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 The bcache wiki can be found at:
9 https://bcache.evilpiepirate.org
10
11 This is the git repository of bcache-tools:
12 https://git.kernel.org/pub/scm/linux/kernel/git/colyli/bcache-tools.git/
13
14 The latest bcache kernel code can be found from mainline Linux kernel:
15 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
16
17 It's designed around the performance characteristics of SSDs - it only allocates
18 in erase block sized buckets, and it uses a hybrid btree/log to track cached
19 extents (which can be anywhere from a single sector to the bucket size). It's
20 designed to avoid random writes at all costs; it fills up an erase block
21 sequentially, then issues a discard before reusing it.
22
23 Both writethrough and writeback caching are supported. Writeback defaults to
24 off, but can be switched on and off arbitrarily at runtime. Bcache goes to
25 great lengths to protect your data - it reliably handles unclean shutdown. (It
26 doesn't even have a notion of a clean shutdown; bcache simply doesn't return
27 writes as completed until they're on stable storage).
28
29 Writeback caching can use most of the cache for buffering writes - writing
30 dirty data to the backing device is always done sequentially, scanning from the
31 start to the end of the index.
32
33 Since random IO is what SSDs excel at, there generally won't be much benefit
34 to caching large sequential IO. Bcache detects sequential IO and skips it;
35 it also keeps a rolling average of the IO sizes per task, and as long as the
36 average is above the cutoff it will skip all IO from that task - instead of
37 caching the first 512k after every seek. Backups and large file copies should
38 thus entirely bypass the cache.
39
40 In the event of a data IO error on the flash it will try to recover by reading
41 from disk or invalidating cache entries. For unrecoverable errors (meta data
42 or dirty data), caching is automatically disabled; if dirty data was present
43 in the cache it first disables writeback caching and waits for all dirty data
44 to be flushed.
45
46 Getting started:
47 You'll need bcache util from the bcache-tools repository. Both the cache device
48 and backing device must be formatted before use::
49
50 bcache make -B /dev/sdb
51 bcache make -C /dev/sdc
52
53 `bcache make` has the ability to format multiple devices at the same time - if
54 you format your backing devices and cache device at the same time, you won't
55 have to manually attach::
56
57 bcache make -B /dev/sda /dev/sdb -C /dev/sdc
58
59 If your bcache-tools is not updated to latest version and does not have the
60 unified `bcache` utility, you may use the legacy `make-bcache` utility to format
61 bcache device with same -B and -C parameters.
62
63 bcache-tools now ships udev rules, and bcache devices are known to the kernel
64 immediately. Without udev, you can manually register devices like this::
65
66 echo /dev/sdb > /sys/fs/bcache/register
67 echo /dev/sdc > /sys/fs/bcache/register
68
69 Registering the backing device makes the bcache device show up in /dev; you can
70 now format it and use it as normal. But the first time using a new bcache
71 device, it'll be running in passthrough mode until you attach it to a cache.
72 If you are thinking about using bcache later, it is recommended to setup all your
73 slow devices as bcache backing devices without a cache, and you can choose to add
74 a caching device later.
75 See 'ATTACHING' section below.
76
77 The devices show up as::
78
79 /dev/bcache<N>
80
81 As well as (with udev)::
82
83 /dev/bcache/by-uuid/<uuid>
84 /dev/bcache/by-label/<label>
85
86 To get started::
87
88 mkfs.ext4 /dev/bcache0
89 mount /dev/bcache0 /mnt
90
91 You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
92 You can also control them through /sys/fs//bcache/<cset-uuid>/ .
93
94 Cache devices are managed as sets; multiple caches per set isn't supported yet
95 but will allow for mirroring of metadata and dirty data in the future. Your new
96 cache set shows up as /sys/fs/bcache/<UUID>
97
98 Attaching
99 ---------
100
101 After your cache device and backing device are registered, the backing device
102 must be attached to your cache set to enable caching. Attaching a backing
103 device to a cache set is done thusly, with the UUID of the cache set in
104 /sys/fs/bcache::
105
106 echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
107
108 This only has to be done once. The next time you reboot, just reregister all
109 your bcache devices. If a backing device has data in a cache somewhere, the
110 /dev/bcache<N> device won't be created until the cache shows up - particularly
111 important if you have writeback caching turned on.
