1 Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be
2 nice if you could use them as cache... Hence bcache.
4 Wiki and git repositories are at:
5 http://bcache.evilpiepirate.org
6 http://evilpiepirate.org/git/linux-bcache.git
7 http://evilpiepirate.org/git/bcache-tools.git
9 It's designed around the performance characteristics of SSDs - it only allocates
10 in erase block sized buckets, and it uses a hybrid btree/log to track cached
11 extants (which can be anywhere from a single sector to the bucket size). It's
12 designed to avoid random writes at all costs; it fills up an erase block
13 sequentially, then issues a discard before reusing it.
15 Both writethrough and writeback caching are supported. Writeback defaults to
16 off, but can be switched on and off arbitrarily at runtime. Bcache goes to
17 great lengths to protect your data - it reliably handles unclean shutdown. (It
18 doesn't even have a notion of a clean shutdown; bcache simply doesn't return
19 writes as completed until they're on stable storage).
21 Writeback caching can use most of the cache for buffering writes - writing
22 dirty data to the backing device is always done sequentially, scanning from the
23 start to the end of the index.
25 Since random IO is what SSDs excel at, there generally won't be much benefit
26 to caching large sequential IO. Bcache detects sequential IO and skips it;
27 it also keeps a rolling average of the IO sizes per task, and as long as the
28 average is above the cutoff it will skip all IO from that task - instead of
29 caching the first 512k after every seek. Backups and large file copies should
30 thus entirely bypass the cache.
32 In the event of a data IO error on the flash it will try to recover by reading
33 from disk or invalidating cache entries. For unrecoverable errors (meta data
34 or dirty data), caching is automatically disabled; if dirty data was present
35 in the cache it first disables writeback caching and waits for all dirty data
39 You'll need make-bcache from the bcache-tools repository. Both the cache device
40 and backing device must be formatted before use.
41 make-bcache -B /dev/sdb
42 make-bcache -C /dev/sdc
44 make-bcache has the ability to format multiple devices at the same time - if
45 you format your backing devices and cache device at the same time, you won't
46 have to manually attach:
47 make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
49 bcache-tools now ships udev rules, and bcache devices are known to the kernel
50 immediately. Without udev, you can manually register devices like this:
52 echo /dev/sdb > /sys/fs/bcache/register
53 echo /dev/sdc > /sys/fs/bcache/register
55 Registering the backing device makes the bcache device show up in /dev; you can
56 now format it and use it as normal. But the first time using a new bcache
57 device, it'll be running in passthrough mode until you attach it to a cache.
58 See the section on attaching.
60 The devices show up as:
64 As well as (with udev):
66 /dev/bcache/by-uuid/<uuid>
67 /dev/bcache/by-label/<label>
71 mkfs.ext4 /dev/bcache0
72 mount /dev/bcache0 /mnt
74 You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
76 Cache devices are managed as sets; multiple caches per set isn't supported yet
77 but will allow for mirroring of metadata and dirty data in the future. Your new
78 cache set shows up as /sys/fs/bcache/<UUID>
82 After your cache device and backing device are registered, the backing device
83 must be attached to your cache set to enable caching. Attaching a backing
84 device to a cache set is done thusly, with the UUID of the cache set in
87 echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
89 This only has to be done once. The next time you reboot, just reregister all
90 your bcache devices. If a backing device has data in a cache somewhere, the
91 /dev/bcache<N> device won't be created until the cache shows up - particularly
92 important if you have writeback caching turned on.
94 If you're booting up and your cache device is gone and never coming back, you
95 can force run the backing device:
97 echo 1 > /sys/block/sdb/bcache/running
99 (You need to use /sys/block/sdb (or whatever your backing device is called), not
100 /sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
101 partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
103 The backing device will still use that cache set if it shows up in the future,
104 but all the cached data will be invalidated. If there was dirty data in the
105 cache, don't expect the filesystem to be recoverable - you will have massive
106 filesystem corruption, though ext4's fsck does work miracles.
110 Bcache tries to transparently handle IO errors to/from the cache device without
111 affecting normal operation; if it sees too many errors (the threshold is
112 configurable, and defaults to 0) it shuts down the cache device and switches all
113 the backing devices to passthrough mode.
115 - For reads from the cache, if they error we just retry the read from the
118 - For writethrough writes, if the write to the cache errors we just switch to
119 invalidating the data at that lba in the cache (i.e. the same thing we do for
120 a write that bypasses the cache)
122 - For writeback writes, we currently pass that error back up to the
123 filesystem/userspace. This could be improved - we could retry it as a write
124 that skips the cache so we don't have to error the write.
