n1/nvme0n1p4/bcache/
         host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
         kernel: [  917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
 
 Now we can wipe it::
 
         host:~# wipefs -a /dev/nvme0n1p4
         /dev/nvme0n1p4: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
 
 
 G) dm-crypt and bcache
 
 First setup bcache unencrypted and then install dmcrypt on top of
 /dev/bcache<N> This will work faster than if you dmcrypt both the backing
 and caching devices and then install bcache on top. [benchmarks?]
 
 
 H) Stop/free a registered bcache to wipe and/or recreate it
 
 Suppose that you need to free up all bcache references so that you can
 fdisk run and re-register a changed partition table, which won't work
 if there are any active backing or caching devices left on it:
 
 1) Is it present in /dev/bcache* ? (there are times where it won't be)
 
    If so, it's easy::
 
         host:/sys/block/bcache0/bcache# echo 1 > stop
 
 2) But if your backing device is gone, this won't work::
 
         host:/sys/block/bcache0# cd bcache
         bash: cd: bcache: No such file or directory
 
    In this case, you may have to unregister the dmcrypt block device that
    references this bcache to free it up::
 
         host:~# dmsetup remove oldds1
         bcache: bcache_device_free() bcache0 stopped
         bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
 
    This causes the backing bcache to be removed from /sys/fs/bcache and
    then it can be reused.  This would be true of any block device stacking
    where bcache is a lower device.
 
 3) In other cases, you can also look in /sys/fs/bcache/::
 
         host:/sys/fs/bcache# ls -l */{cache?,bdev?}
         lrwxrwxrwx 1 root root 0 Mar  5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/
         lrwxrwxrwx 1 root root 0 Mar  5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/
         lrwxrwxrwx 1 root root 0 Mar  5 09:39 5bc072a8-ab17-446d-9744-e247949913c1/cache0 -> ../../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/ata10/host9/target9:0:0/9:0:0:0/block/sdl/sdl2/bcache/
 
    The device names will show which UUID is relevant, cd in that directory
    and stop the cache::
 
         host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
 
    This will free up bcache references and let you reuse the partition for
    other purposes.
 
 
 
 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.
 
  - Backing device alignment
 
    The default metadata size in bcache is 8k.  If your backing device is
    RAID based, then be sure to align this by a multiple of your stride
    width using `bcache make --data-offset`. If you intend to expand your
    disk array in the future, then multiply a series of primes by your
    raid stripe size to get the disk multiples that you would like.
 
    For example:  If you have a 64k stripe size, then the following offset
    would provide alignment for many common RAID5 data spindle counts::
 
         64k * 2*2*2*3*3*5*7 bytes = 161280k
 
    That space is wasted, but for only 157.5MB you can grow your RAID 5
    volume to the following data-spindle counts without re-aligning::
 
         3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...
 
  - 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/bcache/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.

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