112
113 If you're booting up and your cache device is gone and never coming back, you
114 can force run the backing device::
115
116 echo 1 > /sys/block/sdb/bcache/running
117
118 (You need to use /sys/block/sdb (or whatever your backing device is called), not
119 /sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
120 partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
121
122 The backing device will still use that cache set if it shows up in the future,
123 but all the cached data will be invalidated. If there was dirty data in the
124 cache, don't expect the filesystem to be recoverable - you will have massive
125 filesystem corruption, though ext4's fsck does work miracles.
126
127 Error Handling
128 --------------
129
130 Bcache tries to transparently handle IO errors to/from the cache device without
131 affecting normal operation; if it sees too many errors (the threshold is
132 configurable, and defaults to 0) it shuts down the cache device and switches all
133 the backing devices to passthrough mode.
134
135 - For reads from the cache, if they error we just retry the read from the
136 backing device.
137
138 - For writethrough writes, if the write to the cache errors we just switch to
139 invalidating the data at that lba in the cache (i.e. the same thing we do for
140 a write that bypasses the cache)
141
142 - For writeback writes, we currently pass that error back up to the
143 filesystem/userspace. This could be improved - we could retry it as a write
144 that skips the cache so we don't have to error the write.
145
146 - When we detach, we first try to flush any dirty data (if we were running in
147 writeback mode). It currently doesn't do anything intelligent if it fails to
148 read some of the dirty data, though.
149
150
151 Howto/cookbook
152 --------------
153
154 A) Starting a bcache with a missing caching device
155
156 If registering the backing device doesn't help, it's already there, you just need
157 to force it to run without the cache::
158
159 host:~# echo /dev/sdb1 > /sys/fs/bcache/register
160 [ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
161
162 Next, you try to register your caching device if it's present. However
163 if it's absent, or registration fails for some reason, you can still
164 start your bcache without its cache, like so::
165
166 host:/sys/block/sdb/sdb1/bcache# echo 1 > running
167
168 Note that this may cause data loss if you were running in writeback mode.
169
170
171 B) Bcache does not find its cache::
172
173 host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
174 [ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
175 [ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
176 [ 1933.478179] : cache set not found
177
178 In this case, the caching device was simply not registered at boot
179 or disappeared and came back, and needs to be (re-)registered::
180
181 host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
182
183
184 C) Corrupt bcache crashes the kernel at device registration time:
185
186 This should never happen. If it does happen, then you have found a bug!
187 Please report it to the bcache development list: linux-bcache@vger.kernel.org
188
189 Be sure to provide as much information that you can including kernel dmesg
190 output if available so that we may assist.
191
192
193 D) Recovering data without bcache:
194
195 If bcache is not available in the kernel, a filesystem on the backing
196 device is still available at an 8KiB offset. So either via a loopdev
197 of the backing device created with --offset 8K, or any value defined by
198 --data-offset when you originally formatted bcache with `bcache make`.
199
200 For example::
201
202 losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
203
204 This should present your unmodified backing device data in /dev/loop0
205
206 If your cache is in writethrough mode, then you can safely discard the
207 cache device without losing data.
208
209
210 E) Wiping a cache device
211
212 ::
213
214 host:~# wipefs -a /dev/sdh2
215 16 bytes were erased at offset 0x1018 (bcache)
216 they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
217
218 After you boot back with bcache enabled, you recreate the cache and attach it::
219
220 host:~# bcache make -C /dev/sdh2
221 UUID: 7be7e175-8f4c-4f99-94b2-9c904d227045
222 Set UUID: 5bc072a8-ab17-446d-9744-e247949913c1
223 version: 0
224 nbuckets: 106874
225 block_size: 1
226 bucket_size: 1024
227 nr_in_set: 1
228 nr_this_dev: 0
229 first_bucket: 1
230 [ 650.511912] bcache: run_cache_set() invalidating existing data
231 [ 650.549228] bcache: register_cache() registered cache device sdh2
232
233 start backing device with missing cache::
234
235 host:/sys/block/md5/bcache# echo 1 > running
236
237 attach new cache::
238
239 host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach
240 [ 865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1
241
242
243 F) Remove or replace a caching device::
244
245 host:/sys/block/sda/sda7/bcache# echo 1 > detach
246 [ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
247
248 host:~# wipefs -a /dev/nvme0n1p4
249 wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
250 Ooops, it's disabled, but not unregistered, so it's still protected
251
252 We need to go and unregister it::
253
254 host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
255 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/
256 host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
257 kernel: [ 917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
258
259 Now we can wipe it::
260
261 host:~# wipefs -a /dev/nvme0n1p4
262 /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
263
264
265 G) dm-crypt and bcache
266
267 First setup bcache unencrypted and then install dmcrypt on top of
268 /dev/bcache<N> This will work faster than if you dmcrypt both the backing
269 and caching devices and then install bcache on top. [benchmarks?]
