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c9b2ffc0 | 1 | Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be |
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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 | |
c9b2ffc0 | 11 | extents (which can be anywhere from a single sector to the bucket size). It's |
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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 | ||
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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: | |
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51 | |
52 | echo /dev/sdb > /sys/fs/bcache/register | |
53 | echo /dev/sdc > /sys/fs/bcache/register | |
54 | ||
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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. | |
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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. | |
cafe5635 | 62 | |
cecd628d | 63 | The devices show up as: |
cafe5635 | 64 | |
cecd628d | 65 | /dev/bcache<N> |
cafe5635 | 66 | |
cecd628d | 67 | As well as (with udev): |
cafe5635 | 68 | |
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69 | /dev/bcache/by-uuid/<uuid> |
70 | /dev/bcache/by-label/<label> | |
71 | ||
72 | To get started: | |
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73 | |
74 | mkfs.ext4 /dev/bcache0 | |
75 | mount /dev/bcache0 /mnt | |
76 | ||
cecd628d | 77 | You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache . |
c9b2ffc0 | 78 | You can also control them through /sys/fs//bcache/<cset-uuid>/ . |
cecd628d | 79 | |
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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 | ||
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84 | ATTACHING |
85 | --------- | |
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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 | ||
cecd628d | 92 | echo <CSET-UUID> > /sys/block/bcache0/bcache/attach |
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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 | |
cecd628d | 96 | /dev/bcache<N> device won't be created until the cache shows up - particularly |
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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 | ||
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113 | ERROR HANDLING |
114 | -------------- | |
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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 | ||
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136 | |
137 | HOWTO/COOKBOOK | |
138 | -------------- | |
139 | ||
140 | A) Your bcache doesn't start. | |
141 | Starting and starting a bcache with a missing caching device | |
142 | ||
143 | Registering the backing device doesn't help, it's already there, you just need | |
144 | to force it to run without the cache: | |
145 | host:~# echo /dev/sdb1 > /sys/fs/bcache/register | |
146 | [ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered | |
147 | ||
148 | Next, you try to register your caching device if it's present. However if it's | |
149 | absent, or registration fails for some reason, you can still start your bcache | |
150 | without its cache, like so: | |
151 | host:/sys/block/sdb/sdb1/bcache# echo 1 > running | |
152 | ||
153 | ||
154 | B) Bcache not finding its cache and not starting | |
155 | ||
156 | This does not work: | |
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 or | |
163 | 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 caching device crashes the kernel on startup/boot | |
168 | ||
169 | You'll have to wipe the caching device, start the backing device without the | |
170 | cache, and you can re-attach the cleaned up caching device then. This does | |
171 | require booting with a kernel/rescue media where bcache is disabled | |
172 | since it will otherwise try to access your device and probably crash | |
173 | again before you have a chance to wipe it. | |
174 | (or if you plan ahead, compile a backup kernel with bcache disabled and keep it | |
175 | in your grub config for a rainy day) | |
176 | If bcache is not available in the kernel, a filesystem on the backing device is | |
177 | still available at an 8KiB offset. So either via a loopdev of the backing device | |
178 | created with --offset 8K or by temporarily increasing the start sector of the | |
179 | partition by 16 (512byte sectors). | |
180 | ||
181 | This is how you wipe the caching device: | |
182 | host:~# wipefs -a /dev/sdh2 | |
183 | 16 bytes were erased at offset 0x1018 (bcache) | |
184 | they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81 | |
