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1 ============================
2 A block layer cache (bcache)
3 ============================
4
5 Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be
6 nice if you could use them as cache... Hence bcache.
7
8 Wiki and git repositories are at:
9
10 - http://bcache.evilpiepirate.org
11 - http://evilpiepirate.org/git/linux-bcache.git
12 - http://evilpiepirate.org/git/bcache-tools.git
13
14 It's designed around the performance characteristics of SSDs - it only allocates
15 in erase block sized buckets, and it uses a hybrid btree/log to track cached
16 extents (which can be anywhere from a single sector to the bucket size). It's
17 designed to avoid random writes at all costs; it fills up an erase block
18 sequentially, then issues a discard before reusing it.
19
20 Both writethrough and writeback caching are supported. Writeback defaults to
21 off, but can be switched on and off arbitrarily at runtime. Bcache goes to
22 great lengths to protect your data - it reliably handles unclean shutdown. (It
23 doesn't even have a notion of a clean shutdown; bcache simply doesn't return
24 writes as completed until they're on stable storage).
25
26 Writeback caching can use most of the cache for buffering writes - writing
27 dirty data to the backing device is always done sequentially, scanning from the
28 start to the end of the index.
29
30 Since random IO is what SSDs excel at, there generally won't be much benefit
31 to caching large sequential IO. Bcache detects sequential IO and skips it;
32 it also keeps a rolling average of the IO sizes per task, and as long as the
33 average is above the cutoff it will skip all IO from that task - instead of
34 caching the first 512k after every seek. Backups and large file copies should
35 thus entirely bypass the cache.
36
37 In the event of a data IO error on the flash it will try to recover by reading
38 from disk or invalidating cache entries. For unrecoverable errors (meta data
39 or dirty data), caching is automatically disabled; if dirty data was present
40 in the cache it first disables writeback caching and waits for all dirty data
41 to be flushed.
42
43 Getting started:
44 You'll need make-bcache from the bcache-tools repository. Both the cache device
45 and backing device must be formatted before use::
46
47 make-bcache -B /dev/sdb
48 make-bcache -C /dev/sdc
49
50 make-bcache has the ability to format multiple devices at the same time - if
51 you format your backing devices and cache device at the same time, you won't
52 have to manually attach::
53
54 make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
55
56 bcache-tools now ships udev rules, and bcache devices are known to the kernel
57 immediately. Without udev, you can manually register devices like this::
58
59 echo /dev/sdb > /sys/fs/bcache/register
60 echo /dev/sdc > /sys/fs/bcache/register
61
62 Registering the backing device makes the bcache device show up in /dev; you can
63 now format it and use it as normal. But the first time using a new bcache
64 device, it'll be running in passthrough mode until you attach it to a cache.
65 If you are thinking about using bcache later, it is recommended to setup all your
66 slow devices as bcache backing devices without a cache, and you can choose to add
67 a caching device later.
68 See 'ATTACHING' section below.
69
70 The devices show up as::
71
72 /dev/bcache<N>
73
74 As well as (with udev)::
75
76 /dev/bcache/by-uuid/<uuid>
77 /dev/bcache/by-label/<label>
78
79 To get started::
80
81 mkfs.ext4 /dev/bcache0
82 mount /dev/bcache0 /mnt
83
84 You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
85 You can also control them through /sys/fs//bcache/<cset-uuid>/ .
86
87 Cache devices are managed as sets; multiple caches per set isn't supported yet
88 but will allow for mirroring of metadata and dirty data in the future. Your new
89 cache set shows up as /sys/fs/bcache/<UUID>
90
91 Attaching
92 ---------
93
94 After your cache device and backing device are registered, the backing device
95 must be attached to your cache set to enable caching. Attaching a backing
96 device to a cache set is done thusly, with the UUID of the cache set in
97 /sys/fs/bcache::
98
99 echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
100
101 This only has to be done once. The next time you reboot, just reregister all
102 your bcache devices. If a backing device has data in a cache somewhere, the
103 /dev/bcache<N> device won't be created until the cache shows up - particularly
104 important if you have writeback caching turned on.
105
106 If you're booting up and your cache device is gone and never coming back, you
107 can force run the backing device::
108
109 echo 1 > /sys/block/sdb/bcache/running
110
111 (You need to use /sys/block/sdb (or whatever your backing device is called), not
112 /sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
113 partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
114
115 The backing device will still use that cache set if it shows up in the future,
116 but all the cached data will be invalidated. If there was dirty data in the
117 cache, don't expect the filesystem to be recoverable - you will have massive
118 filesystem corruption, though ext4's fsck does work miracles.