126 - When we detach, we first try to flush any dirty data (if we were running in
127 writeback mode). It currently doesn't do anything intelligent if it fails to
128 read some of the dirty data, though.
130 TROUBLESHOOTING PERFORMANCE:
132 Bcache has a bunch of config options and tunables. The defaults are intended to
133 be reasonable for typical desktop and server workloads, but they're not what you
134 want for getting the best possible numbers when benchmarking.
136 - Bad write performance
138 If write performance is not what you expected, you probably wanted to be
139 running in writeback mode, which isn't the default (not due to a lack of
140 maturity, but simply because in writeback mode you'll lose data if something
143 # echo writeback > /sys/block/bcache0/cache_mode
145 - Bad performance, or traffic not going to the SSD that you'd expect
147 By default, bcache doesn't cache everything. It tries to skip sequential IO -
148 because you really want to be caching the random IO, and if you copy a 10
149 gigabyte file you probably don't want that pushing 10 gigabytes of randomly
150 accessed data out of your cache.
152 But if you want to benchmark reads from cache, and you start out with fio
153 writing an 8 gigabyte test file - so you want to disable that.
155 # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
157 To set it back to the default (4 mb), do
159 # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
161 - Traffic's still going to the spindle/still getting cache misses
163 In the real world, SSDs don't always keep up with disks - particularly with
164 slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
165 you want to avoid being bottlenecked by the SSD and having it slow everything
168 To avoid that bcache tracks latency to the cache device, and gradually
169 throttles traffic if the latency exceeds a threshold (it does this by
170 cranking down the sequential bypass).
172 You can disable this if you need to by setting the thresholds to 0:
174 # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
175 # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
177 The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
179 - Still getting cache misses, of the same data
181 One last issue that sometimes trips people up is actually an old bug, due to
182 the way cache coherency is handled for cache misses. If a btree node is full,
183 a cache miss won't be able to insert a key for the new data and the data
184 won't be written to the cache.
186 In practice this isn't an issue because as soon as a write comes along it'll
187 cause the btree node to be split, and you need almost no write traffic for
188 this to not show up enough to be noticeable (especially since bcache's btree
189 nodes are huge and index large regions of the device). But when you're
190 benchmarking, if you're trying to warm the cache by reading a bunch of data
191 and there's no other traffic - that can be a problem.
193 Solution: warm the cache by doing writes, or use the testing branch (there's
194 a fix for the issue there).
196 SYSFS - BACKING DEVICE:
198 Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
199 (if attached) /sys/fs/bcache/<cset-uuid>/bdev*
202 Echo the UUID of a cache set to this file to enable caching.
205 Can be one of either writethrough, writeback, writearound or none.
208 Writing to this file resets the running total stats (not the day/hour/5 minute
212 Write to this file to detach from a cache set. If there is dirty data in the
213 cache, it will be flushed first.
216 Amount of dirty data for this backing device in the cache. Continuously
217 updated unlike the cache set's version, but may be slightly off.
220 Name of underlying device.
223 Size of readahead that should be performed. Defaults to 0. If set to e.g.
224 1M, it will round cache miss reads up to that size, but without overlapping
225 existing cache entries.
228 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
229 it's in passthrough mode or caching).
232 A sequential IO will bypass the cache once it passes this threshold; the
233 most recent 128 IOs are tracked so sequential IO can be detected even when
234 it isn't all done at once.
237 If non zero, bcache keeps a list of the last 128 requests submitted to compare
238 against all new requests to determine which new requests are sequential
239 continuations of previous requests for the purpose of determining sequential
240 cutoff. This is necessary if the sequential cutoff value is greater than the
241 maximum acceptable sequential size for any single request.
244 The backing device can be in one of four different states:
246 no cache: Has never been attached to a cache set.
248 clean: Part of a cache set, and there is no cached dirty data.
250 dirty: Part of a cache set, and there is cached dirty data.
252 inconsistent: The backing device was forcibly run by the user when there was
253 dirty data cached but the cache set was unavailable; whatever data was on the
254 backing device has likely been corrupted.
257 Write to this file to shut down the bcache device and close the backing
261 When dirty data is written to the cache and it previously did not contain
262 any, waits some number of seconds before initiating writeback. Defaults to
266 If nonzero, bcache tries to keep around this percentage of the cache dirty by
267 throttling background writeback and using a PD controller to smoothly adjust
271 Rate in sectors per second - if writeback_percent is nonzero, background
272 writeback is throttled to this rate. Continuously adjusted by bcache but may
273 also be set by the user.
