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Based on kernel version 4.9. Page generated on 2016-12-21 14:28 EST.

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