Based on kernel version 4.7.2. Page generated on 2016-08-22 22:39 EST.
1 Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be 2 nice if you could use them as cache... Hence bcache. 3 4 Wiki and git repositories are at: 5 http://bcache.evilpiepirate.org 6 http://evilpiepirate.org/git/linux-bcache.git 7 http://evilpiepirate.org/git/bcache-tools.git 8 9 It's designed around the performance characteristics of SSDs - it only allocates 10 in erase block sized buckets, and it uses a hybrid btree/log to track cached 11 extants (which can be anywhere from a single sector to the bucket size). It's 12 designed to avoid random writes at all costs; it fills up an erase block 13 sequentially, then issues a discard before reusing it. 14 15 Both writethrough and writeback caching are supported. Writeback defaults to 16 off, but can be switched on and off arbitrarily at runtime. Bcache goes to 17 great lengths to protect your data - it reliably handles unclean shutdown. (It 18 doesn't even have a notion of a clean shutdown; bcache simply doesn't return 19 writes as completed until they're on stable storage). 20 21 Writeback caching can use most of the cache for buffering writes - writing 22 dirty data to the backing device is always done sequentially, scanning from the 23 start to the end of the index. 24 25 Since random IO is what SSDs excel at, there generally won't be much benefit 26 to caching large sequential IO. Bcache detects sequential IO and skips it; 27 it also keeps a rolling average of the IO sizes per task, and as long as the 28 average is above the cutoff it will skip all IO from that task - instead of 29 caching the first 512k after every seek. Backups and large file copies should 30 thus entirely bypass the cache. 31 32 In the event of a data IO error on the flash it will try to recover by reading 33 from disk or invalidating cache entries. For unrecoverable errors (meta data 34 or dirty data), caching is automatically disabled; if dirty data was present 35 in the cache it first disables writeback caching and waits for all dirty data 36 to be flushed. 37 38 Getting started: 39 You'll need make-bcache from the bcache-tools repository. Both the cache device 40 and backing device must be formatted before use. 41 make-bcache -B /dev/sdb 42 make-bcache -C /dev/sdc 43 44 make-bcache has the ability to format multiple devices at the same time - if 45 you format your backing devices and cache device at the same time, you won't 46 have to manually attach: 47 make-bcache -B /dev/sda /dev/sdb -C /dev/sdc 48 49 bcache-tools now ships udev rules, and bcache devices are known to the kernel 50 immediately. Without udev, you can manually register devices like this: 51 52 echo /dev/sdb > /sys/fs/bcache/register 53 echo /dev/sdc > /sys/fs/bcache/register 54 55 Registering the backing device makes the bcache device show up in /dev; you can 56 now format it and use it as normal. But the first time using a new bcache 57 device, it'll be running in passthrough mode until you attach it to a cache. 58 See the section on attaching. 59 60 The devices show up as: 61 62 /dev/bcache<N> 63 64 As well as (with udev): 65 66 /dev/bcache/by-uuid/<uuid> 67 /dev/bcache/by-label/<label> 68 69 To get started: 70 71 mkfs.ext4 /dev/bcache0 72 mount /dev/bcache0 /mnt 73 74 You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache . 75 76 Cache devices are managed as sets; multiple caches per set isn't supported yet 77 but will allow for mirroring of metadata and dirty data in the future. Your new 78 cache set shows up as /sys/fs/bcache/<UUID> 79 80 ATTACHING: 81 82 After your cache device and backing device are registered, the backing device 83 must be attached to your cache set to enable caching. Attaching a backing 84 device to a cache set is done thusly, with the UUID of the cache set in 85 /sys/fs/bcache: 86 87 echo <CSET-UUID> > /sys/block/bcache0/bcache/attach 88 89 This only has to be done once. The next time you reboot, just reregister all 90 your bcache devices. If a backing device has data in a cache somewhere, the 91 /dev/bcache<N> device won't be created until the cache shows up - particularly 92 important if you have writeback caching turned on. 93 94 If you're booting up and your cache device is gone and never coming back, you 95 can force run the backing device: 96 97 echo 1 > /sys/block/sdb/bcache/running 98 99 (You need to use /sys/block/sdb (or whatever your backing device is called), not 100 /sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a 101 partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache) 102 103 The backing device will still use that cache set if it shows up in the future, 104 but all the cached data will be invalidated. If there was dirty data in the 105 cache, don't expect the filesystem to be recoverable - you will have massive 106 filesystem corruption, though ext4's fsck does work miracles. 