270
271
272 H) Stop/free a registered bcache to wipe and/or recreate it
273
274 Suppose that you need to free up all bcache references so that you can
275 fdisk run and re-register a changed partition table, which won't work
276 if there are any active backing or caching devices left on it:
277
278 1) Is it present in /dev/bcache* ? (there are times where it won't be)
279
280 If so, it's easy::
281
282 host:/sys/block/bcache0/bcache# echo 1 > stop
283
284 2) But if your backing device is gone, this won't work::
285
286 host:/sys/block/bcache0# cd bcache
287 bash: cd: bcache: No such file or directory
288
289 In this case, you may have to unregister the dmcrypt block device that
290 references this bcache to free it up::
291
292 host:~# dmsetup remove oldds1
293 bcache: bcache_device_free() bcache0 stopped
294 bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
295
296 This causes the backing bcache to be removed from /sys/fs/bcache and
297 then it can be reused. This would be true of any block device stacking
298 where bcache is a lower device.
299
300 3) In other cases, you can also look in /sys/fs/bcache/::
301
302 host:/sys/fs/bcache# ls -l */{cache?,bdev?}
303 lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/
304 lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/
305 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/
306
307 The device names will show which UUID is relevant, cd in that directory
308 and stop the cache::
309
310 host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
311
312 This will free up bcache references and let you reuse the partition for
313 other purposes.
314
315
316
317 Troubleshooting performance
318 ---------------------------
319
320 Bcache has a bunch of config options and tunables. The defaults are intended to
321 be reasonable for typical desktop and server workloads, but they're not what you
322 want for getting the best possible numbers when benchmarking.
323
324 - Backing device alignment
325
326 The default metadata size in bcache is 8k. If your backing device is
327 RAID based, then be sure to align this by a multiple of your stride
328 width using `bcache make --data-offset`. If you intend to expand your
329 disk array in the future, then multiply a series of primes by your
330 raid stripe size to get the disk multiples that you would like.
331
332 For example: If you have a 64k stripe size, then the following offset
333 would provide alignment for many common RAID5 data spindle counts::
334
335 64k * 2*2*2*3*3*5*7 bytes = 161280k
336
337 That space is wasted, but for only 157.5MB you can grow your RAID 5
338 volume to the following data-spindle counts without re-aligning::
339
340 3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...
341
342 - Bad write performance
343
344 If write performance is not what you expected, you probably wanted to be
345 running in writeback mode, which isn't the default (not due to a lack of
346 maturity, but simply because in writeback mode you'll lose data if something
347 happens to your SSD)::
348
349 # echo writeback > /sys/block/bcache0/bcache/cache_mode
350
351 - Bad performance, or traffic not going to the SSD that you'd expect
352
353 By default, bcache doesn't cache everything. It tries to skip sequential IO -
354 because you really want to be caching the random IO, and if you copy a 10
355 gigabyte file you probably don't want that pushing 10 gigabytes of randomly
356 accessed data out of your cache.
357
358 But if you want to benchmark reads from cache, and you start out with fio
359 writing an 8 gigabyte test file - so you want to disable that::
360
361 # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
362
363 To set it back to the default (4 mb), do::
364
365 # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
366
367 - Traffic's still going to the spindle/still getting cache misses
368
369 In the real world, SSDs don't always keep up with disks - particularly with
370 slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
371 you want to avoid being bottlenecked by the SSD and having it slow everything
372 down.
373
374 To avoid that bcache tracks latency to the cache device, and gradually
375 throttles traffic if the latency exceeds a threshold (it does this by
376 cranking down the sequential bypass).
377
378 You can disable this if you need to by setting the thresholds to 0::
379
380 # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
381 # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
382
383 The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
384
385 - Still getting cache misses, of the same data
386
387 One last issue that sometimes trips people up is actually an old bug, due to
388 the way cache coherency is handled for cache misses. If a btree node is full,
389 a cache miss won't be able to insert a key for the new data and the data
390 won't be written to the cache.
