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Kent Overstreetcafe5632013-03-23 16:11:31 -07001Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be
2nice if you could use them as cache... Hence bcache.
3
4Wiki 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
8
9It's designed around the performance characteristics of SSDs - it only allocates
10in erase block sized buckets, and it uses a hybrid btree/log to track cached
11extants (which can be anywhere from a single sector to the bucket size). It's
12designed to avoid random writes at all costs; it fills up an erase block
13sequentially, then issues a discard before reusing it.
14
15Both writethrough and writeback caching are supported. Writeback defaults to
16off, but can be switched on and off arbitrarily at runtime. Bcache goes to
17great lengths to protect your data - it reliably handles unclean shutdown. (It
18doesn't even have a notion of a clean shutdown; bcache simply doesn't return
19writes as completed until they're on stable storage).
20
21Writeback caching can use most of the cache for buffering writes - writing
22dirty data to the backing device is always done sequentially, scanning from the
23start to the end of the index.
24
25Since random IO is what SSDs excel at, there generally won't be much benefit
26to caching large sequential IO. Bcache detects sequential IO and skips it;
27it also keeps a rolling average of the IO sizes per task, and as long as the
28average is above the cutoff it will skip all IO from that task - instead of
29caching the first 512k after every seek. Backups and large file copies should
30thus entirely bypass the cache.
31
32In the event of a data IO error on the flash it will try to recover by reading
33from disk or invalidating cache entries. For unrecoverable errors (meta data
34or dirty data), caching is automatically disabled; if dirty data was present
35in the cache it first disables writeback caching and waits for all dirty data
36to be flushed.
37
38Getting started:
39You'll need make-bcache from the bcache-tools repository. Both the cache device
40and backing device must be formatted before use.
41 make-bcache -B /dev/sdb
42 make-bcache -C /dev/sdc
43
44make-bcache has the ability to format multiple devices at the same time - if
45you format your backing devices and cache device at the same time, you won't
46have to manually attach:
47 make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
48
49To make bcache devices known to the kernel, echo them to /sys/fs/bcache/register:
50
51 echo /dev/sdb > /sys/fs/bcache/register
52 echo /dev/sdc > /sys/fs/bcache/register
53
54To register your bcache devices automatically, you could add something like
55this to an init script:
56
57 echo /dev/sd* > /sys/fs/bcache/register_quiet
58
59It'll look for bcache superblocks and ignore everything that doesn't have one.
60
61Registering the backing device makes the bcache show up in /dev; you can now
62format it and use it as normal. But the first time using a new bcache device,
63it'll be running in passthrough mode until you attach it to a cache. See the
64section on attaching.
65
66The devices show up at /dev/bcacheN, and can be controlled via sysfs from
67/sys/block/bcacheN/bcache:
68
69 mkfs.ext4 /dev/bcache0
70 mount /dev/bcache0 /mnt
71
72Cache devices are managed as sets; multiple caches per set isn't supported yet
73but will allow for mirroring of metadata and dirty data in the future. Your new
74cache set shows up as /sys/fs/bcache/<UUID>
75
76ATTACHING:
77
78After your cache device and backing device are registered, the backing device
79must be attached to your cache set to enable caching. Attaching a backing
80device to a cache set is done thusly, with the UUID of the cache set in
81/sys/fs/bcache:
82
83 echo <UUID> > /sys/block/bcache0/bcache/attach
84
85This only has to be done once. The next time you reboot, just reregister all
86your bcache devices. If a backing device has data in a cache somewhere, the
87/dev/bcache# device won't be created until the cache shows up - particularly
88important if you have writeback caching turned on.
89
90If you're booting up and your cache device is gone and never coming back, you
91can force run the backing device:
92
93 echo 1 > /sys/block/sdb/bcache/running
94
95(You need to use /sys/block/sdb (or whatever your backing device is called), not
96/sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
97partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
98
99The backing device will still use that cache set if it shows up in the future,
100but all the cached data will be invalidated. If there was dirty data in the
101cache, don't expect the filesystem to be recoverable - you will have massive
102filesystem corruption, though ext4's fsck does work miracles.
