1 ============================ 1 ============================
2 A block layer cache (bcache) 2 A block layer cache (bcache)
3 ============================ 3 ============================
4 4
5 Say you've got a big slow raid 6, and an ssd o 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 b 6 nice if you could use them as cache... Hence bcache.
7 7
8 The bcache wiki can be found at: 8 The bcache wiki can be found at:
9 https://bcache.evilpiepirate.org 9 https://bcache.evilpiepirate.org
10 10
11 This is the git repository of bcache-tools: 11 This is the git repository of bcache-tools:
12 https://git.kernel.org/pub/scm/linux/kernel/ 12 https://git.kernel.org/pub/scm/linux/kernel/git/colyli/bcache-tools.git/
13 13
14 The latest bcache kernel code can be found fro 14 The latest bcache kernel code can be found from mainline Linux kernel:
15 https://git.kernel.org/pub/scm/linux/kernel/ 15 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
16 16
17 It's designed around the performance character 17 It's designed around the performance characteristics of SSDs - it only allocates
18 in erase block sized buckets, and it uses a hy 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 s 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; 20 designed to avoid random writes at all costs; it fills up an erase block
21 sequentially, then issues a discard before reu 21 sequentially, then issues a discard before reusing it.
22 22
23 Both writethrough and writeback caching are su 23 Both writethrough and writeback caching are supported. Writeback defaults to
24 off, but can be switched on and off arbitraril 24 off, but can be switched on and off arbitrarily at runtime. Bcache goes to
25 great lengths to protect your data - it reliab 25 great lengths to protect your data - it reliably handles unclean shutdown. (It
26 doesn't even have a notion of a clean shutdown 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 st 27 writes as completed until they're on stable storage).
28 28
29 Writeback caching can use most of the cache fo 29 Writeback caching can use most of the cache for buffering writes - writing
30 dirty data to the backing device is always don 30 dirty data to the backing device is always done sequentially, scanning from the
31 start to the end of the index. 31 start to the end of the index.
32 32
33 Since random IO is what SSDs excel at, there g 33 Since random IO is what SSDs excel at, there generally won't be much benefit
34 to caching large sequential IO. Bcache detects 34 to caching large sequential IO. Bcache detects sequential IO and skips it;
35 it also keeps a rolling average of the IO size 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 I 36 average is above the cutoff it will skip all IO from that task - instead of
37 caching the first 512k after every seek. Backu 37 caching the first 512k after every seek. Backups and large file copies should
38 thus entirely bypass the cache. 38 thus entirely bypass the cache.
39 39
40 In the event of a data IO error on the flash i 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 41 from disk or invalidating cache entries. For unrecoverable errors (meta data
42 or dirty data), caching is automatically disab 42 or dirty data), caching is automatically disabled; if dirty data was present
43 in the cache it first disables writeback cachi 43 in the cache it first disables writeback caching and waits for all dirty data
44 to be flushed. 44 to be flushed.
45 45
46 Getting started: 46 Getting started:
47 You'll need bcache util from the bcache-tools 47 You'll need bcache util from the bcache-tools repository. Both the cache device
48 and backing device must be formatted before us 48 and backing device must be formatted before use::
49 49
50 bcache make -B /dev/sdb 50 bcache make -B /dev/sdb
51 bcache make -C /dev/sdc 51 bcache make -C /dev/sdc
52 52
53 `bcache make` has the ability to format multip 53 `bcache make` has the ability to format multiple devices at the same time - if
54 you format your backing devices and cache devi 54 you format your backing devices and cache device at the same time, you won't
55 have to manually attach:: 55 have to manually attach::
56 56
57 bcache make -B /dev/sda /dev/sdb -C /dev/sdc 57 bcache make -B /dev/sda /dev/sdb -C /dev/sdc
58 58
59 If your bcache-tools is not updated to latest 59 If your bcache-tools is not updated to latest version and does not have the
60 unified `bcache` utility, you may use the lega 60 unified `bcache` utility, you may use the legacy `make-bcache` utility to format
61 bcache device with same -B and -C parameters. 61 bcache device with same -B and -C parameters.
62 62
63 bcache-tools now ships udev rules, and bcache 63 bcache-tools now ships udev rules, and bcache devices are known to the kernel
64 immediately. Without udev, you can manually r 64 immediately. Without udev, you can manually register devices like this::
65 65
66 echo /dev/sdb > /sys/fs/bcache/register 66 echo /dev/sdb > /sys/fs/bcache/register
67 echo /dev/sdc > /sys/fs/bcache/register 67 echo /dev/sdc > /sys/fs/bcache/register
68 68
69 Registering the backing device makes the bcach 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 fi 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 u 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, 72 If you are thinking about using bcache later, it is recommended to setup all your
73 slow devices as bcache backing devices without 73 slow devices as bcache backing devices without a cache, and you can choose to add
74 a caching device later. 74 a caching device later.
