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1 | ============================ |
2 | A block layer cache (bcache) | |
3 | ============================ | |
4 | ||
c9b2ffc0 | 5 | Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be |
cafe5635 KO |
6 | nice if you could use them as cache... Hence bcache. |
7 | ||
27c8700b CL |
8 | The bcache wiki can be found at: |
9 | https://bcache.evilpiepirate.org | |
a966ac73 | 10 | |
27c8700b CL |
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/ | |
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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 | |
c9b2ffc0 | 19 | extents (which can be anywhere from a single sector to the bucket size). It's |
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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: | |
27c8700b | 47 | You'll need bcache util from the bcache-tools repository. Both the cache device |
a966ac73 MCC |
48 | and backing device must be formatted before use:: |
49 | ||
27c8700b CL |
50 | bcache make -B /dev/sdb |
51 | bcache make -C /dev/sdc | |
cafe5635 | 52 | |
27c8700b | 53 | `bcache make` has the ability to format multiple devices at the same time - if |
cafe5635 | 54 | you format your backing devices and cache device at the same time, you won't |
a966ac73 MCC |
55 | have to manually attach:: |
56 | ||
27c8700b CL |
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. | |
cafe5635 | 62 | |
cecd628d | 63 | bcache-tools now ships udev rules, and bcache devices are known to the kernel |
a966ac73 | 64 | immediately. Without udev, you can manually register devices like this:: |
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65 | |
66 | echo /dev/sdb > /sys/fs/bcache/register | |
67 | echo /dev/sdc > /sys/fs/bcache/register | |
68 | ||
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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. | |
c9b2ffc0 MM |
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. | |
cafe5635 | 76 | |
a966ac73 | 77 | The devices show up as:: |
cafe5635 | 78 | |
cecd628d | 79 | /dev/bcache<N> |
cafe5635 | 80 | |
a966ac73 | 81 | As well as (with udev):: |
cafe5635 | 82 | |
cecd628d GP |
83 | /dev/bcache/by-uuid/<uuid> |
84 | /dev/bcache/by-label/<label> | |
85 | ||
a966ac73 | 86 | To get started:: |
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87 | |
88 | mkfs.ext4 /dev/bcache0 | |
89 | mount /dev/bcache0 /mnt | |
90 | ||
cecd628d | 91 | You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache . |
c9b2ffc0 | 92 | You can also control them through /sys/fs//bcache/<cset-uuid>/ . |
cecd628d | 93 | |
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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 | ||
a966ac73 | 98 | Attaching |
c9b2ffc0 | 99 | --------- |
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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 | |
a966ac73 | 104 | /sys/fs/bcache:: |
cafe5635 | 105 | |
cecd628d | 106 | echo <CSET-UUID> > /sys/block/bcache0/bcache/attach |
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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 | |
cecd628d | 110 | /dev/bcache<N> device won't be created until the cache shows up - particularly |
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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 | |
a966ac73 | 114 | can force run the backing device:: |
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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 | ||
a966ac73 | 127 | Error Handling |
c9b2ffc0 | 128 | -------------- |
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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 | ||
c9b2ffc0 | 150 | |
a966ac73 | 151 | Howto/cookbook |
c9b2ffc0 MM |
152 | -------------- |
153 | ||
c0b8c9a3 | 154 | A) Starting a bcache with a missing caching device |
c9b2ffc0 | 155 | |
c0b8c9a3 | 156 | If registering the backing device doesn't help, it's already there, you just need |
a966ac73 MCC |
157 | to force it to run without the cache:: |
158 | ||
c0b8c9a3 EW |
159 | host:~# echo /dev/sdb1 > /sys/fs/bcache/register |
160 | [ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered | |
c9b2ffc0 | 161 | |
c0b8c9a3 EW |
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 | |
a966ac73 MCC |
164 | start your bcache without its cache, like so:: |
165 | ||
c0b8c9a3 | 166 | host:/sys/block/sdb/sdb1/bcache# echo 1 > running |
c9b2ffc0 | 167 | |
c0b8c9a3 | 168 | Note that this may cause data loss if you were running in writeback mode. |