103
Kent Overstreet7b41b512013-03-27 12:24:17 -0700104ERROR HANDLING:
105
106Bcache tries to transparently handle IO errors to/from the cache device without
107affecting normal operation; if it sees too many errors (the threshold is
108configurable, and defaults to 0) it shuts down the cache device and switches all
109the backing devices to passthrough mode.
110
111 - For reads from the cache, if they error we just retry the read from the
112 backing device.
113
114 - For writethrough writes, if the write to the cache errors we just switch to
115 invalidating the data at that lba in the cache (i.e. the same thing we do for
116 a write that bypasses the cache)
117
118 - For writeback writes, we currently pass that error back up to the
119 filesystem/userspace. This could be improved - we could retry it as a write
120 that skips the cache so we don't have to error the write.
121
122 - When we detach, we first try to flush any dirty data (if we were running in
123 writeback mode). It currently doesn't do anything intelligent if it fails to
124 read some of the dirty data, though.
125
126TROUBLESHOOTING PERFORMANCE:
127
128Bcache has a bunch of config options and tunables. The defaults are intended to
129be reasonable for typical desktop and server workloads, but they're not what you
130want for getting the best possible numbers when benchmarking.
131
132 - Bad write performance
133
134 If write performance is not what you expected, you probably wanted to be
135 running in writeback mode, which isn't the default (not due to a lack of
136 maturity, but simply because in writeback mode you'll lose data if something
137 happens to your SSD)
138
139 # echo writeback > /sys/block/bcache0/cache_mode
140
141 - Bad performance, or traffic not going to the SSD that you'd expect
142
143 By default, bcache doesn't cache everything. It tries to skip sequential IO -
144 because you really want to be caching the random IO, and if you copy a 10
145 gigabyte file you probably don't want that pushing 10 gigabytes of randomly
146 accessed data out of your cache.
147
148 But if you want to benchmark reads from cache, and you start out with fio
149 writing an 8 gigabyte test file - so you want to disable that.
150
151 # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
152
153 To set it back to the default (4 mb), do
154
155 # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
156
157 - Traffic's still going to the spindle/still getting cache misses
158
159 In the real world, SSDs don't always keep up with disks - particularly with
160 slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
161 you want to avoid being bottlenecked by the SSD and having it slow everything
162 down.
163
164 To avoid that bcache tracks latency to the cache device, and gradually
165 throttles traffic if the latency exceeds a threshold (it does this by
166 cranking down the sequential bypass).
167
168 You can disable this if you need to by setting the thresholds to 0:
169
170 # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
171 # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
172
173 The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
174
175 - Still getting cache misses, of the same data
176
177 One last issue that sometimes trips people up is actually an old bug, due to
178 the way cache coherency is handled for cache misses. If a btree node is full,
179 a cache miss won't be able to insert a key for the new data and the data
180 won't be written to the cache.
181
182 In practice this isn't an issue because as soon as a write comes along it'll
183 cause the btree node to be split, and you need almost no write traffic for
Anatol Pomozovf884ab12013-05-08 16:56:16 -0700184 this to not show up enough to be noticeable (especially since bcache's btree
Kent Overstreet7b41b512013-03-27 12:24:17 -0700185 nodes are huge and index large regions of the device). But when you're
186 benchmarking, if you're trying to warm the cache by reading a bunch of data
187 and there's no other traffic - that can be a problem.
188
189 Solution: warm the cache by doing writes, or use the testing branch (there's
190 a fix for the issue there).
191
Kent Overstreetcafe5632013-03-23 16:11:31 -0700192SYSFS - BACKING DEVICE:
193
194attach
195 Echo the UUID of a cache set to this file to enable caching.
196
197cache_mode
198 Can be one of either writethrough, writeback, writearound or none.
199
200clear_stats
201 Writing to this file resets the running total stats (not the day/hour/5 minute
202 decaying versions).
203
204detach
205 Write to this file to detach from a cache set. If there is dirty data in the
206 cache, it will be flushed first.