391
392 In practice this isn't an issue because as soon as a write comes along it'll
393 cause the btree node to be split, and you need almost no write traffic for
394 this to not show up enough to be noticeable (especially since bcache's btree
395 nodes are huge and index large regions of the device). But when you're
396 benchmarking, if you're trying to warm the cache by reading a bunch of data
397 and there's no other traffic - that can be a problem.
398
399 Solution: warm the cache by doing writes, or use the testing branch (there's
400 a fix for the issue there).
401
402
403 Sysfs - backing device
404 ----------------------
405
406 Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
407 (if attached) /sys/fs/bcache/<cset-uuid>/bdev*
408
409 attach
410 Echo the UUID of a cache set to this file to enable caching.
411
412 cache_mode
413 Can be one of either writethrough, writeback, writearound or none.
414
415 clear_stats
416 Writing to this file resets the running total stats (not the day/hour/5 minute
417 decaying versions).
418
419 detach
420 Write to this file to detach from a cache set. If there is dirty data in the
421 cache, it will be flushed first.
422
423 dirty_data
424 Amount of dirty data for this backing device in the cache. Continuously
425 updated unlike the cache set's version, but may be slightly off.
426
427 label
428 Name of underlying device.
429
430 readahead
431 Size of readahead that should be performed. Defaults to 0. If set to e.g.
432 1M, it will round cache miss reads up to that size, but without overlapping
433 existing cache entries.
434
435 running
436 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
437 it's in passthrough mode or caching).
438
439 sequential_cutoff
440 A sequential IO will bypass the cache once it passes this threshold; the
441 most recent 128 IOs are tracked so sequential IO can be detected even when
442 it isn't all done at once.
443
444 sequential_merge
445 If non zero, bcache keeps a list of the last 128 requests submitted to compare
446 against all new requests to determine which new requests are sequential
447 continuations of previous requests for the purpose of determining sequential
448 cutoff. This is necessary if the sequential cutoff value is greater than the
449 maximum acceptable sequential size for any single request.
450
451 state
452 The backing device can be in one of four different states:
453
454 no cache: Has never been attached to a cache set.
455
456 clean: Part of a cache set, and there is no cached dirty data.
457
458 dirty: Part of a cache set, and there is cached dirty data.
459
460 inconsistent: The backing device was forcibly run by the user when there was
461 dirty data cached but the cache set was unavailable; whatever data was on the
462 backing device has likely been corrupted.
463
464 stop
465 Write to this file to shut down the bcache device and close the backing
466 device.
467
468 writeback_delay
469 When dirty data is written to the cache and it previously did not contain
470 any, waits some number of seconds before initiating writeback. Defaults to
471 30.
472
473 writeback_percent
474 If nonzero, bcache tries to keep around this percentage of the cache dirty by
475 throttling background writeback and using a PD controller to smoothly adjust
476 the rate.
477
478 writeback_rate
479 Rate in sectors per second - if writeback_percent is nonzero, background
480 writeback is throttled to this rate. Continuously adjusted by bcache but may
481 also be set by the user.
482
483 writeback_running
484 If off, writeback of dirty data will not take place at all. Dirty data will
485 still be added to the cache until it is mostly full; only meant for
486 benchmarking. Defaults to on.
487
488 Sysfs - backing device stats
489 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
490
491 There are directories with these numbers for a running total, as well as
492 versions that decay over the past day, hour and 5 minutes; they're also
493 aggregated in the cache set directory as well.
494
495 bypassed
496 Amount of IO (both reads and writes) that has bypassed the cache
497
498 cache_hits, cache_misses, cache_hit_ratio
499 Hits and misses are counted per individual IO as bcache sees them; a
500 partial hit is counted as a miss.
501
502 cache_bypass_hits, cache_bypass_misses
503 Hits and misses for IO that is intended to skip the cache are still counted,
504 but broken out here.