75 See 'ATTACHING' section below. 75 See 'ATTACHING' section below.
76 76
77 The devices show up as:: 77 The devices show up as::
78 78
79 /dev/bcache<N> 79 /dev/bcache<N>
80 80
81 As well as (with udev):: 81 As well as (with udev)::
82 82
83 /dev/bcache/by-uuid/<uuid> 83 /dev/bcache/by-uuid/<uuid>
84 /dev/bcache/by-label/<label> 84 /dev/bcache/by-label/<label>
85 85
86 To get started:: 86 To get started::
87 87
88 mkfs.ext4 /dev/bcache0 88 mkfs.ext4 /dev/bcache0
89 mount /dev/bcache0 /mnt 89 mount /dev/bcache0 /mnt
90 90
91 You can control bcache devices through sysfs a 91 You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
92 You can also control them through /sys/fs//bca 92 You can also control them through /sys/fs//bcache/<cset-uuid>/ .
93 93
94 Cache devices are managed as sets; multiple ca 94 Cache devices are managed as sets; multiple caches per set isn't supported yet
95 but will allow for mirroring of metadata and d 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> 96 cache set shows up as /sys/fs/bcache/<UUID>
97 97
98 Attaching 98 Attaching
99 --------- 99 ---------
100 100
101 After your cache device and backing device are 101 After your cache device and backing device are registered, the backing device
102 must be attached to your cache set to enable c 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 103 device to a cache set is done thusly, with the UUID of the cache set in
104 /sys/fs/bcache:: 104 /sys/fs/bcache::
105 105
106 echo <CSET-UUID> > /sys/block/bcache0/bcache 106 echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
107 107
108 This only has to be done once. The next time y 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 d 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 t 110 /dev/bcache<N> device won't be created until the cache shows up - particularly
111 important if you have writeback caching turned 111 important if you have writeback caching turned on.
112 112
113 If you're booting up and your cache device is 113 If you're booting up and your cache device is gone and never coming back, you
114 can force run the backing device:: 114 can force run the backing device::
115 115
116 echo 1 > /sys/block/sdb/bcache/running 116 echo 1 > /sys/block/sdb/bcache/running
117 117
118 (You need to use /sys/block/sdb (or whatever y 118 (You need to use /sys/block/sdb (or whatever your backing device is called), not
119 /sys/block/bcache0, because bcache0 doesn't ex 119 /sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
120 partition, the bcache directory would be at /s 120 partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
121 121
122 The backing device will still use that cache s 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. I 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 recov 124 cache, don't expect the filesystem to be recoverable - you will have massive
125 filesystem corruption, though ext4's fsck does 125 filesystem corruption, though ext4's fsck does work miracles.
126 126
127 Error Handling 127 Error Handling
128 -------------- 128 --------------
129 129
130 Bcache tries to transparently handle IO errors 130 Bcache tries to transparently handle IO errors to/from the cache device without
131 affecting normal operation; if it sees too man 131 affecting normal operation; if it sees too many errors (the threshold is
132 configurable, and defaults to 0) it shuts down 132 configurable, and defaults to 0) it shuts down the cache device and switches all
133 the backing devices to passthrough mode. 133 the backing devices to passthrough mode.
134 134
135 - For reads from the cache, if they error we 135 - For reads from the cache, if they error we just retry the read from the
136 backing device. 136 backing device.
137 137
138 - For writethrough writes, if the write to th 138 - For writethrough writes, if the write to the cache errors we just switch to
139 invalidating the data at that lba in the ca 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) 140 a write that bypasses the cache)
141 141
142 - For writeback writes, we currently pass tha 142 - For writeback writes, we currently pass that error back up to the
143 filesystem/userspace. This could be improve 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 er 144 that skips the cache so we don't have to error the write.
145 145
146 - When we detach, we first try to flush any d 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 an 147 writeback mode). It currently doesn't do anything intelligent if it fails to
148 read some of the dirty data, though. 148 read some of the dirty data, though.
149 149
150 150
151 Howto/cookbook 151 Howto/cookbook
152 -------------- 152 --------------
153 153
154 A) Starting a bcache with a missing caching de 154 A) Starting a bcache with a missing caching device
155 155
156 If registering the backing device doesn't help 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:: 157 to force it to run without the cache::
158 158
159 host:~# echo /dev/sdb1 > /sys/fs/bcach 159 host:~# echo /dev/sdb1 > /sys/fs/bcache/register
160 [ 119.844831] bcache: register_bcache 160 [ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
161 161
162 Next, you try to register your caching device 162 Next, you try to register your caching device if it's present. However
163 if it's absent, or registration fails for some 163 if it's absent, or registration fails for some reason, you can still
164 start your bcache without its cache, like so:: 164 start your bcache without its cache, like so::
165 165
166 host:/sys/block/sdb/sdb1/bcache# echo 166 host:/sys/block/sdb/sdb1/bcache# echo 1 > running
167 167
168 Note that this may cause data loss if you were 168 Note that this may cause data loss if you were running in writeback mode.