185 | ||
186 | After you boot back with bcache enabled, you recreate the cache and attach it: | |
187 | host:~# make-bcache -C /dev/sdh2 | |
188 | UUID: 7be7e175-8f4c-4f99-94b2-9c904d227045 | |
189 | Set UUID: 5bc072a8-ab17-446d-9744-e247949913c1 | |
190 | version: 0 | |
191 | nbuckets: 106874 | |
192 | block_size: 1 | |
193 | bucket_size: 1024 | |
194 | nr_in_set: 1 | |
195 | nr_this_dev: 0 | |
196 | first_bucket: 1 | |
197 | [ 650.511912] bcache: run_cache_set() invalidating existing data | |
198 | [ 650.549228] bcache: register_cache() registered cache device sdh2 | |
199 | ||
200 | start backing device with missing cache: | |
201 | host:/sys/block/md5/bcache# echo 1 > running | |
202 | ||
203 | attach new cache: | |
204 | host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach | |
205 | [ 865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1 | |
206 | ||
207 | ||
208 | D) Remove or replace a caching device | |
209 | ||
210 | host:/sys/block/sda/sda7/bcache# echo 1 > detach | |
211 | [ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7 | |
212 | ||
213 | host:~# wipefs -a /dev/nvme0n1p4 | |
214 | wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy | |
215 | Ooops, it's disabled, but not unregistered, so it's still protected | |
216 | ||
217 | We need to go and unregister it: | |
218 | host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0 | |
219 | 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/ | |
220 | host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop | |
221 | kernel: [ 917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered | |
222 | ||
223 | Now we can wipe it: | |
224 | host:~# wipefs -a /dev/nvme0n1p4 | |
225 | /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 | |
226 | ||
227 | ||
228 | E) dmcrypt and bcache | |
229 | ||
230 | First setup bcache unencrypted and then install dmcrypt on top of /dev/bcache<N> | |
231 | This will work faster than if you dmcrypt both the backing and caching | |
232 | devices and then install bcache on top. | |
233 | ||
234 | ||
235 | F) Stop/free a registered bcache to wipe and/or recreate it | |
236 | (or maybe you need to free up all bcache references so that you can have fdisk | |
237 | run and re-register a changed partition table, which won't work if there are any | |
238 | active backing or caching devices left on it) | |
239 | ||
240 | 1) Is it present in /dev/bcache* ? (there are times where it won't be) | |
241 | If so, it's easy: | |
242 | host:/sys/block/bcache0/bcache# echo 1 > stop | |
243 | ||
244 | 2) But if your backing device is gone, this won't work: | |
245 | host:/sys/block/bcache0# cd bcache | |
246 | bash: cd: bcache: No such file or directory | |
247 | ||
248 | In this case, you may have to unregister the dmcrypt block device that | |
249 | references this bcache to free it up: | |
250 | host:~# dmsetup remove oldds1 | |
251 | bcache: bcache_device_free() bcache0 stopped | |
252 | bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered | |
253 | ||
254 | This causes the backing bcache to be removed from /sys/fs/bcache and then it can | |
255 | be reused | |
256 | ||
257 | 3) In other cases, you can also look in /sys/fs/bcache/: | |
258 | host:/sys/fs/bcache# ls -l */{cache?,bdev?} | |
259 | lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/ | |
260 | lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/ | |
261 | 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/ | |
262 | ||
263 | The device names will show which UUID is relevant, cd in that directory | |
264 | and stop the cache: | |
265 | host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop | |
266 | this will free up bcache references and let you reuse the partition for other | |
267 | purposes. | |
268 | ||
269 | ||
270 | ||
271 | TROUBLESHOOTING PERFORMANCE | |
272 | --------------------------- | |
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273 | |
274 | Bcache has a bunch of config options and tunables. The defaults are intended to | |
275 | be reasonable for typical desktop and server workloads, but they're not what you | |
276 | want for getting the best possible numbers when benchmarking. | |
277 | ||
278 | - Bad write performance | |
279 | ||
280 | If write performance is not what you expected, you probably wanted to be | |
281 | running in writeback mode, which isn't the default (not due to a lack of | |
282 | maturity, but simply because in writeback mode you'll lose data if something | |
283 | happens to your SSD) | |
284 | ||
c9b2ffc0 | 285 | # echo writeback > /sys/block/bcache0/bcache/cache_mode |