119
120 Error Handling
121 --------------
122
123 Bcache tries to transparently handle IO errors to/from the cache device without
124 affecting normal operation; if it sees too many errors (the threshold is
125 configurable, and defaults to 0) it shuts down the cache device and switches all
126 the backing devices to passthrough mode.
127
128 - For reads from the cache, if they error we just retry the read from the
129 backing device.
130
131 - For writethrough writes, if the write to the cache errors we just switch to
132 invalidating the data at that lba in the cache (i.e. the same thing we do for
133 a write that bypasses the cache)
134
135 - For writeback writes, we currently pass that error back up to the
136 filesystem/userspace. This could be improved - we could retry it as a write
137 that skips the cache so we don't have to error the write.
138
139 - When we detach, we first try to flush any dirty data (if we were running in
140 writeback mode). It currently doesn't do anything intelligent if it fails to
141 read some of the dirty data, though.
142
143
144 Howto/cookbook
145 --------------
146
147 A) Starting a bcache with a missing caching device
148
149 If registering the backing device doesn't help, it's already there, you just need
150 to force it to run without the cache::
151
152 host:~# echo /dev/sdb1 > /sys/fs/bcache/register
153 [ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
154
155 Next, you try to register your caching device if it's present. However
156 if it's absent, or registration fails for some reason, you can still
157 start your bcache without its cache, like so::
158
159 host:/sys/block/sdb/sdb1/bcache# echo 1 > running
160
161 Note that this may cause data loss if you were running in writeback mode.
162
163
164 B) Bcache does not find its cache::
165
166 host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
167 [ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
168 [ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
169 [ 1933.478179] : cache set not found
170
171 In this case, the caching device was simply not registered at boot
172 or disappeared and came back, and needs to be (re-)registered::
173
174 host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
175
176
177 C) Corrupt bcache crashes the kernel at device registration time:
178
179 This should never happen. If it does happen, then you have found a bug!
180 Please report it to the bcache development list: linux-bcache@vger.kernel.org
181
182 Be sure to provide as much information that you can including kernel dmesg
183 output if available so that we may assist.
184
185
186 D) Recovering data without bcache:
187
188 If bcache is not available in the kernel, a filesystem on the backing
189 device is still available at an 8KiB offset. So either via a loopdev
190 of the backing device created with --offset 8K, or any value defined by
191 --data-offset when you originally formatted bcache with `make-bcache`.
192
193 For example::
194
195 losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
196
197 This should present your unmodified backing device data in /dev/loop0
198
199 If your cache is in writethrough mode, then you can safely discard the
200 cache device without loosing data.
201
202
203 E) Wiping a cache device
204
205 ::
206
207 host:~# wipefs -a /dev/sdh2
208 16 bytes were erased at offset 0x1018 (bcache)
209 they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
210
211 After you boot back with bcache enabled, you recreate the cache and attach it::
212
213 host:~# make-bcache -C /dev/sdh2
214 UUID: 7be7e175-8f4c-4f99-94b2-9c904d227045
215 Set UUID: 5bc072a8-ab17-446d-9744-e247949913c1
216 version: 0
217 nbuckets: 106874
218 block_size: 1
219 bucket_size: 1024
220 nr_in_set: 1
221 nr_this_dev: 0
222 first_bucket: 1
223 [ 650.511912] bcache: run_cache_set() invalidating existing data
224 [ 650.549228] bcache: register_cache() registered cache device sdh2
225
226 start backing device with missing cache::
227
228 host:/sys/block/md5/bcache# echo 1 > running
229
230 attach new cache::
231
232 host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach
233 [ 865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1
234
235
236 F) Remove or replace a caching device::
237
238 host:/sys/block/sda/sda7/bcache# echo 1 > detach
239 [ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
240
241 host:~# wipefs -a /dev/nvme0n1p4
242 wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
243 Ooops, it's disabled, but not unregistered, so it's still protected
244
245 We need to go and unregister it::
246
247 host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
248 lrwxrwxrwx 1 root root 0 Feb 25 18:33 cache0 -> ../../../devices/pci0000:00/0000:00:1d.0/0000:70:00.0/nvme/nvme0/nvme0n1/nvme0n1p4/bcache/
249 host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
250 kernel: [ 917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
251
252 Now we can wipe it::
253
254 host:~# wipefs -a /dev/nvme0n1p4
255 /dev/nvme0n1p4: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
256
257
258 G) dm-crypt and bcache
259
260 First setup bcache unencrypted and then install dmcrypt on top of
261 /dev/bcache<N> This will work faster than if you dmcrypt both the backing
262 and caching devices and then install bcache on top. [benchmarks?]