c9b2ffc0 | 169 | |
c9b2ffc0 | 170 | |
a966ac73 | 171 | B) Bcache does not find its cache:: |
c9b2ffc0 | 172 | |
c0b8c9a3 EW |
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 | |
c9b2ffc0 | 177 | |
c0b8c9a3 | 178 | In this case, the caching device was simply not registered at boot |
a966ac73 MCC |
179 | or disappeared and came back, and needs to be (re-)registered:: |
180 | ||
c0b8c9a3 | 181 | host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register |
c9b2ffc0 | 182 | |
c9b2ffc0 | 183 | |
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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 | |
27c8700b | 198 | --data-offset when you originally formatted bcache with `bcache make`. |
c0b8c9a3 | 199 | |
a966ac73 MCC |
200 | For example:: |
201 | ||
c0b8c9a3 EW |
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 loosing data. | |
208 | ||
209 | ||
210 | E) Wiping a cache device | |
c9b2ffc0 | 211 | |
a966ac73 MCC |
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:: | |
c9b2ffc0 | 219 | |
27c8700b | 220 | host:~# bcache make -C /dev/sdh2 |
a966ac73 MCC |
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 | |
c9b2ffc0 | 232 | |
a966ac73 | 233 | start backing device with missing cache:: |
c9b2ffc0 | 234 | |
a966ac73 | 235 | host:/sys/block/md5/bcache# echo 1 > running |
c9b2ffc0 | 236 | |
a966ac73 | 237 | attach new cache:: |
c9b2ffc0 | 238 | |
a966ac73 MCC |
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:: | |
c9b2ffc0 | 244 | |
c0b8c9a3 EW |
245 | host:/sys/block/sda/sda7/bcache# echo 1 > detach |
246 | [ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7 | |
c9b2ffc0 | 247 | |
c0b8c9a3 EW |
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 | |
c9b2ffc0 | 251 | |
a966ac73 MCC |
252 | We need to go and unregister it:: |
253 | ||
c0b8c9a3 EW |
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 | |
c9b2ffc0 | 258 | |
a966ac73 MCC |
259 | Now we can wipe it:: |
260 | ||
c0b8c9a3 EW |
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 | ||
c9b2ffc0 | 264 | |
c0b8c9a3 | 265 | G) dm-crypt and bcache |
c9b2ffc0 | 266 | |
c0b8c9a3 EW |
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?] | |
c9b2ffc0 | 270 | |
c9b2ffc0 | 271 | |
c0b8c9a3 | 272 | H) Stop/free a registered bcache to wipe and/or recreate it |
c9b2ffc0 | 273 | |
c0b8c9a3 EW |
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: | |
c9b2ffc0 MM |
277 | |
278 | 1) Is it present in /dev/bcache* ? (there are times where it won't be) | |
c0b8c9a3 | 279 | |
a966ac73 MCC |
280 | If so, it's easy:: |
281 | ||
c0b8c9a3 | 282 | host:/sys/block/bcache0/bcache# echo 1 > stop |
c9b2ffc0 | 283 | |
a966ac73 MCC |
284 | 2) But if your backing device is gone, this won't work:: |
285 | ||
c0b8c9a3 EW |
286 | host:/sys/block/bcache0# cd bcache |
287 | bash: cd: bcache: No such file or directory | |
c9b2ffc0 | 288 | |
a966ac73 MCC |
289 | In this case, you may have to unregister the dmcrypt block device that |
290 | references this bcache to free it up:: | |
291 | ||
c0b8c9a3 EW |
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 | |
c9b2ffc0 | 295 | |
a966ac73 MCC |
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/:: | |
c9b2ffc0 | 301 | |
a966ac73 MCC |
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/ | |
c0b8c9a3 | 306 | |
a966ac73 MCC |
307 | The device names will show which UUID is relevant, cd in that directory |
308 | and stop the cache:: | |
c9b2ffc0 | 309 | |
c0b8c9a3 EW |
310 | host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop |
311 | ||
a966ac73 MCC |
312 | This will free up bcache references and let you reuse the partition for |
313 | other purposes. | |
c9b2ffc0 MM |
314 | |
315 | ||
316 | ||
a966ac73 | 317 | Troubleshooting performance |
c9b2ffc0 | 318 | --------------------------- |
7b41b51a KO |
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 | ||
c0b8c9a3 EW |
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 | |
27c8700b | 328 | width using `bcache make --data-offset`. If you intend to expand your |
c0b8c9a3 EW |
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 | |
a966ac73 MCC |
333 | would provide alignment for many common RAID5 data spindle counts:: |
334 | ||
c0b8c9a3 EW |
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 | |
a966ac73 MCC |
338 | volume to the following data-spindle counts without re-aligning:: |
339 | ||
c0b8c9a3 EW |
340 | 3,4,5,6,7,8,9,10,12,14,15,18,20,21 ... |
341 | ||
7b41b51a KO |
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 | |
a966ac73 | 347 | happens to your SSD):: |
