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diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt new file mode 100644 index 00000000000..77db8809bd9 --- /dev/null +++ b/Documentation/bcache.txt @@ -0,0 +1,431 @@ +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 + +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 +/sys/block/bcacheN/bcache: + + 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> + +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 <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. + +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 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: + +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 threshhold; 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 occured. + +SYSFS - CACHE SET: + +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 free. + +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 occurence 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: + +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 occured, 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. + +written + Sum of all data that has been written to the cache; comparison with + btree_written gives the amount of write inflation in bcache. |