263
264
265 H) Stop/free a registered bcache to wipe and/or recreate it
266
267 Suppose that you need to free up all bcache references so that you can
268 fdisk run and re-register a changed partition table, which won't work
269 if there are any active backing or caching devices left on it:
270
271 1) Is it present in /dev/bcache* ? (there are times where it won't be)
272
273 If so, it's easy::
274
275 host:/sys/block/bcache0/bcache# echo 1 > stop
276
277 2) But if your backing device is gone, this won't work::
278
279 host:/sys/block/bcache0# cd bcache
280 bash: cd: bcache: No such file or directory
281
282 In this case, you may have to unregister the dmcrypt block device that
283 references this bcache to free it up::
284
285 host:~# dmsetup remove oldds1
286 bcache: bcache_device_free() bcache0 stopped
287 bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
288
289 This causes the backing bcache to be removed from /sys/fs/bcache and
290 then it can be reused. This would be true of any block device stacking
291 where bcache is a lower device.
292
293 3) In other cases, you can also look in /sys/fs/bcache/::
294
295 host:/sys/fs/bcache# ls -l */{cache?,bdev?}
296 lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/
297 lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/
298 lrwxrwxrwx 1 root root 0 Mar 5 09:39 5bc072a8-ab17-446d-9744-e247949913c1/cache0 -> ../../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/ata10/host9/target9:0:0/9:0:0:0/block/sdl/sdl2/bcache/
299
300 The device names will show which UUID is relevant, cd in that directory
301 and stop the cache::
302
303 host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
304
305 This will free up bcache references and let you reuse the partition for
306 other purposes.
307
308
309
310 Troubleshooting performance
311 ---------------------------
312
313 Bcache has a bunch of config options and tunables. The defaults are intended to
314 be reasonable for typical desktop and server workloads, but they're not what you
315 want for getting the best possible numbers when benchmarking.
316
317 - Backing device alignment
318
319 The default metadata size in bcache is 8k. If your backing device is
320 RAID based, then be sure to align this by a multiple of your stride
321 width using `make-bcache --data-offset`. If you intend to expand your
322 disk array in the future, then multiply a series of primes by your
323 raid stripe size to get the disk multiples that you would like.
324
325 For example: If you have a 64k stripe size, then the following offset
326 would provide alignment for many common RAID5 data spindle counts::
327
328 64k * 2*2*2*3*3*5*7 bytes = 161280k
329
330 That space is wasted, but for only 157.5MB you can grow your RAID 5
331 volume to the following data-spindle counts without re-aligning::
332
333 3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...
334
335 - Bad write performance
336
337 If write performance is not what you expected, you probably wanted to be
338 running in writeback mode, which isn't the default (not due to a lack of
339 maturity, but simply because in writeback mode you'll lose data if something
340 happens to your SSD)::
341
342 # echo writeback > /sys/block/bcache0/bcache/cache_mode
343
344 - Bad performance, or traffic not going to the SSD that you'd expect
345
346 By default, bcache doesn't cache everything. It tries to skip sequential IO -
347 because you really want to be caching the random IO, and if you copy a 10
348 gigabyte file you probably don't want that pushing 10 gigabytes of randomly
349 accessed data out of your cache.
350
351 But if you want to benchmark reads from cache, and you start out with fio
352 writing an 8 gigabyte test file - so you want to disable that::
353
354 # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
355
356 To set it back to the default (4 mb), do::
357
358 # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
359
360 - Traffic's still going to the spindle/still getting cache misses
361
362 In the real world, SSDs don't always keep up with disks - particularly with
363 slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
364 you want to avoid being bottlenecked by the SSD and having it slow everything
365 down.
366
367 To avoid that bcache tracks latency to the cache device, and gradually
368 throttles traffic if the latency exceeds a threshold (it does this by
369 cranking down the sequential bypass).
370
371 You can disable this if you need to by setting the thresholds to 0::
372
373 # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
374 # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
375
376 The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
377
378 - Still getting cache misses, of the same data
379
380 One last issue that sometimes trips people up is actually an old bug, due to
381 the way cache coherency is handled for cache misses. If a btree node is full,
382 a cache miss won't be able to insert a key for the new data and the data
383 won't be written to the cache.
