============================A block layer cache (bcache)============================ Say you've got a big slow raid 6, and an ssd or three. Wouldn't it benice if you could use them as cache... Hence bcache. The bcache wiki can be found at:  https://bcache.evilpiepirate.org This is the git repository of bcache-tools:  https://git.kernel.org/pub/scm/linux/kernel/git/colyli/bcache-tools.git/ The latest bcache kernel code can be found from mainline Linux kernel:  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ It's designed around the performance characteristics of SSDs - it only allocatesin erase block sized buckets, and it uses a hybrid btree/log to track cachedextents (which can be anywhere from a single sector to the bucket size). It'sdesigned to avoid random writes at all costs. Both writethrough and writeback caching are supported. Writeback defaults tooff, but can be switched on and off arbitrarily at runtime. Bcache goes togreat lengths to protect your data - it reliably handles unclean shutdown. (Itdoesn't even have a notion of a clean shutdown; bcache simply doesn't returnwrites as completed until they're on stable storage). Writeback caching can use most of the cache for buffering writes - writingdirty data to the backing device is always done sequentially, scanning from thestart to the end of the index. Since random IO is what SSDs excel at, there generally won't be much benefitto caching large sequential IO. Bcache detects sequential IO and skips it;it also keeps a rolling average of the IO sizes per task, and as long as theaverage is above the cutoff it will skip all IO from that task - instead ofcaching the first 512k after every seek. Backups and large file copies shouldthus entirely bypass the cache. In the event of a data IO error on the flash it will try to recover by readingfrom disk or invalidating cache entries.  For unrecoverable errors (meta dataor dirty data), caching is automatically disabled; if dirty data was presentin the cache it first disables writeback caching and waits for all dirty datato be flushed. Getting started:You'll need bcache util from the bcache-tools repository. Both the cache deviceand backing device must be formatted before use::   bcache make -B /dev/sdb  bcache make -C /dev/sdc `bcache make` has the ability to format multiple devices at the same time - ifyou format your backing devices and cache device at the same time, you won'thave to manually attach::   bcache make -B /dev/sda /dev/sdb -C /dev/sdc If your bcache-tools is not updated to latest version and does not have theunified `bcache` utility, you may use the legacy `make-bcache` utility to formatbcache device with same -B and -C parameters. bcache-tools now ships udev rules, and bcache devices are known to the kernelimmediately.  Without udev, you can manually register devices like this::   echo /dev/sdb > /sys/fs/bcache/register  echo /dev/sdc > /sys/fs/bcache/register Registering the backing device makes the bcache device show up in /dev; you cannow format it and use it as normal. But the first time using a new bcachedevice, it'll be running in passthrough mode until you attach it to a cache.If you are thinking about using bcache later, it is recommended to setup all yourslow devices as bcache backing devices without a cache, and you can choose to adda caching device later.See 'ATTACHING' section below. The devices show up as::   /dev/bcache<N> As well as (with udev)::   /dev/bcache/by-uuid/<uuid>  /dev/bcache/by-label/<label> To get started::   mkfs.ext4 /dev/bcache0  mount /dev/bcache0 /mnt You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .You can also control them through /sys/fs//bcache/<cset-uuid>/ . Cache devices are managed as sets; multiple caches per set isn't supported yetbut will allow for mirroring of metadata and dirty data in the future. Your newcache set shows up as /sys/fs/bcache/<UUID> Attaching--------- After your cache device and backing device are registered, the backing devicemust be attached to your cache set to enable caching. Attaching a backingdevice to a cache set is done thusly, with the UUID of the cache set in/sys/fs/bcache::   echo <CSET-UUID> > /sys/block/bcache0/bcache/attach This only has to be done once. The next time you reboot, just reregister allyour bcache devices. If a backing device has data in a cache somewhere, the/dev/bcache<N> device won't be created until the cache shows up - particularlyimportant if you have writeback caching turned on. If you're booting up and your cache device is gone and never coming back, youcan force run the backing device::   echo 1 > /sys/block/sdb/bcache/running (You need to use /sys/block/sdb (or whatever your backing device is called), not/sys/block/bcache0, because bcache0 doesn't exist yet. If you're using apartition, the bcache directory would be at /sys/block/sdb/sdb2/bcache) The backing device will still use that cache set if it shows up in the future,but all the cached data will be invalidated. If there was dirty data in thecache, don't expect the filesystem to be recoverable - you will have massivefilesystem corruption, though ext4's fsck does work miracles. Error Handling-------------- Bcache tries to transparently handle IO errors to/from the cache device withoutaffecting normal operation; if it sees too many errors (the threshold isconfigurable, and defaults to 0) it shuts down the cache device and switches allthe backing devices to passthrough mode.  - For reads from the cache, if they error we just retry the read from the   backing device.  - For writethrough writes, if the write to the cache errors we just switch to   invalidating the data at that lba in the cache (i.e. the same thing we do for   a write that bypasses the cache)  - For writeback writes, we currently pass that error back up to the   filesystem/userspace. This could be improved - we could retry it as a write   that skips the cache so we don't have to error the write.  - When we detach, we first try to flush any dirty data (if we were running in   writeback mode). It currently doesn't do anything intelligent if it fails to   read some of the dirty data, though.  Howto/cookbook-------------- A) Starting a bcache with a missing caching device If registering the backing device doesn't help, it's already there, you just needto force it to run without the cache:: 	host:~# echo /dev/sdb1 > /sys/fs/bcache/register	[  119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered Next, you try to register your caching device if it's present. Howeverif it's absent, or registration fails for some reason, you can stillstart your bcache without its cache, like so:: 	host:/sys/block/sdb/sdb1/bcache# echo 1 > running Note that this may cause data loss if you were running in writeback mode.  