107 108 ERROR HANDLING: 109 110 Bcache tries to transparently handle IO errors to/from the cache device without 111 affecting normal operation; if it sees too many errors (the threshold is 112 configurable, and defaults to 0) it shuts down the cache device and switches all 113 the backing devices to passthrough mode. 114 115 - For reads from the cache, if they error we just retry the read from the 116 backing device. 117 118 - For writethrough writes, if the write to the cache errors we just switch to 119 invalidating the data at that lba in the cache (i.e. the same thing we do for 120 a write that bypasses the cache) 121 122 - For writeback writes, we currently pass that error back up to the 123 filesystem/userspace. This could be improved - we could retry it as a write 124 that skips the cache so we don't have to error the write. 125 126 - When we detach, we first try to flush any dirty data (if we were running in 127 writeback mode). It currently doesn't do anything intelligent if it fails to 128 read some of the dirty data, though. 129 130 TROUBLESHOOTING PERFORMANCE: 131 132 Bcache has a bunch of config options and tunables. The defaults are intended to 133 be reasonable for typical desktop and server workloads, but they're not what you 134 want for getting the best possible numbers when benchmarking. 135 136 - Bad write performance 137 138 If write performance is not what you expected, you probably wanted to be 139 running in writeback mode, which isn't the default (not due to a lack of 140 maturity, but simply because in writeback mode you'll lose data if something 141 happens to your SSD) 142 143 # echo writeback > /sys/block/bcache0/cache_mode 144 145 - Bad performance, or traffic not going to the SSD that you'd expect 146 147 By default, bcache doesn't cache everything. It tries to skip sequential IO - 148 because you really want to be caching the random IO, and if you copy a 10 149 gigabyte file you probably don't want that pushing 10 gigabytes of randomly 150 accessed data out of your cache. 151 152 But if you want to benchmark reads from cache, and you start out with fio 153 writing an 8 gigabyte test file - so you want to disable that. 154 155 # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff 156 157 To set it back to the default (4 mb), do 158 159 # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff 160 161 - Traffic's still going to the spindle/still getting cache misses 162 163 In the real world, SSDs don't always keep up with disks - particularly with 164 slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So 165 you want to avoid being bottlenecked by the SSD and having it slow everything 166 down. 167 168 To avoid that bcache tracks latency to the cache device, and gradually 169 throttles traffic if the latency exceeds a threshold (it does this by 170 cranking down the sequential bypass). 171 172 You can disable this if you need to by setting the thresholds to 0: 173 174 # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us 175 # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us 176 177 The default is 2000 us (2 milliseconds) for reads, and 20000 for writes. 178 179 - Still getting cache misses, of the same data 180 181 One last issue that sometimes trips people up is actually an old bug, due to 182 the way cache coherency is handled for cache misses. If a btree node is full, 183 a cache miss won't be able to insert a key for the new data and the data 184 won't be written to the cache. 185 186 In practice this isn't an issue because as soon as a write comes along it'll 187 cause the btree node to be split, and you need almost no write traffic for 188 this to not show up enough to be noticeable (especially since bcache's btree 189 nodes are huge and index large regions of the device). But when you're 190 benchmarking, if you're trying to warm the cache by reading a bunch of data 191 and there's no other traffic - that can be a problem. 192 193 Solution: warm the cache by doing writes, or use the testing branch (there's 194 a fix for the issue there). 