384
385 In practice this isn't an issue because as soon as a write comes along it'll
386 cause the btree node to be split, and you need almost no write traffic for
387 this to not show up enough to be noticeable (especially since bcache's btree
388 nodes are huge and index large regions of the device). But when you're
389 benchmarking, if you're trying to warm the cache by reading a bunch of data
390 and there's no other traffic - that can be a problem.
391
392 Solution: warm the cache by doing writes, or use the testing branch (there's
393 a fix for the issue there).
394
395
396 Sysfs - backing device
397 ----------------------
398
399 Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
400 (if attached) /sys/fs/bcache/<cset-uuid>/bdev*
401
402 attach
403 Echo the UUID of a cache set to this file to enable caching.
404
405 cache_mode
406 Can be one of either writethrough, writeback, writearound or none.
407
408 clear_stats
409 Writing to this file resets the running total stats (not the day/hour/5 minute
410 decaying versions).
411
412 detach
413 Write to this file to detach from a cache set. If there is dirty data in the
414 cache, it will be flushed first.
415
416 dirty_data
417 Amount of dirty data for this backing device in the cache. Continuously
418 updated unlike the cache set's version, but may be slightly off.
419
420 label
421 Name of underlying device.
422
423 readahead
424 Size of readahead that should be performed. Defaults to 0. If set to e.g.
425 1M, it will round cache miss reads up to that size, but without overlapping
426 existing cache entries.
427
428 running
429 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
430 it's in passthrough mode or caching).
431
432 sequential_cutoff
433 A sequential IO will bypass the cache once it passes this threshold; the
434 most recent 128 IOs are tracked so sequential IO can be detected even when
435 it isn't all done at once.
436
437 sequential_merge
438 If non zero, bcache keeps a list of the last 128 requests submitted to compare
439 against all new requests to determine which new requests are sequential
440 continuations of previous requests for the purpose of determining sequential
441 cutoff. This is necessary if the sequential cutoff value is greater than the
442 maximum acceptable sequential size for any single request.
443
444 state
445 The backing device can be in one of four different states:
446
447 no cache: Has never been attached to a cache set.
448
449 clean: Part of a cache set, and there is no cached dirty data.
450
451 dirty: Part of a cache set, and there is cached dirty data.
452
453 inconsistent: The backing device was forcibly run by the user when there was
454 dirty data cached but the cache set was unavailable; whatever data was on the
455 backing device has likely been corrupted.
456
457 stop
458 Write to this file to shut down the bcache device and close the backing
459 device.
460
461 writeback_delay
462 When dirty data is written to the cache and it previously did not contain
463 any, waits some number of seconds before initiating writeback. Defaults to
464 30.
465
466 writeback_percent
467 If nonzero, bcache tries to keep around this percentage of the cache dirty by
468 throttling background writeback and using a PD controller to smoothly adjust
469 the rate.
470
471 writeback_rate
472 Rate in sectors per second - if writeback_percent is nonzero, background
473 writeback is throttled to this rate. Continuously adjusted by bcache but may
474 also be set by the user.
475
476 writeback_running
477 If off, writeback of dirty data will not take place at all. Dirty data will
478 still be added to the cache until it is mostly full; only meant for
479 benchmarking. Defaults to on.
480
481 Sysfs - backing device stats
482 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
483
484 There are directories with these numbers for a running total, as well as
485 versions that decay over the past day, hour and 5 minutes; they're also
486 aggregated in the cache set directory as well.
487
488 bypassed
489 Amount of IO (both reads and writes) that has bypassed the cache
490
491 cache_hits, cache_misses, cache_hit_ratio
492 Hits and misses are counted per individual IO as bcache sees them; a
493 partial hit is counted as a miss.
494
495 cache_bypass_hits, cache_bypass_misses
496 Hits and misses for IO that is intended to skip the cache are still counted,
497 but broken out here.