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286 | |
287 | - Bad performance, or traffic not going to the SSD that you'd expect | |
288 | ||
289 | By default, bcache doesn't cache everything. It tries to skip sequential IO - | |
290 | because you really want to be caching the random IO, and if you copy a 10 | |
291 | gigabyte file you probably don't want that pushing 10 gigabytes of randomly | |
292 | accessed data out of your cache. | |
293 | ||
294 | But if you want to benchmark reads from cache, and you start out with fio | |
295 | writing an 8 gigabyte test file - so you want to disable that. | |
296 | ||
297 | # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff | |
298 | ||
299 | To set it back to the default (4 mb), do | |
300 | ||
301 | # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff | |
302 | ||
303 | - Traffic's still going to the spindle/still getting cache misses | |
304 | ||
305 | In the real world, SSDs don't always keep up with disks - particularly with | |
306 | slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So | |
307 | you want to avoid being bottlenecked by the SSD and having it slow everything | |
308 | down. | |
309 | ||
310 | To avoid that bcache tracks latency to the cache device, and gradually | |
311 | throttles traffic if the latency exceeds a threshold (it does this by | |
312 | cranking down the sequential bypass). | |
313 | ||
314 | You can disable this if you need to by setting the thresholds to 0: | |
315 | ||
316 | # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us | |
317 | # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us | |
318 | ||
319 | The default is 2000 us (2 milliseconds) for reads, and 20000 for writes. | |
320 | ||
321 | - Still getting cache misses, of the same data | |
322 | ||
323 | One last issue that sometimes trips people up is actually an old bug, due to | |
324 | the way cache coherency is handled for cache misses. If a btree node is full, | |
325 | a cache miss won't be able to insert a key for the new data and the data | |
326 | won't be written to the cache. | |
327 | ||
328 | In practice this isn't an issue because as soon as a write comes along it'll | |
329 | cause the btree node to be split, and you need almost no write traffic for | |
bd206b51 | 330 | this to not show up enough to be noticeable (especially since bcache's btree |
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331 | nodes are huge and index large regions of the device). But when you're |
332 | benchmarking, if you're trying to warm the cache by reading a bunch of data | |
333 | and there's no other traffic - that can be a problem. | |
334 | ||
335 | Solution: warm the cache by doing writes, or use the testing branch (there's | |
336 | a fix for the issue there). | |
337 | ||
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338 | |
339 | SYSFS - BACKING DEVICE | |
340 | ---------------------- | |
cafe5635 | 341 | |
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342 | Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and |
343 | (if attached) /sys/fs/bcache/<cset-uuid>/bdev* | |
344 | ||
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345 | attach |
346 | Echo the UUID of a cache set to this file to enable caching. | |
347 | ||
348 | cache_mode | |
349 | Can be one of either writethrough, writeback, writearound or none. | |
350 | ||
351 | clear_stats | |
352 | Writing to this file resets the running total stats (not the day/hour/5 minute | |
353 | decaying versions). | |
354 | ||
355 | detach | |
356 | Write to this file to detach from a cache set. If there is dirty data in the | |
357 | cache, it will be flushed first. | |
358 | ||
359 | dirty_data | |
360 | Amount of dirty data for this backing device in the cache. Continuously | |
361 | updated unlike the cache set's version, but may be slightly off. | |
362 | ||
363 | label | |
364 | Name of underlying device. | |
365 | ||
366 | readahead | |
367 | Size of readahead that should be performed. Defaults to 0. If set to e.g. | |
368 | 1M, it will round cache miss reads up to that size, but without overlapping | |
369 | existing cache entries. | |
370 | ||
371 | running | |
372 | 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether | |
373 | it's in passthrough mode or caching). | |
374 | ||
375 | sequential_cutoff | |
bd206b51 | 376 | A sequential IO will bypass the cache once it passes this threshold; the |
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377 | most recent 128 IOs are tracked so sequential IO can be detected even when |
378 | it isn't all done at once. | |
379 | ||
380 | sequential_merge | |