169 169
170 170
171 B) Bcache does not find its cache:: 171 B) Bcache does not find its cache::
172 172
173 host:/sys/block/md5/bcache# echo 02265 173 host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
174 [ 1933.455082] bcache: bch_cached_dev_ 174 [ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
175 [ 1933.478179] bcache: __cached_dev_st 175 [ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
176 [ 1933.478179] : cache set not found 176 [ 1933.478179] : cache set not found
177 177
178 In this case, the caching device was simply no 178 In this case, the caching device was simply not registered at boot
179 or disappeared and came back, and needs to be 179 or disappeared and came back, and needs to be (re-)registered::
180 180
181 host:/sys/block/md5/bcache# echo /dev/ 181 host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
182 182
183 183
184 C) Corrupt bcache crashes the kernel at device 184 C) Corrupt bcache crashes the kernel at device registration time:
185 185
186 This should never happen. If it does happen, 186 This should never happen. If it does happen, then you have found a bug!
187 Please report it to the bcache development lis 187 Please report it to the bcache development list: linux-bcache@vger.kernel.org
188 188
189 Be sure to provide as much information that yo 189 Be sure to provide as much information that you can including kernel dmesg
190 output if available so that we may assist. 190 output if available so that we may assist.
191 191
192 192
193 D) Recovering data without bcache: 193 D) Recovering data without bcache:
194 194
195 If bcache is not available in the kernel, a fi 195 If bcache is not available in the kernel, a filesystem on the backing
196 device is still available at an 8KiB offset. S 196 device is still available at an 8KiB offset. So either via a loopdev
197 of the backing device created with --offset 8K 197 of the backing device created with --offset 8K, or any value defined by
198 --data-offset when you originally formatted bc 198 --data-offset when you originally formatted bcache with `bcache make`.
199 199
200 For example:: 200 For example::
201 201
202 losetup -o 8192 /dev/loop0 /dev/your_b 202 losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
203 203
204 This should present your unmodified backing de 204 This should present your unmodified backing device data in /dev/loop0
205 205
206 If your cache is in writethrough mode, then yo 206 If your cache is in writethrough mode, then you can safely discard the
207 cache device without losing data. !! 207 cache device without loosing data.
208 208
209 209
210 E) Wiping a cache device 210 E) Wiping a cache device
211 211
212 :: 212 ::
213 213
214 host:~# wipefs -a /dev/sdh2 214 host:~# wipefs -a /dev/sdh2
215 16 bytes were erased at offset 0x1018 215 16 bytes were erased at offset 0x1018 (bcache)
216 they were: c6 85 73 f6 4e 1a 45 ca 82 216 they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
217 217
218 After you boot back with bcache enabled, you r 218 After you boot back with bcache enabled, you recreate the cache and attach it::
219 219
220 host:~# bcache make -C /dev/sdh2 220 host:~# bcache make -C /dev/sdh2
221 UUID: 7be7e175-8f4c- 221 UUID: 7be7e175-8f4c-4f99-94b2-9c904d227045
222 Set UUID: 5bc072a8-ab17- 222 Set UUID: 5bc072a8-ab17-446d-9744-e247949913c1
223 version: 0 223 version: 0
224 nbuckets: 106874 224 nbuckets: 106874
225 block_size: 1 225 block_size: 1
226 bucket_size: 1024 226 bucket_size: 1024
227 nr_in_set: 1 227 nr_in_set: 1
228 nr_this_dev: 0 228 nr_this_dev: 0
229 first_bucket: 1 229 first_bucket: 1
230 [ 650.511912] bcache: run_cache_set() 230 [ 650.511912] bcache: run_cache_set() invalidating existing data
231 [ 650.549228] bcache: register_cache( 231 [ 650.549228] bcache: register_cache() registered cache device sdh2
232 232
233 start backing device with missing cache:: 233 start backing device with missing cache::
234 234
235 host:/sys/block/md5/bcache# echo 1 > r 235 host:/sys/block/md5/bcache# echo 1 > running
236 236
237 attach new cache:: 237 attach new cache::
238 238
239 host:/sys/block/md5/bcache# echo 5bc07 239 host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach
240 [ 865.276616] bcache: bch_cached_dev_ 240 [ 865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1
241 241
242 242
243 F) Remove or replace a caching device:: 243 F) Remove or replace a caching device::
244 244
245 host:/sys/block/sda/sda7/bcache# echo 245 host:/sys/block/sda/sda7/bcache# echo 1 > detach
246 [ 695.872542] bcache: cached_dev_deta 246 [ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
247 247
248 host:~# wipefs -a /dev/nvme0n1p4 248 host:~# wipefs -a /dev/nvme0n1p4
