Root/Documentation/bcache.txt

Source at commit fbf123cd4cc0c097fe9a99c90109ebb2a5e94a50 created 10 years 3 months ago.
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1Say 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
49bcache-tools now ships udev rules, and bcache devices are known to the kernel
50immediately. Without udev, you can manually register devices like this:
51
52  echo /dev/sdb > /sys/fs/bcache/register
53  echo /dev/sdc > /sys/fs/bcache/register
54
55Registering the backing device makes the bcache device show up in /dev; you can
56now format it and use it as normal. But the first time using a new bcache
57device, it'll be running in passthrough mode until you attach it to a cache.
58See the section on attaching.
59
60The devices show up as:
61
62  /dev/bcache<N>
63
64As well as (with udev):
65
66  /dev/bcache/by-uuid/<uuid>
67  /dev/bcache/by-label/<label>
68
69To get started:
70
71  mkfs.ext4 /dev/bcache0
72  mount /dev/bcache0 /mnt
73
74You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
75
76Cache devices are managed as sets; multiple caches per set isn't supported yet
77but will allow for mirroring of metadata and dirty data in the future. Your new
78cache set shows up as /sys/fs/bcache/<UUID>
79
80ATTACHING:
81
82After your cache device and backing device are registered, the backing device
83must be attached to your cache set to enable caching. Attaching a backing
84device to a cache set is done thusly, with the UUID of the cache set in
85/sys/fs/bcache:
86
87  echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
88
89This only has to be done once. The next time you reboot, just reregister all
90your 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
92important if you have writeback caching turned on.
93
94If you're booting up and your cache device is gone and never coming back, you
95can force run the backing device:
96
97  echo 1 > /sys/block/sdb/bcache/running
98
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
101partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
102
103The backing device will still use that cache set if it shows up in the future,
104but all the cached data will be invalidated. If there was dirty data in the
105cache, don't expect the filesystem to be recoverable - you will have massive
106filesystem corruption, though ext4's fsck does work miracles.
107
108ERROR HANDLING:
109
110Bcache tries to transparently handle IO errors to/from the cache device without
111affecting normal operation; if it sees too many errors (the threshold is
112configurable, and defaults to 0) it shuts down the cache device and switches all
113the backing devices to passthrough mode.
114
115 - For reads from the cache, if they error we just retry the read from the
116   backing device.
117
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)
121
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.
125
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.
129
130TROUBLESHOOTING PERFORMANCE:
131
132Bcache has a bunch of config options and tunables. The defaults are intended to
133be reasonable for typical desktop and server workloads, but they're not what you
134want for getting the best possible numbers when benchmarking.
135
136 - Bad write performance
137
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
141   happens to your SSD)
142
143   # echo writeback > /sys/block/bcache0/cache_mode
144
145 - Bad performance, or traffic not going to the SSD that you'd expect
146
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.
151
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.
154
155   # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
156
157   To set it back to the default (4 mb), do
158
159   # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
160
161 - Traffic's still going to the spindle/still getting cache misses
162
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
166   down.
167
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).
171
172   You can disable this if you need to by setting the thresholds to 0:
173
174   # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
175   # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
176
177   The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
178
179 - Still getting cache misses, of the same data
180
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.
185
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.
192
193   Solution: warm the cache by doing writes, or use the testing branch (there's
194   a fix for the issue there).
195
196SYSFS - BACKING DEVICE:
197
198Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
199(if attached) /sys/fs/bcache/<cset-uuid>/bdev*
200
201attach
202  Echo the UUID of a cache set to this file to enable caching.
203
204cache_mode
205  Can be one of either writethrough, writeback, writearound or none.
206
207clear_stats
208  Writing to this file resets the running total stats (not the day/hour/5 minute
209  decaying versions).
210
211detach
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.
214
215dirty_data
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.
218
219label
220  Name of underlying device.
221
222readahead
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.
226
227running
228  1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
229  it's in passthrough mode or caching).