381 | If non zero, bcache keeps a list of the last 128 requests submitted to compare | |
382 | against all new requests to determine which new requests are sequential | |
383 | continuations of previous requests for the purpose of determining sequential | |
384 | cutoff. This is necessary if the sequential cutoff value is greater than the | |
385 | maximum acceptable sequential size for any single request. | |
386 | ||
387 | state | |
388 | The backing device can be in one of four different states: | |
389 | ||
390 | no cache: Has never been attached to a cache set. | |
391 | ||
392 | clean: Part of a cache set, and there is no cached dirty data. | |
393 | ||
394 | dirty: Part of a cache set, and there is cached dirty data. | |
395 | ||
396 | inconsistent: The backing device was forcibly run by the user when there was | |
397 | dirty data cached but the cache set was unavailable; whatever data was on the | |
398 | backing device has likely been corrupted. | |
399 | ||
400 | stop | |
401 | Write to this file to shut down the bcache device and close the backing | |
402 | device. | |
403 | ||
404 | writeback_delay | |
405 | When dirty data is written to the cache and it previously did not contain | |
406 | any, waits some number of seconds before initiating writeback. Defaults to | |
407 | 30. | |
408 | ||
409 | writeback_percent | |
410 | If nonzero, bcache tries to keep around this percentage of the cache dirty by | |
411 | throttling background writeback and using a PD controller to smoothly adjust | |
412 | the rate. | |
413 | ||
414 | writeback_rate | |
415 | Rate in sectors per second - if writeback_percent is nonzero, background | |
416 | writeback is throttled to this rate. Continuously adjusted by bcache but may | |
417 | also be set by the user. | |
418 | ||
419 | writeback_running | |
420 | If off, writeback of dirty data will not take place at all. Dirty data will | |
421 | still be added to the cache until it is mostly full; only meant for | |
422 | benchmarking. Defaults to on. | |
423 | ||
424 | SYSFS - BACKING DEVICE STATS: | |
425 | ||
426 | There are directories with these numbers for a running total, as well as | |
427 | versions that decay over the past day, hour and 5 minutes; they're also | |
428 | aggregated in the cache set directory as well. | |
429 | ||
430 | bypassed | |
431 | Amount of IO (both reads and writes) that has bypassed the cache | |
432 | ||
433 | cache_hits | |
434 | cache_misses | |
435 | cache_hit_ratio | |
436 | Hits and misses are counted per individual IO as bcache sees them; a | |
437 | partial hit is counted as a miss. | |
438 | ||
439 | cache_bypass_hits | |
440 | cache_bypass_misses | |
441 | Hits and misses for IO that is intended to skip the cache are still counted, | |
442 | but broken out here. | |
443 | ||
444 | cache_miss_collisions | |
445 | Counts instances where data was going to be inserted into the cache from a | |
446 | cache miss, but raced with a write and data was already present (usually 0 | |
447 | since the synchronization for cache misses was rewritten) | |
448 | ||
449 | cache_readaheads | |
bd206b51 | 450 | Count of times readahead occurred. |
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451 | |
452 | SYSFS - CACHE SET: | |
453 | ||
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454 | Available at /sys/fs/bcache/<cset-uuid> |
455 | ||
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456 | average_key_size |
457 | Average data per key in the btree. | |
458 | ||
459 | bdev<0..n> | |
460 | Symlink to each of the attached backing devices. | |
461 | ||
462 | block_size | |
463 | Block size of the cache devices. | |
464 | ||
465 | btree_cache_size | |
466 | Amount of memory currently used by the btree cache | |
467 | ||
468 | bucket_size | |
469 | Size of buckets | |
470 | ||
471 | cache<0..n> | |
472 | Symlink to each of the cache devices comprising this cache set. | |
473 | ||
474 | cache_available_percent | |
fe0a797a G |
475 | Percentage of cache device which doesn't contain dirty data, and could |
476 | potentially be used for writeback. This doesn't mean this space isn't used | |
477 | for clean cached data; the unused statistic (in priority_stats) is typically | |
478 | much lower. | |
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479 | |
480 | clear_stats | |
481 | Clears the statistics associated with this cache | |
482 | ||
483 | dirty_data | |
484 | Amount of dirty data is in the cache (updated when garbage collection runs). | |
485 | ||
486 | flash_vol_create | |
487 | Echoing a size to this file (in human readable units, k/M/G) creates a thinly | |
488 | provisioned volume backed by the cache set. | |