249 wipefs: error: /dev/nvme0n1p4: probing 249 wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
250 Ooops, it's disabled, but not unregist 250 Ooops, it's disabled, but not unregistered, so it's still protected
251 251
252 We need to go and unregister it:: 252 We need to go and unregister it::
253 253
254 host:/sys/fs/bcache/b7ba27a1-2398-4649 254 host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
255 lrwxrwxrwx 1 root root 0 Feb 25 18:33 255 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 256 host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
257 kernel: [ 917.041908] bcache: cache_s 257 kernel: [ 917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
258 258
259 Now we can wipe it:: 259 Now we can wipe it::
260 260
261 host:~# wipefs -a /dev/nvme0n1p4 261 host:~# wipefs -a /dev/nvme0n1p4
262 /dev/nvme0n1p4: 16 bytes were erased a 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
263 263
264 264
265 G) dm-crypt and bcache 265 G) dm-crypt and bcache
266 266
267 First setup bcache unencrypted and then instal 267 First setup bcache unencrypted and then install dmcrypt on top of
268 /dev/bcache<N> This will work faster than if y 268 /dev/bcache<N> This will work faster than if you dmcrypt both the backing
269 and caching devices and then install bcache on 269 and caching devices and then install bcache on top. [benchmarks?]
270 270
271 271
272 H) Stop/free a registered bcache to wipe and/o 272 H) Stop/free a registered bcache to wipe and/or recreate it
273 273
274 Suppose that you need to free up all bcache re 274 Suppose that you need to free up all bcache references so that you can
275 fdisk run and re-register a changed partition 275 fdisk run and re-register a changed partition table, which won't work
276 if there are any active backing or caching dev 276 if there are any active backing or caching devices left on it:
277 277
278 1) Is it present in /dev/bcache* ? (there are 278 1) Is it present in /dev/bcache* ? (there are times where it won't be)
279 279
280 If so, it's easy:: 280 If so, it's easy::
281 281
282 host:/sys/block/bcache0/bcache# echo 1 282 host:/sys/block/bcache0/bcache# echo 1 > stop
283 283
284 2) But if your backing device is gone, this wo 284 2) But if your backing device is gone, this won't work::
285 285
286 host:/sys/block/bcache0# cd bcache 286 host:/sys/block/bcache0# cd bcache
287 bash: cd: bcache: No such file or dire 287 bash: cd: bcache: No such file or directory
288 288
289 In this case, you may have to unregister th 289 In this case, you may have to unregister the dmcrypt block device that
290 references this bcache to free it up:: 290 references this bcache to free it up::
291 291
292 host:~# dmsetup remove oldds1 292 host:~# dmsetup remove oldds1
293 bcache: bcache_device_free() bcache0 s 293 bcache: bcache_device_free() bcache0 stopped
294 bcache: cache_set_free() Cache set 5bc 294 bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
295 295
296 This causes the backing bcache to be remove 296 This causes the backing bcache to be removed from /sys/fs/bcache and
297 then it can be reused. This would be true 297 then it can be reused. This would be true of any block device stacking
298 where bcache is a lower device. 298 where bcache is a lower device.
299 299
300 3) In other cases, you can also look in /sys/f 300 3) In other cases, you can also look in /sys/fs/bcache/::
301 301
302 host:/sys/fs/bcache# ls -l */{cache?,b 302 host:/sys/fs/bcache# ls -l */{cache?,bdev?}
303 lrwxrwxrwx 1 root root 0 Mar 5 09:39 303 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 09:39 304 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 09:39 305 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/
306 306
307 The device names will show which UUID is re 307 The device names will show which UUID is relevant, cd in that directory
308 and stop the cache:: 308 and stop the cache::
309 309
310 host:/sys/fs/bcache/5bc072a8-ab17-446d 310 host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
311 311
312 This will free up bcache references and let 312 This will free up bcache references and let you reuse the partition for
313 other purposes. 313 other purposes.
314 314
315 315
316 316
317 Troubleshooting performance 317 Troubleshooting performance
318 --------------------------- 318 ---------------------------
319 319
320 Bcache has a bunch of config options and tunab 320 Bcache has a bunch of config options and tunables. The defaults are intended to
321 be reasonable for typical desktop and server w 321 be reasonable for typical desktop and server workloads, but they're not what you
322 want for getting the best possible numbers whe 322 want for getting the best possible numbers when benchmarking.
