|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.
|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
|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.
|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
|- 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.
|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
|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 noticable (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:
|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
|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
|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
|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
|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.
|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.
|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.
|SYSFS - CACHE SET:
|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 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
|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.
|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
|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.
|SYSFS - CACHE DEVICE:
|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
|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
|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. 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.
|Sum of all data that has been written to the cache; comparison with
|btree_written gives the amount of write inflation in bcache.