489 | ||
490 | io_error_halflife | |
491 | io_error_limit | |
492 | These determines how many errors we accept before disabling the cache. | |
493 | Each error is decayed by the half life (in # ios). If the decaying count | |
494 | reaches io_error_limit dirty data is written out and the cache is disabled. | |
495 | ||
496 | journal_delay_ms | |
497 | Journal writes will delay for up to this many milliseconds, unless a cache | |
498 | flush happens sooner. Defaults to 100. | |
499 | ||
500 | root_usage_percent | |
501 | Percentage of the root btree node in use. If this gets too high the node | |
502 | will split, increasing the tree depth. | |
503 | ||
504 | stop | |
505 | Write to this file to shut down the cache set - waits until all attached | |
506 | backing devices have been shut down. | |
507 | ||
508 | tree_depth | |
509 | Depth of the btree (A single node btree has depth 0). | |
510 | ||
511 | unregister | |
512 | Detaches all backing devices and closes the cache devices; if dirty data is | |
513 | present it will disable writeback caching and wait for it to be flushed. | |
514 | ||
515 | SYSFS - CACHE SET INTERNAL: | |
516 | ||
517 | This directory also exposes timings for a number of internal operations, with | |
bd206b51 | 518 | separate files for average duration, average frequency, last occurrence and max |
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519 | duration: garbage collection, btree read, btree node sorts and btree splits. |
520 | ||
521 | active_journal_entries | |
522 | Number of journal entries that are newer than the index. | |
523 | ||
524 | btree_nodes | |
525 | Total nodes in the btree. | |
526 | ||
527 | btree_used_percent | |
528 | Average fraction of btree in use. | |
529 | ||
530 | bset_tree_stats | |
531 | Statistics about the auxiliary search trees | |
532 | ||
533 | btree_cache_max_chain | |
534 | Longest chain in the btree node cache's hash table | |
535 | ||
536 | cache_read_races | |
537 | Counts instances where while data was being read from the cache, the bucket | |
538 | was reused and invalidated - i.e. where the pointer was stale after the read | |
539 | completed. When this occurs the data is reread from the backing device. | |
540 | ||
541 | trigger_gc | |
542 | Writing to this file forces garbage collection to run. | |
543 | ||
544 | SYSFS - CACHE DEVICE: | |
545 | ||
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546 | Available at /sys/block/<cdev>/bcache |
547 | ||
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548 | block_size |
549 | Minimum granularity of writes - should match hardware sector size. | |
550 | ||
551 | btree_written | |
552 | Sum of all btree writes, in (kilo/mega/giga) bytes | |
553 | ||
554 | bucket_size | |
555 | Size of buckets | |
556 | ||
557 | cache_replacement_policy | |
558 | One of either lru, fifo or random. | |
559 | ||
560 | discard | |
561 | Boolean; if on a discard/TRIM will be issued to each bucket before it is | |
562 | reused. Defaults to off, since SATA TRIM is an unqueued command (and thus | |
563 | slow). | |
564 | ||
565 | freelist_percent | |
566 | Size of the freelist as a percentage of nbuckets. Can be written to to | |
567 | increase the number of buckets kept on the freelist, which lets you | |
568 | artificially reduce the size of the cache at runtime. Mostly for testing | |
569 | purposes (i.e. testing how different size caches affect your hit rate), but | |
570 | since buckets are discarded when they move on to the freelist will also make | |
571 | the SSD's garbage collection easier by effectively giving it more reserved | |
572 | space. | |
573 | ||
574 | io_errors | |
bd206b51 | 575 | Number of errors that have occurred, decayed by io_error_halflife. |
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576 | |
577 | metadata_written | |
578 | Sum of all non data writes (btree writes and all other metadata). | |
579 | ||
580 | nbuckets | |
581 | Total buckets in this cache | |
582 | ||
583 | priority_stats | |
fe0a797a G |
584 | Statistics about how recently data in the cache has been accessed. |
585 | This can reveal your working set size. Unused is the percentage of | |
586 | the cache that doesn't contain any data. Metadata is bcache's | |
587 | metadata overhead. Average is the average priority of cache buckets. | |
588 | Next is a list of quantiles with the priority threshold of each. | |
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589 | |
590 | written | |
591 | Sum of all data that has been written to the cache; comparison with | |
592 | btree_written gives the amount of write inflation in bcache. |