276 If off, writeback of dirty data will not take place at all. Dirty data will
277 still be added to the cache until it is mostly full; only meant for
278 benchmarking. Defaults to on.
280 SYSFS - BACKING DEVICE STATS:
282 There are directories with these numbers for a running total, as well as
283 versions that decay over the past day, hour and 5 minutes; they're also
284 aggregated in the cache set directory as well.
287 Amount of IO (both reads and writes) that has bypassed the cache
292 Hits and misses are counted per individual IO as bcache sees them; a
293 partial hit is counted as a miss.
297 Hits and misses for IO that is intended to skip the cache are still counted,
300 cache_miss_collisions
301 Counts instances where data was going to be inserted into the cache from a
302 cache miss, but raced with a write and data was already present (usually 0
303 since the synchronization for cache misses was rewritten)
306 Count of times readahead occurred.
310 Available at /sys/fs/bcache/<cset-uuid>
313 Average data per key in the btree.
316 Symlink to each of the attached backing devices.
319 Block size of the cache devices.
322 Amount of memory currently used by the btree cache
328 Symlink to each of the cache devices comprising this cache set.
330 cache_available_percent
331 Percentage of cache device which doesn't contain dirty data, and could
332 potentially be used for writeback. This doesn't mean this space isn't used
333 for clean cached data; the unused statistic (in priority_stats) is typically
337 Clears the statistics associated with this cache
340 Amount of dirty data is in the cache (updated when garbage collection runs).
343 Echoing a size to this file (in human readable units, k/M/G) creates a thinly
344 provisioned volume backed by the cache set.
348 These determines how many errors we accept before disabling the cache.
349 Each error is decayed by the half life (in # ios). If the decaying count
350 reaches io_error_limit dirty data is written out and the cache is disabled.
353 Journal writes will delay for up to this many milliseconds, unless a cache
354 flush happens sooner. Defaults to 100.
357 Percentage of the root btree node in use. If this gets too high the node
358 will split, increasing the tree depth.
361 Write to this file to shut down the cache set - waits until all attached
362 backing devices have been shut down.
365 Depth of the btree (A single node btree has depth 0).
368 Detaches all backing devices and closes the cache devices; if dirty data is
369 present it will disable writeback caching and wait for it to be flushed.
371 SYSFS - CACHE SET INTERNAL:
373 This directory also exposes timings for a number of internal operations, with
374 separate files for average duration, average frequency, last occurrence and max
375 duration: garbage collection, btree read, btree node sorts and btree splits.
377 active_journal_entries
378 Number of journal entries that are newer than the index.
381 Total nodes in the btree.
384 Average fraction of btree in use.
387 Statistics about the auxiliary search trees
389 btree_cache_max_chain
390 Longest chain in the btree node cache's hash table
393 Counts instances where while data was being read from the cache, the bucket
394 was reused and invalidated - i.e. where the pointer was stale after the read
395 completed. When this occurs the data is reread from the backing device.
398 Writing to this file forces garbage collection to run.
400 SYSFS - CACHE DEVICE:
402 Available at /sys/block/<cdev>/bcache
405 Minimum granularity of writes - should match hardware sector size.
408 Sum of all btree writes, in (kilo/mega/giga) bytes
413 cache_replacement_policy
414 One of either lru, fifo or random.
417 Boolean; if on a discard/TRIM will be issued to each bucket before it is
418 reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
422 Size of the freelist as a percentage of nbuckets. Can be written to to
423 increase the number of buckets kept on the freelist, which lets you
424 artificially reduce the size of the cache at runtime. Mostly for testing
425 purposes (i.e. testing how different size caches affect your hit rate), but
426 since buckets are discarded when they move on to the freelist will also make
427 the SSD's garbage collection easier by effectively giving it more reserved
431 Number of errors that have occurred, decayed by io_error_halflife.
434 Sum of all non data writes (btree writes and all other metadata).
437 Total buckets in this cache
440 Statistics about how recently data in the cache has been accessed.
441 This can reveal your working set size. Unused is the percentage of
442 the cache that doesn't contain any data. Metadata is bcache's
443 metadata overhead. Average is the average priority of cache buckets.
444 Next is a list of quantiles with the priority threshold of each.
447 Sum of all data that has been written to the cache; comparison with
448 btree_written gives the amount of write inflation in bcache.