7b41b51a | 348 | |
a966ac73 | 349 | # echo writeback > /sys/block/bcache0/bcache/cache_mode |
7b41b51a KO |
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 | |
a966ac73 | 359 | writing an 8 gigabyte test file - so you want to disable that:: |
7b41b51a | 360 | |
a966ac73 | 361 | # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff |
7b41b51a | 362 | |
a966ac73 | 363 | To set it back to the default (4 mb), do:: |
7b41b51a | 364 | |
a966ac73 | 365 | # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff |
7b41b51a KO |
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 | ||
a966ac73 | 378 | You can disable this if you need to by setting the thresholds to 0:: |
7b41b51a | 379 | |
a966ac73 MCC |
380 | # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us |
381 | # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us | |
7b41b51a KO |
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 | |
bd206b51 | 394 | this to not show up enough to be noticeable (especially since bcache's btree |
7b41b51a KO |
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 | ||
c9b2ffc0 | 402 | |
a966ac73 | 403 | Sysfs - backing device |
c9b2ffc0 | 404 | ---------------------- |
cafe5635 | 405 | |
cecd628d GP |
406 | Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and |
407 | (if attached) /sys/fs/bcache/<cset-uuid>/bdev* | |
408 | ||
cafe5635 KO |
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 | |
bd206b51 | 440 | A sequential IO will bypass the cache once it passes this threshold; the |
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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 | |
c0b8c9a3 | 449 | maximum acceptable sequential size for any single request. |
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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 | ||
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488 | Sysfs - backing device stats |
489 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
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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 | ||
a966ac73 | 498 | cache_hits, cache_misses, cache_hit_ratio |
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499 | Hits and misses are counted per individual IO as bcache sees them; a |
500 | partial hit is counted as a miss. | |
501 | ||
a966ac73 | 502 | cache_bypass_hits, cache_bypass_misses |
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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 | cache_readaheads | |
bd206b51 | 512 | Count of times readahead occurred. |
cafe5635 | 513 | |
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514 | Sysfs - cache set |
515 | ~~~~~~~~~~~~~~~~~ | |
cafe5635 | 516 | |
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517 | Available at /sys/fs/bcache/<cset-uuid> |
518 | ||
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519 | average_key_size |
520 | Average data per key in the btree. | |
521 | ||
522 | bdev<0..n> | |
523 | Symlink to each of the attached backing devices. | |
524 | ||
525 | block_size | |
526 | Block size of the cache devices. | |
527 | ||
528 | btree_cache_size | |
529 | Amount of memory currently used by the btree cache | |
530 | ||
531 | bucket_size | |
532 | Size of buckets | |
533 | ||
534 | cache<0..n> | |
c0b8c9a3 | 535 | Symlink to each of the cache devices comprising this cache set. |
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536 | |
537 | cache_available_percent | |
fe0a797a G |
538 | Percentage of cache device which doesn't contain dirty data, and could |
539 | potentially be used for writeback. This doesn't mean this space isn't used | |
540 | for clean cached data; the unused statistic (in priority_stats) is typically | |
541 | much lower. | |
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542 | |
543 | clear_stats | |
544 | Clears the statistics associated with this cache | |
545 | ||
546 | dirty_data | |
547 | Amount of dirty data is in the cache (updated when garbage collection runs). | |
548 | ||
549 | flash_vol_create | |
550 | Echoing a size to this file (in human readable units, k/M/G) creates a thinly | |
551 | provisioned volume backed by the cache set. | |
552 | ||
a966ac73 | 553 | io_error_halflife, io_error_limit |
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554 | These determines how many errors we accept before disabling the cache. |
555 | Each error is decayed by the half life (in # ios). If the decaying count | |
556 | reaches io_error_limit dirty data is written out and the cache is disabled. | |
