docs: admin-guide: add bcache documentation

The bcache.txt is already in ReST format. So, move it to the
admin guide, where it belongs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 649 deletions

diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt
deleted file mode 100644
index c0ce64d75bbf..000000000000
--- a/Documentation/bcache.txt
+++ /dev/null
@@ -1,649 +0,0 @@
-============================
-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.
-
-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
-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 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.
-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 `make-bcache`.
-
-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:~# make-bcache -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 `make-bcache --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.