323 323
324 - Backing device alignment 324 - Backing device alignment
325 325
326 The default metadata size in bcache is 8k. 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 327 RAID based, then be sure to align this by a multiple of your stride
328 width using `bcache make --data-offset`. If 328 width using `bcache make --data-offset`. If you intend to expand your
329 disk array in the future, then multiply a s 329 disk array in the future, then multiply a series of primes by your
330 raid stripe size to get the disk multiples 330 raid stripe size to get the disk multiples that you would like.
331 331
332 For example: If you have a 64k stripe size 332 For example: If you have a 64k stripe size, then the following offset
333 would provide alignment for many common RAI 333 would provide alignment for many common RAID5 data spindle counts::
334 334
335 64k * 2*2*2*3*3*5*7 bytes = 161280k 335 64k * 2*2*2*3*3*5*7 bytes = 161280k
336 336
337 That space is wasted, but for only 157.5MB 337 That space is wasted, but for only 157.5MB you can grow your RAID 5
338 volume to the following data-spindle counts 338 volume to the following data-spindle counts without re-aligning::
339 339
340 3,4,5,6,7,8,9,10,12,14,15,18,20,21 ... 340 3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...
341 341
342 - Bad write performance 342 - Bad write performance
343 343
344 If write performance is not what you expect 344 If write performance is not what you expected, you probably wanted to be
345 running in writeback mode, which isn't the 345 running in writeback mode, which isn't the default (not due to a lack of
346 maturity, but simply because in writeback m 346 maturity, but simply because in writeback mode you'll lose data if something
347 happens to your SSD):: 347 happens to your SSD)::
348 348
349 # echo writeback > /sys/block/bcache0/ 349 # echo writeback > /sys/block/bcache0/bcache/cache_mode
350 350
351 - Bad performance, or traffic not going to th 351 - Bad performance, or traffic not going to the SSD that you'd expect
352 352
353 By default, bcache doesn't cache everything 353 By default, bcache doesn't cache everything. It tries to skip sequential IO -
354 because you really want to be caching the r 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 355 gigabyte file you probably don't want that pushing 10 gigabytes of randomly
356 accessed data out of your cache. 356 accessed data out of your cache.
357 357
358 But if you want to benchmark reads from cac 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 wa 359 writing an 8 gigabyte test file - so you want to disable that::
360 360
361 # echo 0 > /sys/block/bcache0/bcache/s 361 # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
362 362
363 To set it back to the default (4 mb), do:: 363 To set it back to the default (4 mb), do::
364 364
365 # echo 4M > /sys/block/bcache0/bcache/ 365 # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
366 366
367 - Traffic's still going to the spindle/still 367 - Traffic's still going to the spindle/still getting cache misses
368 368
369 In the real world, SSDs don't always keep u 369 In the real world, SSDs don't always keep up with disks - particularly with
370 slower SSDs, many disks being cached by one 370 slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
371 you want to avoid being bottlenecked by the 371 you want to avoid being bottlenecked by the SSD and having it slow everything
372 down. 372 down.
373 373
374 To avoid that bcache tracks latency to the 374 To avoid that bcache tracks latency to the cache device, and gradually
375 throttles traffic if the latency exceeds a 375 throttles traffic if the latency exceeds a threshold (it does this by
376 cranking down the sequential bypass). 376 cranking down the sequential bypass).
377 377
378 You can disable this if you need to by sett 378 You can disable this if you need to by setting the thresholds to 0::
379 379
380 # echo 0 > /sys/fs/bcache/<cache set>/ 380 # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
381 # echo 0 > /sys/fs/bcache/<cache set>/ 381 # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
382 382
383 The default is 2000 us (2 milliseconds) for 383 The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
384 384
385 - Still getting cache misses, of the same dat 385 - Still getting cache misses, of the same data
386 386
387 One last issue that sometimes trips people 387 One last issue that sometimes trips people up is actually an old bug, due to
388 the way cache coherency is handled for cach 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 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. 390 won't be written to the cache.
391 391
392 In practice this isn't an issue because as 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 n 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 394 this to not show up enough to be noticeable (especially since bcache's btree
395 nodes are huge and index large regions of t 395 nodes are huge and index large regions of the device). But when you're
396 benchmarking, if you're trying to warm the 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 397 and there's no other traffic - that can be a problem.
398 398
399 Solution: warm the cache by doing writes, o 399 Solution: warm the cache by doing writes, or use the testing branch (there's
400 a fix for the issue there). 400 a fix for the issue there).