195 196 SYSFS - BACKING DEVICE: 197 198 Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and 199 (if attached) /sys/fs/bcache/<cset-uuid>/bdev* 200 201 attach 202 Echo the UUID of a cache set to this file to enable caching. 203 204 cache_mode 205 Can be one of either writethrough, writeback, writearound or none. 206 207 clear_stats 208 Writing to this file resets the running total stats (not the day/hour/5 minute 209 decaying versions). 210 211 detach 212 Write to this file to detach from a cache set. If there is dirty data in the 213 cache, it will be flushed first. 214 215 dirty_data 216 Amount of dirty data for this backing device in the cache. Continuously 217 updated unlike the cache set's version, but may be slightly off. 218 219 label 220 Name of underlying device. 221 222 readahead 223 Size of readahead that should be performed. Defaults to 0. If set to e.g. 224 1M, it will round cache miss reads up to that size, but without overlapping 225 existing cache entries. 226 227 running 228 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether 229 it's in passthrough mode or caching). 230 231 sequential_cutoff 232 A sequential IO will bypass the cache once it passes this threshold; the 233 most recent 128 IOs are tracked so sequential IO can be detected even when 234 it isn't all done at once. 235 236 sequential_merge 237 If non zero, bcache keeps a list of the last 128 requests submitted to compare 238 against all new requests to determine which new requests are sequential 239 continuations of previous requests for the purpose of determining sequential 240 cutoff. This is necessary if the sequential cutoff value is greater than the 241 maximum acceptable sequential size for any single request. 242 243 state 244 The backing device can be in one of four different states: 245 246 no cache: Has never been attached to a cache set. 247 248 clean: Part of a cache set, and there is no cached dirty data. 249 250 dirty: Part of a cache set, and there is cached dirty data. 251 252 inconsistent: The backing device was forcibly run by the user when there was 253 dirty data cached but the cache set was unavailable; whatever data was on the 254 backing device has likely been corrupted. 255 256 stop 257 Write to this file to shut down the bcache device and close the backing 258 device. 259 260 writeback_delay 261 When dirty data is written to the cache and it previously did not contain 262 any, waits some number of seconds before initiating writeback. Defaults to 263 30. 264 265 writeback_percent 266 If nonzero, bcache tries to keep around this percentage of the cache dirty by 267 throttling background writeback and using a PD controller to smoothly adjust 268 the rate. 269 270 writeback_rate 271 Rate in sectors per second - if writeback_percent is nonzero, background 272 writeback is throttled to this rate. Continuously adjusted by bcache but may 273 also be set by the user. 274 275 writeback_running 276 If off, writeback of dirty data will not take place at all. Dirty data will 277 still be added to the cache until it is mostly full; only meant for 278 benchmarking. Defaults to on. 279 280 SYSFS - BACKING DEVICE STATS: 281 282 There are directories with these numbers for a running total, as well as 283 versions that decay over the past day, hour and 5 minutes; they're also 284 aggregated in the cache set directory as well. 285 286 bypassed 287 Amount of IO (both reads and writes) that has bypassed the cache 288 289 cache_hits 290 cache_misses 291 cache_hit_ratio 292 Hits and misses are counted per individual IO as bcache sees them; a 293 partial hit is counted as a miss. 294 295 cache_bypass_hits 296 cache_bypass_misses 297 Hits and misses for IO that is intended to skip the cache are still counted, 298 but broken out here. 299 300 cache_miss_collisions 301 Counts instances where data was going to be inserted into the cache from a 302 cache miss, but raced with a write and data was already present (usually 0 303 since the synchronization for cache misses was rewritten) 304 305 cache_readaheads 306 Count of times readahead occurred. 307 308 SYSFS - CACHE SET: 309 310 Available at /sys/fs/bcache/<cset-uuid> 311 312 average_key_size 313 Average data per key in the btree. 314 315 bdev<0..n> 316 Symlink to each of the attached backing devices. 317 318 block_size 319 Block size of the cache devices. 