498
499 cache_miss_collisions
500 Counts instances where data was going to be inserted into the cache from a
501 cache miss, but raced with a write and data was already present (usually 0
502 since the synchronization for cache misses was rewritten)
503
504 cache_readaheads
505 Count of times readahead occurred.
506
507 Sysfs - cache set
508 ~~~~~~~~~~~~~~~~~
509
510 Available at /sys/fs/bcache/<cset-uuid>
511
512 average_key_size
513 Average data per key in the btree.
514
515 bdev<0..n>
516 Symlink to each of the attached backing devices.
517
518 block_size
519 Block size of the cache devices.
520
521 btree_cache_size
522 Amount of memory currently used by the btree cache
523
524 bucket_size
525 Size of buckets
526
527 cache<0..n>
528 Symlink to each of the cache devices comprising this cache set.
529
530 cache_available_percent
531 Percentage of cache device which doesn't contain dirty data, and could
532 potentially be used for writeback. This doesn't mean this space isn't used
533 for clean cached data; the unused statistic (in priority_stats) is typically
534 much lower.
535
536 clear_stats
537 Clears the statistics associated with this cache
538
539 dirty_data
540 Amount of dirty data is in the cache (updated when garbage collection runs).
541
542 flash_vol_create
543 Echoing a size to this file (in human readable units, k/M/G) creates a thinly
544 provisioned volume backed by the cache set.
545
546 io_error_halflife, io_error_limit
547 These determines how many errors we accept before disabling the cache.
548 Each error is decayed by the half life (in # ios). If the decaying count
549 reaches io_error_limit dirty data is written out and the cache is disabled.
550
551 journal_delay_ms
552 Journal writes will delay for up to this many milliseconds, unless a cache
553 flush happens sooner. Defaults to 100.
554
555 root_usage_percent
556 Percentage of the root btree node in use. If this gets too high the node
557 will split, increasing the tree depth.
558
559 stop
560 Write to this file to shut down the cache set - waits until all attached
561 backing devices have been shut down.
562
563 tree_depth
564 Depth of the btree (A single node btree has depth 0).
565
566 unregister
567 Detaches all backing devices and closes the cache devices; if dirty data is
568 present it will disable writeback caching and wait for it to be flushed.
569
570 Sysfs - cache set internal
571 ~~~~~~~~~~~~~~~~~~~~~~~~~~
572
573 This directory also exposes timings for a number of internal operations, with
574 separate files for average duration, average frequency, last occurrence and max
575 duration: garbage collection, btree read, btree node sorts and btree splits.
576
577 active_journal_entries
578 Number of journal entries that are newer than the index.
579
580 btree_nodes
581 Total nodes in the btree.
582
583 btree_used_percent
584 Average fraction of btree in use.
585
586 bset_tree_stats
587 Statistics about the auxiliary search trees
588
589 btree_cache_max_chain
590 Longest chain in the btree node cache's hash table
591
592 cache_read_races
593 Counts instances where while data was being read from the cache, the bucket
594 was reused and invalidated - i.e. where the pointer was stale after the read
595 completed. When this occurs the data is reread from the backing device.
596
597 trigger_gc
598 Writing to this file forces garbage collection to run.
599
600 Sysfs - Cache device
601 ~~~~~~~~~~~~~~~~~~~~
602
603 Available at /sys/block/<cdev>/bcache
604
605 block_size
606 Minimum granularity of writes - should match hardware sector size.
607
608 btree_written
609 Sum of all btree writes, in (kilo/mega/giga) bytes
610
611 bucket_size
612 Size of buckets
613
614 cache_replacement_policy
615 One of either lru, fifo or random.
616
617 discard
618 Boolean; if on a discard/TRIM will be issued to each bucket before it is
619 reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
620 slow).
621
622 freelist_percent
623 Size of the freelist as a percentage of nbuckets. Can be written to to
624 increase the number of buckets kept on the freelist, which lets you
625 artificially reduce the size of the cache at runtime. Mostly for testing
626 purposes (i.e. testing how different size caches affect your hit rate), but
627 since buckets are discarded when they move on to the freelist will also make
628 the SSD's garbage collection easier by effectively giving it more reserved
629 space.
630
631 io_errors
632 Number of errors that have occurred, decayed by io_error_halflife.
633
634 metadata_written
635 Sum of all non data writes (btree writes and all other metadata).
636
637 nbuckets
638 Total buckets in this cache
639
640 priority_stats
641 Statistics about how recently data in the cache has been accessed.
642 This can reveal your working set size. Unused is the percentage of
643 the cache that doesn't contain any data. Metadata is bcache's
644 metadata overhead. Average is the average priority of cache buckets.
645 Next is a list of quantiles with the priority threshold of each.
646
647 written
648 Sum of all data that has been written to the cache; comparison with
649 btree_written gives the amount of write inflation in bcache.