401 401
402 402
403 Sysfs - backing device 403 Sysfs - backing device
404 ---------------------- 404 ----------------------
405 405
406 Available at /sys/block/<bdev>/bcache, /sys/bl 406 Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
407 (if attached) /sys/fs/bcache/<cset-uuid>/bdev* 407 (if attached) /sys/fs/bcache/<cset-uuid>/bdev*
408 408
409 attach 409 attach
410 Echo the UUID of a cache set to this file to 410 Echo the UUID of a cache set to this file to enable caching.
411 411
412 cache_mode 412 cache_mode
413 Can be one of either writethrough, writeback 413 Can be one of either writethrough, writeback, writearound or none.
414 414
415 clear_stats 415 clear_stats
416 Writing to this file resets the running tota 416 Writing to this file resets the running total stats (not the day/hour/5 minute
417 decaying versions). 417 decaying versions).
418 418
419 detach 419 detach
420 Write to this file to detach from a cache se 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. 421 cache, it will be flushed first.
422 422
423 dirty_data 423 dirty_data
424 Amount of dirty data for this backing device 424 Amount of dirty data for this backing device in the cache. Continuously
425 updated unlike the cache set's version, but 425 updated unlike the cache set's version, but may be slightly off.
426 426
427 label 427 label
428 Name of underlying device. 428 Name of underlying device.
429 429
430 readahead 430 readahead
431 Size of readahead that should be performed. 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 tha 432 1M, it will round cache miss reads up to that size, but without overlapping
433 existing cache entries. 433 existing cache entries.
434 434
435 running 435 running
436 1 if bcache is running (i.e. whether the /de 436 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
437 it's in passthrough mode or caching). 437 it's in passthrough mode or caching).
438 438
439 sequential_cutoff 439 sequential_cutoff
440 A sequential IO will bypass the cache once i 440 A sequential IO will bypass the cache once it passes this threshold; the
441 most recent 128 IOs are tracked so sequentia 441 most recent 128 IOs are tracked so sequential IO can be detected even when
442 it isn't all done at once. 442 it isn't all done at once.
443 443
444 sequential_merge 444 sequential_merge
445 If non zero, bcache keeps a list of the last 445 If non zero, bcache keeps a list of the last 128 requests submitted to compare
446 against all new requests to determine which 446 against all new requests to determine which new requests are sequential
447 continuations of previous requests for the p 447 continuations of previous requests for the purpose of determining sequential
448 cutoff. This is necessary if the sequential 448 cutoff. This is necessary if the sequential cutoff value is greater than the
449 maximum acceptable sequential size for any s 449 maximum acceptable sequential size for any single request.
450 450
451 state 451 state
452 The backing device can be in one of four dif 452 The backing device can be in one of four different states:
453 453
454 no cache: Has never been attached to a cache 454 no cache: Has never been attached to a cache set.
455 455
456 clean: Part of a cache set, and there is no 456 clean: Part of a cache set, and there is no cached dirty data.
457 457
458 dirty: Part of a cache set, and there is cac 458 dirty: Part of a cache set, and there is cached dirty data.
459 459
460 inconsistent: The backing device was forcibl 460 inconsistent: The backing device was forcibly run by the user when there was
461 dirty data cached but the cache set was unav 461 dirty data cached but the cache set was unavailable; whatever data was on the
462 backing device has likely been corrupted. 462 backing device has likely been corrupted.
463 463
464 stop 464 stop
465 Write to this file to shut down the bcache d 465 Write to this file to shut down the bcache device and close the backing
466 device. 466 device.
467 467
468 writeback_delay 468 writeback_delay
469 When dirty data is written to the cache and 469 When dirty data is written to the cache and it previously did not contain
470 any, waits some number of seconds before ini 470 any, waits some number of seconds before initiating writeback. Defaults to
471 30. 471 30.
472 472
473 writeback_percent 473 writeback_percent
474 If nonzero, bcache tries to keep around this 474 If nonzero, bcache tries to keep around this percentage of the cache dirty by
475 throttling background writeback and using a 475 throttling background writeback and using a PD controller to smoothly adjust
476 the rate. 476 the rate.
477 477
478 writeback_rate 478 writeback_rate
479 Rate in sectors per second - if writeback_pe 479 Rate in sectors per second - if writeback_percent is nonzero, background
480 writeback is throttled to this rate. Continu 480 writeback is throttled to this rate. Continuously adjusted by bcache but may
481 also be set by the user. 481 also be set by the user.
482 482
483 writeback_running 483 writeback_running
484 If off, writeback of dirty data will not tak 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 most 485 still be added to the cache until it is mostly full; only meant for
486 benchmarking. Defaults to on. 486 benchmarking. Defaults to on.
487 487
488 Sysfs - backing device stats 488 Sysfs - backing device stats
489 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 489 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
490 490
491 There are directories with these numbers for a 491 There are directories with these numbers for a running total, as well as
492 versions that decay over the past day, hour an 492 versions that decay over the past day, hour and 5 minutes; they're also
493 aggregated in the cache set directory as well. 493 aggregated in the cache set directory as well.