207
208dirty_data
209 Amount of dirty data for this backing device in the cache. Continuously
210 updated unlike the cache set's version, but may be slightly off.
211
212label
213 Name of underlying device.
214
215readahead
216 Size of readahead that should be performed. Defaults to 0. If set to e.g.
217 1M, it will round cache miss reads up to that size, but without overlapping
218 existing cache entries.
219
220running
221 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
222 it's in passthrough mode or caching).
223
224sequential_cutoff
Anatol Pomozovf884ab12013-05-08 16:56:16 -0700225 A sequential IO will bypass the cache once it passes this threshold; the
Kent Overstreetcafe5632013-03-23 16:11:31 -0700226 most recent 128 IOs are tracked so sequential IO can be detected even when
227 it isn't all done at once.
228
229sequential_merge
230 If non zero, bcache keeps a list of the last 128 requests submitted to compare
231 against all new requests to determine which new requests are sequential
232 continuations of previous requests for the purpose of determining sequential
233 cutoff. This is necessary if the sequential cutoff value is greater than the
234 maximum acceptable sequential size for any single request.
235
236state
237 The backing device can be in one of four different states:
238
239 no cache: Has never been attached to a cache set.
240
241 clean: Part of a cache set, and there is no cached dirty data.
242
243 dirty: Part of a cache set, and there is cached dirty data.
244
245 inconsistent: The backing device was forcibly run by the user when there was
246 dirty data cached but the cache set was unavailable; whatever data was on the
247 backing device has likely been corrupted.
248
249stop
250 Write to this file to shut down the bcache device and close the backing
251 device.
252
253writeback_delay
254 When dirty data is written to the cache and it previously did not contain
255 any, waits some number of seconds before initiating writeback. Defaults to
256 30.
257
258writeback_percent
259 If nonzero, bcache tries to keep around this percentage of the cache dirty by
260 throttling background writeback and using a PD controller to smoothly adjust
261 the rate.
262
263writeback_rate
264 Rate in sectors per second - if writeback_percent is nonzero, background
265 writeback is throttled to this rate. Continuously adjusted by bcache but may
266 also be set by the user.
267
268writeback_running
269 If off, writeback of dirty data will not take place at all. Dirty data will
270 still be added to the cache until it is mostly full; only meant for
271 benchmarking. Defaults to on.
272
273SYSFS - BACKING DEVICE STATS:
274
275There are directories with these numbers for a running total, as well as
276versions that decay over the past day, hour and 5 minutes; they're also
277aggregated in the cache set directory as well.
278
279bypassed
280 Amount of IO (both reads and writes) that has bypassed the cache
281
282cache_hits
283cache_misses
284cache_hit_ratio
285 Hits and misses are counted per individual IO as bcache sees them; a
286 partial hit is counted as a miss.
287
288cache_bypass_hits
289cache_bypass_misses
290 Hits and misses for IO that is intended to skip the cache are still counted,
291 but broken out here.
292
293cache_miss_collisions
294 Counts instances where data was going to be inserted into the cache from a
295 cache miss, but raced with a write and data was already present (usually 0
296 since the synchronization for cache misses was rewritten)
297
298cache_readaheads
Anatol Pomozovf884ab12013-05-08 16:56:16 -0700299 Count of times readahead occurred.
Kent Overstreetcafe5632013-03-23 16:11:31 -0700300
301SYSFS - CACHE SET:
302
303average_key_size
304 Average data per key in the btree.
305
306bdev<0..n>
307 Symlink to each of the attached backing devices.
308
309block_size
310 Block size of the cache devices.
311
312btree_cache_size
313 Amount of memory currently used by the btree cache
314
315bucket_size
316 Size of buckets
317
318cache<0..n>
319 Symlink to each of the cache devices comprising this cache set.
320
321cache_available_percent
Gabrielfe0a7972013-04-24 19:51:02 +0200322 Percentage of cache device which doesn't contain dirty data, and could
323 potentially be used for writeback. This doesn't mean this space isn't used
324 for clean cached data; the unused statistic (in priority_stats) is typically
325 much lower.