557 | ||
558 | journal_delay_ms | |
559 | Journal writes will delay for up to this many milliseconds, unless a cache | |
560 | flush happens sooner. Defaults to 100. | |
561 | ||
562 | root_usage_percent | |
563 | Percentage of the root btree node in use. If this gets too high the node | |
564 | will split, increasing the tree depth. | |
565 | ||
566 | stop | |
567 | Write to this file to shut down the cache set - waits until all attached | |
568 | backing devices have been shut down. | |
569 | ||
570 | tree_depth | |
571 | Depth of the btree (A single node btree has depth 0). | |
572 | ||
573 | unregister | |
574 | Detaches all backing devices and closes the cache devices; if dirty data is | |
575 | present it will disable writeback caching and wait for it to be flushed. | |
576 | ||
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577 | Sysfs - cache set internal |
578 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
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579 | |
580 | This directory also exposes timings for a number of internal operations, with | |
bd206b51 | 581 | separate files for average duration, average frequency, last occurrence and max |
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582 | duration: garbage collection, btree read, btree node sorts and btree splits. |
583 | ||
584 | active_journal_entries | |
585 | Number of journal entries that are newer than the index. | |
586 | ||
587 | btree_nodes | |
588 | Total nodes in the btree. | |
589 | ||
590 | btree_used_percent | |
591 | Average fraction of btree in use. | |
592 | ||
593 | bset_tree_stats | |
594 | Statistics about the auxiliary search trees | |
595 | ||
596 | btree_cache_max_chain | |
597 | Longest chain in the btree node cache's hash table | |
598 | ||
599 | cache_read_races | |
600 | Counts instances where while data was being read from the cache, the bucket | |
601 | was reused and invalidated - i.e. where the pointer was stale after the read | |
602 | completed. When this occurs the data is reread from the backing device. | |
603 | ||
604 | trigger_gc | |
605 | Writing to this file forces garbage collection to run. | |
606 | ||
a966ac73 MCC |
607 | Sysfs - Cache device |
608 | ~~~~~~~~~~~~~~~~~~~~ | |
cafe5635 | 609 | |
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610 | Available at /sys/block/<cdev>/bcache |
611 | ||
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612 | block_size |
613 | Minimum granularity of writes - should match hardware sector size. | |
614 | ||
615 | btree_written | |
616 | Sum of all btree writes, in (kilo/mega/giga) bytes | |
617 | ||
618 | bucket_size | |
619 | Size of buckets | |
620 | ||
621 | cache_replacement_policy | |
622 | One of either lru, fifo or random. | |
623 | ||
624 | discard | |
625 | Boolean; if on a discard/TRIM will be issued to each bucket before it is | |
626 | reused. Defaults to off, since SATA TRIM is an unqueued command (and thus | |
627 | slow). | |
628 | ||
629 | freelist_percent | |
630 | Size of the freelist as a percentage of nbuckets. Can be written to to | |
631 | increase the number of buckets kept on the freelist, which lets you | |
632 | artificially reduce the size of the cache at runtime. Mostly for testing | |
633 | purposes (i.e. testing how different size caches affect your hit rate), but | |
634 | since buckets are discarded when they move on to the freelist will also make | |
635 | the SSD's garbage collection easier by effectively giving it more reserved | |
636 | space. | |
637 | ||
638 | io_errors | |
bd206b51 | 639 | Number of errors that have occurred, decayed by io_error_halflife. |
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640 | |
641 | metadata_written | |
642 | Sum of all non data writes (btree writes and all other metadata). | |
643 | ||
644 | nbuckets | |
645 | Total buckets in this cache | |
646 | ||
647 | priority_stats | |
fe0a797a G |
648 | Statistics about how recently data in the cache has been accessed. |
649 | This can reveal your working set size. Unused is the percentage of | |
650 | the cache that doesn't contain any data. Metadata is bcache's | |
651 | metadata overhead. Average is the average priority of cache buckets. | |
652 | Next is a list of quantiles with the priority threshold of each. | |
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653 | |
654 | written | |
655 | Sum of all data that has been written to the cache; comparison with | |
656 | btree_written gives the amount of write inflation in bcache. |