494 494
495 bypassed 495 bypassed
496 Amount of IO (both reads and writes) that ha 496 Amount of IO (both reads and writes) that has bypassed the cache
497 497
498 cache_hits, cache_misses, cache_hit_ratio 498 cache_hits, cache_misses, cache_hit_ratio
499 Hits and misses are counted per individual I 499 Hits and misses are counted per individual IO as bcache sees them; a
500 partial hit is counted as a miss. 500 partial hit is counted as a miss.
501 501
502 cache_bypass_hits, cache_bypass_misses 502 cache_bypass_hits, cache_bypass_misses
503 Hits and misses for IO that is intended to s 503 Hits and misses for IO that is intended to skip the cache are still counted,
504 but broken out here. 504 but broken out here.
505 505
506 cache_miss_collisions 506 cache_miss_collisions
507 Counts instances where data was going to be 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 508 cache miss, but raced with a write and data was already present (usually 0
509 since the synchronization for cache misses w 509 since the synchronization for cache misses was rewritten)
>> 510
>> 511 cache_readaheads
>> 512 Count of times readahead occurred.
510 513
511 Sysfs - cache set 514 Sysfs - cache set
512 ~~~~~~~~~~~~~~~~~ 515 ~~~~~~~~~~~~~~~~~
513 516
514 Available at /sys/fs/bcache/<cset-uuid> 517 Available at /sys/fs/bcache/<cset-uuid>
515 518
516 average_key_size 519 average_key_size
517 Average data per key in the btree. 520 Average data per key in the btree.
518 521
519 bdev<0..n> 522 bdev<0..n>
520 Symlink to each of the attached backing devi 523 Symlink to each of the attached backing devices.
521 524
522 block_size 525 block_size
523 Block size of the cache devices. 526 Block size of the cache devices.
524 527
525 btree_cache_size 528 btree_cache_size
526 Amount of memory currently used by the btree 529 Amount of memory currently used by the btree cache
527 530
528 bucket_size 531 bucket_size
529 Size of buckets 532 Size of buckets
530 533
531 cache<0..n> 534 cache<0..n>
532 Symlink to each of the cache devices compris 535 Symlink to each of the cache devices comprising this cache set.
533 536
534 cache_available_percent 537 cache_available_percent
535 Percentage of cache device which doesn't con 538 Percentage of cache device which doesn't contain dirty data, and could
536 potentially be used for writeback. This doe 539 potentially be used for writeback. This doesn't mean this space isn't used
537 for clean cached data; the unused statistic 540 for clean cached data; the unused statistic (in priority_stats) is typically
538 much lower. 541 much lower.
539 542
540 clear_stats 543 clear_stats
541 Clears the statistics associated with this c 544 Clears the statistics associated with this cache
542 545
543 dirty_data 546 dirty_data
544 Amount of dirty data is in the cache (update 547 Amount of dirty data is in the cache (updated when garbage collection runs).
545 548
546 flash_vol_create 549 flash_vol_create
547 Echoing a size to this file (in human readab 550 Echoing a size to this file (in human readable units, k/M/G) creates a thinly
548 provisioned volume backed by the cache set. 551 provisioned volume backed by the cache set.
549 552
550 io_error_halflife, io_error_limit 553 io_error_halflife, io_error_limit
551 These determines how many errors we accept b 554 These determines how many errors we accept before disabling the cache.
552 Each error is decayed by the half life (in # 555 Each error is decayed by the half life (in # ios). If the decaying count
553 reaches io_error_limit dirty data is written 556 reaches io_error_limit dirty data is written out and the cache is disabled.
554 557
555 journal_delay_ms 558 journal_delay_ms
556 Journal writes will delay for up to this man 559 Journal writes will delay for up to this many milliseconds, unless a cache
557 flush happens sooner. Defaults to 100. 560 flush happens sooner. Defaults to 100.
558 561
559 root_usage_percent 562 root_usage_percent
560 Percentage of the root btree node in use. I 563 Percentage of the root btree node in use. If this gets too high the node
561 will split, increasing the tree depth. 564 will split, increasing the tree depth.
562 565
563 stop 566 stop
564 Write to this file to shut down the cache se 567 Write to this file to shut down the cache set - waits until all attached
565 backing devices have been shut down. 568 backing devices have been shut down.
566 569
567 tree_depth 570 tree_depth
568 Depth of the btree (A single node btree has 571 Depth of the btree (A single node btree has depth 0).
569 572
570 unregister 573 unregister
571 Detaches all backing devices and closes the 574 Detaches all backing devices and closes the cache devices; if dirty data is
572 present it will disable writeback caching an 575 present it will disable writeback caching and wait for it to be flushed.