230
231sequential_cutoff
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.
235
236sequential_merge
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.
242
243state
244  The backing device can be in one of four different states:
245
246  no cache: Has never been attached to a cache set.
247
248  clean: Part of a cache set, and there is no cached dirty data.
249
250  dirty: Part of a cache set, and there is cached dirty data.
251
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.
255
256stop
257  Write to this file to shut down the bcache device and close the backing
258  device.
259
260writeback_delay
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
263  30.
264
265writeback_percent
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
268  the rate.
269
270writeback_rate
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.
274
275writeback_running
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.
279
280SYSFS - BACKING DEVICE STATS:
281
282There are directories with these numbers for a running total, as well as
283versions that decay over the past day, hour and 5 minutes; they're also
284aggregated in the cache set directory as well.
285
286bypassed
287  Amount of IO (both reads and writes) that has bypassed the cache
288
289cache_hits
290cache_misses
291cache_hit_ratio
292  Hits and misses are counted per individual IO as bcache sees them; a
293  partial hit is counted as a miss.
294
295cache_bypass_hits
296cache_bypass_misses
297  Hits and misses for IO that is intended to skip the cache are still counted,
298  but broken out here.
299
300cache_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)
304
305cache_readaheads
306  Count of times readahead occurred.
307
308SYSFS - CACHE SET:
309
310Available at /sys/fs/bcache/<cset-uuid>
311
312average_key_size
313  Average data per key in the btree.
314
315bdev<0..n>
316  Symlink to each of the attached backing devices.
317
318block_size
319  Block size of the cache devices.
320
321btree_cache_size
322  Amount of memory currently used by the btree cache
323
324bucket_size
325  Size of buckets
326
327cache<0..n>
328  Symlink to each of the cache devices comprising this cache set.
329
330cache_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
334  much lower.
335
336clear_stats
337  Clears the statistics associated with this cache
338
339dirty_data
340  Amount of dirty data is in the cache (updated when garbage collection runs).
341
342flash_vol_create
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.
345
346io_error_halflife
347io_error_limit
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.
351
352journal_delay_ms
353  Journal writes will delay for up to this many milliseconds, unless a cache
354  flush happens sooner. Defaults to 100.
355
356root_usage_percent
357  Percentage of the root btree node in use. If this gets too high the node
358  will split, increasing the tree depth.
359
360stop
361  Write to this file to shut down the cache set - waits until all attached
362  backing devices have been shut down.
363
364tree_depth
365  Depth of the btree (A single node btree has depth 0).
366
367unregister
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.
370
371SYSFS - CACHE SET INTERNAL:
372
373This directory also exposes timings for a number of internal operations, with
374separate files for average duration, average frequency, last occurrence and max
375duration: garbage collection, btree read, btree node sorts and btree splits.
376
377active_journal_entries
378  Number of journal entries that are newer than the index.
379
380btree_nodes
381  Total nodes in the btree.
382
383btree_used_percent
384  Average fraction of btree in use.
385
386bset_tree_stats
387  Statistics about the auxiliary search trees
388
389btree_cache_max_chain
390  Longest chain in the btree node cache's hash table
391
392cache_read_races
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.
396
397trigger_gc
398  Writing to this file forces garbage collection to run.
399
400SYSFS - CACHE DEVICE:
401
402Available at /sys/block/<cdev>/bcache
403
404block_size
405  Minimum granularity of writes - should match hardware sector size.
406
407btree_written
408  Sum of all btree writes, in (kilo/mega/giga) bytes
409
410bucket_size
411  Size of buckets
412
413cache_replacement_policy
414  One of either lru, fifo or random.
415
416discard
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
419  slow).
420
421freelist_percent
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
428  space.
429
430io_errors
431  Number of errors that have occurred, decayed by io_error_halflife.
432
433metadata_written
434  Sum of all non data writes (btree writes and all other metadata).
435
436nbuckets
437  Total buckets in this cache
438
439priority_stats
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.
445
446written
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.
449

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