Kent Overstreetcafe5632013-03-23 16:11:31 -0700326
327clear_stats
328 Clears the statistics associated with this cache
329
330dirty_data
331 Amount of dirty data is in the cache (updated when garbage collection runs).
332
333flash_vol_create
334 Echoing a size to this file (in human readable units, k/M/G) creates a thinly
335 provisioned volume backed by the cache set.
336
337io_error_halflife
338io_error_limit
339 These determines how many errors we accept before disabling the cache.
340 Each error is decayed by the half life (in # ios). If the decaying count
341 reaches io_error_limit dirty data is written out and the cache is disabled.
342
343journal_delay_ms
344 Journal writes will delay for up to this many milliseconds, unless a cache
345 flush happens sooner. Defaults to 100.
346
347root_usage_percent
348 Percentage of the root btree node in use. If this gets too high the node
349 will split, increasing the tree depth.
350
351stop
352 Write to this file to shut down the cache set - waits until all attached
353 backing devices have been shut down.
354
355tree_depth
356 Depth of the btree (A single node btree has depth 0).
357
358unregister
359 Detaches all backing devices and closes the cache devices; if dirty data is
360 present it will disable writeback caching and wait for it to be flushed.
361
362SYSFS - CACHE SET INTERNAL:
363
364This directory also exposes timings for a number of internal operations, with
Anatol Pomozovf884ab12013-05-08 16:56:16 -0700365separate files for average duration, average frequency, last occurrence and max
Kent Overstreetcafe5632013-03-23 16:11:31 -0700366duration: garbage collection, btree read, btree node sorts and btree splits.
367
368active_journal_entries
369 Number of journal entries that are newer than the index.
370
371btree_nodes
372 Total nodes in the btree.
373
374btree_used_percent
375 Average fraction of btree in use.
376
377bset_tree_stats
378 Statistics about the auxiliary search trees
379
380btree_cache_max_chain
381 Longest chain in the btree node cache's hash table
382
383cache_read_races
384 Counts instances where while data was being read from the cache, the bucket
385 was reused and invalidated - i.e. where the pointer was stale after the read
386 completed. When this occurs the data is reread from the backing device.
387
388trigger_gc
389 Writing to this file forces garbage collection to run.
390
391SYSFS - CACHE DEVICE:
392
393block_size
394 Minimum granularity of writes - should match hardware sector size.
395
396btree_written
397 Sum of all btree writes, in (kilo/mega/giga) bytes
398
399bucket_size
400 Size of buckets
401
402cache_replacement_policy
403 One of either lru, fifo or random.
404
405discard
406 Boolean; if on a discard/TRIM will be issued to each bucket before it is
407 reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
408 slow).
409
410freelist_percent
411 Size of the freelist as a percentage of nbuckets. Can be written to to
412 increase the number of buckets kept on the freelist, which lets you
413 artificially reduce the size of the cache at runtime. Mostly for testing
414 purposes (i.e. testing how different size caches affect your hit rate), but
415 since buckets are discarded when they move on to the freelist will also make
416 the SSD's garbage collection easier by effectively giving it more reserved
417 space.
418
419io_errors
Anatol Pomozovf884ab12013-05-08 16:56:16 -0700420 Number of errors that have occurred, decayed by io_error_halflife.
Kent Overstreetcafe5632013-03-23 16:11:31 -0700421
422metadata_written
423 Sum of all non data writes (btree writes and all other metadata).
424
425nbuckets
426 Total buckets in this cache
427
428priority_stats
Gabrielfe0a7972013-04-24 19:51:02 +0200429 Statistics about how recently data in the cache has been accessed.
430 This can reveal your working set size. Unused is the percentage of
431 the cache that doesn't contain any data. Metadata is bcache's
432 metadata overhead. Average is the average priority of cache buckets.
433 Next is a list of quantiles with the priority threshold of each.
Kent Overstreetcafe5632013-03-23 16:11:31 -0700434
435written
436 Sum of all data that has been written to the cache; comparison with
437 btree_written gives the amount of write inflation in bcache.