573 576
574 Sysfs - cache set internal 577 Sysfs - cache set internal
575 ~~~~~~~~~~~~~~~~~~~~~~~~~~ 578 ~~~~~~~~~~~~~~~~~~~~~~~~~~
576 579
577 This directory also exposes timings for a numb 580 This directory also exposes timings for a number of internal operations, with
578 separate files for average duration, average f 581 separate files for average duration, average frequency, last occurrence and max
579 duration: garbage collection, btree read, btre 582 duration: garbage collection, btree read, btree node sorts and btree splits.
580 583
581 active_journal_entries 584 active_journal_entries
582 Number of journal entries that are newer tha 585 Number of journal entries that are newer than the index.
583 586
584 btree_nodes 587 btree_nodes
585 Total nodes in the btree. 588 Total nodes in the btree.
586 589
587 btree_used_percent 590 btree_used_percent
588 Average fraction of btree in use. 591 Average fraction of btree in use.
589 592
590 bset_tree_stats 593 bset_tree_stats
591 Statistics about the auxiliary search trees 594 Statistics about the auxiliary search trees
592 595
593 btree_cache_max_chain 596 btree_cache_max_chain
594 Longest chain in the btree node cache's hash 597 Longest chain in the btree node cache's hash table
595 598
596 cache_read_races 599 cache_read_races
597 Counts instances where while data was being 600 Counts instances where while data was being read from the cache, the bucket
598 was reused and invalidated - i.e. where the 601 was reused and invalidated - i.e. where the pointer was stale after the read
599 completed. When this occurs the data is rere 602 completed. When this occurs the data is reread from the backing device.
600 603
601 trigger_gc 604 trigger_gc
602 Writing to this file forces garbage collecti 605 Writing to this file forces garbage collection to run.
603 606
604 Sysfs - Cache device 607 Sysfs - Cache device
605 ~~~~~~~~~~~~~~~~~~~~ 608 ~~~~~~~~~~~~~~~~~~~~
606 609
607 Available at /sys/block/<cdev>/bcache 610 Available at /sys/block/<cdev>/bcache
608 611
609 block_size 612 block_size
610 Minimum granularity of writes - should match 613 Minimum granularity of writes - should match hardware sector size.
611 614
612 btree_written 615 btree_written
613 Sum of all btree writes, in (kilo/mega/giga) 616 Sum of all btree writes, in (kilo/mega/giga) bytes
614 617
615 bucket_size 618 bucket_size
616 Size of buckets 619 Size of buckets
617 620
618 cache_replacement_policy 621 cache_replacement_policy
619 One of either lru, fifo or random. 622 One of either lru, fifo or random.
620 623
621 discard 624 discard
622 Boolean; if on a discard/TRIM will be issued 625 Boolean; if on a discard/TRIM will be issued to each bucket before it is
623 reused. Defaults to off, since SATA TRIM is 626 reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
624 slow). 627 slow).
625 628
626 freelist_percent 629 freelist_percent
627 Size of the freelist as a percentage of nbuc 630 Size of the freelist as a percentage of nbuckets. Can be written to to
628 increase the number of buckets kept on the f 631 increase the number of buckets kept on the freelist, which lets you
629 artificially reduce the size of the cache at 632 artificially reduce the size of the cache at runtime. Mostly for testing
630 purposes (i.e. testing how different size ca 633 purposes (i.e. testing how different size caches affect your hit rate), but
631 since buckets are discarded when they move o 634 since buckets are discarded when they move on to the freelist will also make
632 the SSD's garbage collection easier by effec 635 the SSD's garbage collection easier by effectively giving it more reserved
633 space. 636 space.
634 637
635 io_errors 638 io_errors
636 Number of errors that have occurred, decayed 639 Number of errors that have occurred, decayed by io_error_halflife.
637 640
638 metadata_written 641 metadata_written
639 Sum of all non data writes (btree writes and 642 Sum of all non data writes (btree writes and all other metadata).
640 643
641 nbuckets 644 nbuckets
642 Total buckets in this cache 645 Total buckets in this cache
643 646
644 priority_stats 647 priority_stats
645 Statistics about how recently data in the ca 648 Statistics about how recently data in the cache has been accessed.
646 This can reveal your working set size. Unus 649 This can reveal your working set size. Unused is the percentage of
647 the cache that doesn't contain any data. Me 650 the cache that doesn't contain any data. Metadata is bcache's
648 metadata overhead. Average is the average p 651 metadata overhead. Average is the average priority of cache buckets.
649 Next is a list of quantiles with the priorit 652 Next is a list of quantiles with the priority threshold of each.
650 653
651 written 654 written
652 Sum of all data that has been written to the 655 Sum of all data that has been written to the cache; comparison with
653 btree_written gives the amount of write infl 656 btree_written gives the amount of write inflation in bcache.
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