path: root/Documentation/bcache.txt
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authorKent Overstreet <>2013-03-23 16:11:31 -0700
committerKent Overstreet <>2013-03-23 16:11:31 -0700
commitcafe563591446cf80bfbc2fe3bc72a2e36cf1060 (patch)
treec8ae27b13dcdb0219634376ca5e667df32b1173a /Documentation/bcache.txt
parentea6749c705d9e629ed03c7336cc929fc6014b834 (diff)
bcache: A block layer cache
Does writethrough and writeback caching, handles unclean shutdown, and has a bunch of other nifty features motivated by real world usage. See the wiki at for more. Signed-off-by: Kent Overstreet <>
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+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:
+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
+To make bcache devices known to the kernel, echo them to /sys/fs/bcache/register:
+ echo /dev/sdb > /sys/fs/bcache/register
+ echo /dev/sdc > /sys/fs/bcache/register
+To register your bcache devices automatically, you could add something like
+this to an init script:
+ echo /dev/sd* > /sys/fs/bcache/register_quiet
+It'll look for bcache superblocks and ignore everything that doesn't have one.
+Registering the backing device makes the bcache 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 at /dev/bcacheN, and can be controlled via sysfs from
+ mkfs.ext4 /dev/bcache0
+ mount /dev/bcache0 /mnt
+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>
+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
+ echo <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# 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.
+ Echo the UUID of a cache set to this file to enable caching.
+ Can be one of either writethrough, writeback, writearound or none.
+ Writing to this file resets the running total stats (not the day/hour/5 minute
+ decaying versions).
+ Write to this file to detach from a cache set. If there is dirty data in the
+ cache, it will be flushed first.
+ Amount of dirty data for this backing device in the cache. Continuously
+ updated unlike the cache set's version, but may be slightly off.
+ Name of underlying device.
+ 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.
+ 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
+ it's in passthrough mode or caching).
+ A sequential IO will bypass the cache once it passes this threshhold; the
+ most recent 128 IOs are tracked so sequential IO can be detected even when
+ it isn't all done at once.
+ 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.
+ 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.
+ Write to this file to shut down the bcache device and close the backing
+ device.
+ 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.
+ 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.
+ 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.
+ 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.
+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.
+ Amount of IO (both reads and writes) that has bypassed the cache
+ Hits and misses are counted per individual IO as bcache sees them; a
+ partial hit is counted as a miss.
+ Hits and misses for IO that is intended to skip the cache are still counted,
+ but broken out here.
+ 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)
+ Count of times readahead occured.
+ Average data per key in the btree.
+ Symlink to each of the attached backing devices.
+ Block size of the cache devices.
+ Amount of memory currently used by the btree cache
+ Size of buckets
+ Symlink to each of the cache devices comprising this cache set.
+ Percentage of cache device free.
+ Clears the statistics associated with this cache
+ Amount of dirty data is in the cache (updated when garbage collection runs).
+ Echoing a size to this file (in human readable units, k/M/G) creates a thinly
+ provisioned volume backed by the cache set.
+ 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 writes will delay for up to this many milliseconds, unless a cache
+ flush happens sooner. Defaults to 100.
+ Percentage of the root btree node in use. If this gets too high the node
+ will split, increasing the tree depth.
+ Write to this file to shut down the cache set - waits until all attached
+ backing devices have been shut down.
+ Depth of the btree (A single node btree has depth 0).
+ 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.
+This directory also exposes timings for a number of internal operations, with
+separate files for average duration, average frequency, last occurence and max
+duration: garbage collection, btree read, btree node sorts and btree splits.
+ Number of journal entries that are newer than the index.
+ Total nodes in the btree.
+ Average fraction of btree in use.
+ Statistics about the auxiliary search trees
+ Longest chain in the btree node cache's hash table
+ 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.
+ Writing to this file forces garbage collection to run.
+ Minimum granularity of writes - should match hardware sector size.
+ Sum of all btree writes, in (kilo/mega/giga) bytes
+ Size of buckets
+ One of either lru, fifo or random.
+ 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).
+ 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.
+ Number of errors that have occured, decayed by io_error_halflife.
+ Sum of all non data writes (btree writes and all other metadata).
+ Total buckets in this cache
+ Statistics about how recently data in the cache has been accessed. This can
+ reveal your working set size.
+ Sum of all data that has been written to the cache; comparison with
+ btree_written gives the amount of write inflation in bcache.