320 321 btree_cache_size 322 Amount of memory currently used by the btree cache 323 324 bucket_size 325 Size of buckets 326 327 cache<0..n> 328 Symlink to each of the cache devices comprising this cache set. 329 330 cache_available_percent 331 Percentage of cache device which doesn't contain dirty data, and could 332 potentially be used for writeback. This doesn't mean this space isn't used 333 for clean cached data; the unused statistic (in priority_stats) is typically 334 much lower. 335 336 clear_stats 337 Clears the statistics associated with this cache 338 339 dirty_data 340 Amount of dirty data is in the cache (updated when garbage collection runs). 341 342 flash_vol_create 343 Echoing a size to this file (in human readable units, k/M/G) creates a thinly 344 provisioned volume backed by the cache set. 345 346 io_error_halflife 347 io_error_limit 348 These determines how many errors we accept before disabling the cache. 349 Each error is decayed by the half life (in # ios). If the decaying count 350 reaches io_error_limit dirty data is written out and the cache is disabled. 351 352 journal_delay_ms 353 Journal writes will delay for up to this many milliseconds, unless a cache 354 flush happens sooner. Defaults to 100. 355 356 root_usage_percent 357 Percentage of the root btree node in use. If this gets too high the node 358 will split, increasing the tree depth. 359 360 stop 361 Write to this file to shut down the cache set - waits until all attached 362 backing devices have been shut down. 363 364 tree_depth 365 Depth of the btree (A single node btree has depth 0). 366 367 unregister 368 Detaches all backing devices and closes the cache devices; if dirty data is 369 present it will disable writeback caching and wait for it to be flushed. 370 371 SYSFS - CACHE SET INTERNAL: 372 373 This directory also exposes timings for a number of internal operations, with 374 separate files for average duration, average frequency, last occurrence and max 375 duration: garbage collection, btree read, btree node sorts and btree splits. 376 377 active_journal_entries 378 Number of journal entries that are newer than the index. 379 380 btree_nodes 381 Total nodes in the btree. 382 383 btree_used_percent 384 Average fraction of btree in use. 385 386 bset_tree_stats 387 Statistics about the auxiliary search trees 388 389 btree_cache_max_chain 390 Longest chain in the btree node cache's hash table 391 392 cache_read_races 393 Counts instances where while data was being read from the cache, the bucket 394 was reused and invalidated - i.e. where the pointer was stale after the read 395 completed. When this occurs the data is reread from the backing device. 396 397 trigger_gc 398 Writing to this file forces garbage collection to run. 399 400 SYSFS - CACHE DEVICE: 401 402 Available at /sys/block/<cdev>/bcache 403 404 block_size 405 Minimum granularity of writes - should match hardware sector size. 406 407 btree_written 408 Sum of all btree writes, in (kilo/mega/giga) bytes 409 410 bucket_size 411 Size of buckets 412 413 cache_replacement_policy 414 One of either lru, fifo or random. 415 416 discard 417 Boolean; if on a discard/TRIM will be issued to each bucket before it is 418 reused. Defaults to off, since SATA TRIM is an unqueued command (and thus 419 slow). 420 421 freelist_percent 422 Size of the freelist as a percentage of nbuckets. Can be written to to 423 increase the number of buckets kept on the freelist, which lets you 424 artificially reduce the size of the cache at runtime. Mostly for testing 425 purposes (i.e. testing how different size caches affect your hit rate), but 426 since buckets are discarded when they move on to the freelist will also make 427 the SSD's garbage collection easier by effectively giving it more reserved 428 space. 429 430 io_errors 431 Number of errors that have occurred, decayed by io_error_halflife. 432 433 metadata_written 434 Sum of all non data writes (btree writes and all other metadata). 435 436 nbuckets 437 Total buckets in this cache 438 439 priority_stats 440 Statistics about how recently data in the cache has been accessed. 441 This can reveal your working set size. Unused is the percentage of 442 the cache that doesn't contain any data. Metadata is bcache's 443 metadata overhead. Average is the average priority of cache buckets. 444 Next is a list of quantiles with the priority threshold of each. 445 446 written 447 Sum of all data that has been written to the cache; comparison with 448 btree_written gives the amount of write inflation in bcache.