505
506 cache_miss_collisions
507 Counts instances where data was going to be inserted into the cache from a
508 cache miss, but raced with a write and data was already present (usually 0
509 since the synchronization for cache misses was rewritten)
510
511 Sysfs - cache set
512 ~~~~~~~~~~~~~~~~~
513
514 Available at /sys/fs/bcache/<cset-uuid>
515
516 average_key_size
517 Average data per key in the btree.
518
519 bdev<0..n>
520 Symlink to each of the attached backing devices.
521
522 block_size
523 Block size of the cache devices.
524
525 btree_cache_size
526 Amount of memory currently used by the btree cache
527
528 bucket_size
529 Size of buckets
530
531 cache<0..n>
532 Symlink to each of the cache devices comprising this cache set.
533
534 cache_available_percent
535 Percentage of cache device which doesn't contain dirty data, and could
536 potentially be used for writeback. This doesn't mean this space isn't used
537 for clean cached data; the unused statistic (in priority_stats) is typically
538 much lower.
539
540 clear_stats
541 Clears the statistics associated with this cache
542
543 dirty_data
544 Amount of dirty data is in the cache (updated when garbage collection runs).
545
546 flash_vol_create
547 Echoing a size to this file (in human readable units, k/M/G) creates a thinly
548 provisioned volume backed by the cache set.
549
550 io_error_halflife, io_error_limit
551 These determines how many errors we accept before disabling the cache.
552 Each error is decayed by the half life (in # ios). If the decaying count
553 reaches io_error_limit dirty data is written out and the cache is disabled.
554
555 journal_delay_ms
556 Journal writes will delay for up to this many milliseconds, unless a cache
557 flush happens sooner. Defaults to 100.
558
559 root_usage_percent
560 Percentage of the root btree node in use. If this gets too high the node
561 will split, increasing the tree depth.
562
563 stop
564 Write to this file to shut down the cache set - waits until all attached
565 backing devices have been shut down.
566
567 tree_depth
568 Depth of the btree (A single node btree has depth 0).
569
570 unregister
571 Detaches all backing devices and closes the cache devices; if dirty data is
572 present it will disable writeback caching and wait for it to be flushed.
573
574 Sysfs - cache set internal
575 ~~~~~~~~~~~~~~~~~~~~~~~~~~
576
577 This directory also exposes timings for a number of internal operations, with
578 separate files for average duration, average frequency, last occurrence and max
579 duration: garbage collection, btree read, btree node sorts and btree splits.
580
581 active_journal_entries
582 Number of journal entries that are newer than the index.
583
584 btree_nodes
585 Total nodes in the btree.
586
587 btree_used_percent
588 Average fraction of btree in use.
589
590 bset_tree_stats
591 Statistics about the auxiliary search trees
592
593 btree_cache_max_chain
594 Longest chain in the btree node cache's hash table
595
596 cache_read_races
597 Counts instances where while data was being read from the cache, the bucket
598 was reused and invalidated - i.e. where the pointer was stale after the read
599 completed. When this occurs the data is reread from the backing device.
600
601 trigger_gc
602 Writing to this file forces garbage collection to run.
603
604 Sysfs - Cache device
605 ~~~~~~~~~~~~~~~~~~~~
606
607 Available at /sys/block/<cdev>/bcache
608
609 block_size
610 Minimum granularity of writes - should match hardware sector size.
611
612 btree_written
613 Sum of all btree writes, in (kilo/mega/giga) bytes
614
615 bucket_size
616 Size of buckets
617
618 cache_replacement_policy
619 One of either lru, fifo or random.
620
621 discard
622 Boolean; if on a discard/TRIM will be issued to each bucket before it is
623 reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
624 slow).
625
626 freelist_percent
627 Size of the freelist as a percentage of nbuckets. Can be written to to
628 increase the number of buckets kept on the freelist, which lets you
629 artificially reduce the size of the cache at runtime. Mostly for testing
630 purposes (i.e. testing how different size caches affect your hit rate), but
631 since buckets are discarded when they move on to the freelist will also make
632 the SSD's garbage collection easier by effectively giving it more reserved
633 space.
634
635 io_errors
636 Number of errors that have occurred, decayed by io_error_halflife.
637
638 metadata_written
639 Sum of all non data writes (btree writes and all other metadata).
640
641 nbuckets
642 Total buckets in this cache
643
644 priority_stats
645 Statistics about how recently data in the cache has been accessed.
646 This can reveal your working set size. Unused is the percentage of
647 the cache that doesn't contain any data. Metadata is bcache's
648 metadata overhead. Average is the average priority of cache buckets.
649 Next is a list of quantiles with the priority threshold of each.
650
651 written
652 Sum of all data that has been written to the cache; comparison with
653 btree_written gives the amount of write inflation in bcache.
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