B) Bcache does not find its cache:: 	host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach	[ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set	[ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8	[ 1933.478179] : cache set not found In this case, the caching device was simply not registered at bootor disappeared and came back, and needs to be (re-)registered:: 	host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register  C) Corrupt bcache crashes the kernel at device registration time: This should never happen.  If it does happen, then you have found a bug!Please report it to the bcache development list: linux-bcache@vger.kernel.org Be sure to provide as much information that you can including kernel dmesgoutput if available so that we may assist.  D) Recovering data without bcache: If bcache is not available in the kernel, a filesystem on the backingdevice is still available at an 8KiB offset. So either via a loopdevof the backing device created with --offset 8K, or any value defined by--data-offset when you originally formatted bcache with `bcache make`. For example:: 	losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev This should present your unmodified backing device data in /dev/loop0 If your cache is in writethrough mode, then you can safely discard thecache device without losing data.  E) Wiping a cache device :: 	host:~# wipefs -a /dev/sdh2	16 bytes were erased at offset 0x1018 (bcache)	they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81 After you boot back with bcache enabled, you recreate the cache and attach it:: 	host:~# bcache make -C /dev/sdh2	UUID:                   7be7e175-8f4c-4f99-94b2-9c904d227045	Set UUID:               5bc072a8-ab17-446d-9744-e247949913c1	version:                0	nbuckets:               106874	block_size:             1	bucket_size:            1024	nr_in_set:              1	nr_this_dev:            0	first_bucket:           1	[  650.511912] bcache: run_cache_set() invalidating existing data	[  650.549228] bcache: register_cache() registered cache device sdh2 start backing device with missing cache:: 	host:/sys/block/md5/bcache# echo 1 > running attach new cache:: 	host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach	[  865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1  F) Remove or replace a caching device:: 	host:/sys/block/sda/sda7/bcache# echo 1 > detach	[  695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7 	host:~# wipefs -a /dev/nvme0n1p4	wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy	Ooops, it's disabled, but not unregistered, so it's still protected We need to go and unregister it:: 	host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0	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/	host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop	kernel: [  917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered Now we can wipe it:: 	host:~# wipefs -a /dev/nvme0n1p4	/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  G) dm-crypt and bcache First setup bcache unencrypted and then install dmcrypt on top of/dev/bcache<N> This will work faster than if you dmcrypt both the backingand caching devices and then install bcache on top. [benchmarks?]  H) Stop/free a registered bcache to wipe and/or recreate it Suppose that you need to free up all bcache references so that you canfdisk run and re-register a changed partition table, which won't workif there are any active backing or caching devices left on it: 1) Is it present in /dev/bcache* ? (there are times where it won't be)    If so, it's easy:: 	host:/sys/block/bcache0/bcache# echo 1 > stop 2) But if your backing device is gone, this won't work:: 	host:/sys/block/bcache0# cd bcache	bash: cd: bcache: No such file or directory    In this case, you may have to unregister the dmcrypt block device that   references this bcache to free it up:: 	host:~# dmsetup remove oldds1	bcache: bcache_device_free() bcache0 stopped	bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered    This causes the backing bcache to be removed from /sys/fs/bcache and   then it can be reused.  This would be true of any block device stacking   where bcache is a lower device. 3) In other cases, you can also look in /sys/fs/bcache/:: 	host:/sys/fs/bcache# ls -l */{cache?,bdev?}	lrwxrwxrwx 1 root root 0 Mar  5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/	lrwxrwxrwx 1 root root 0 Mar  5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/	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/    The device names will show which UUID is relevant, cd in that directory   and stop the cache:: 	host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop    This will free up bcache references and let you reuse the partition for   other purposes.   Troubleshooting performance--------------------------- Bcache has a bunch of config options and tunables. The defaults are intended tobe reasonable for typical desktop and server workloads, but they're not what youwant for getting the best possible numbers when benchmarking.  - Backing device alignment    The default metadata size in bcache is 8k.  If your backing device is   RAID based, then be sure to align this by a multiple of your stride   width using `bcache make --data-offset`. If you intend to expand your   disk array in the future, then multiply a series of primes by your   raid stripe size to get the disk multiples that you would like.    For example:  If you have a 64k stripe size, then the following offset   would provide alignment for many common RAID5 data spindle counts:: 	64k * 2*2*2*3*3*5*7 bytes = 161280k    That space is wasted, but for only 157.5MB you can grow your RAID 5   volume to the following data-spindle counts without re-aligning:: 	3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...  - Bad write performance    If write performance is not what you expected, you probably wanted to be   running in writeback mode, which isn't the default (not due to a lack of   maturity, but simply because in writeback mode you'll lose data if something   happens to your SSD):: 	# echo writeback > /sys/block/bcache0/bcache/cache_mode  - Bad performance, or traffic not going to the SSD that you'd expect    By default, bcache doesn't cache everything. It tries to skip sequential IO -   because you really want to be caching the random IO, and if you copy a 10   gigabyte file you probably don't want that pushing 10 gigabytes of randomly   accessed data out of your cache.    But if you want to benchmark reads from cache, and you start out with fio   writing an 8 gigabyte test file - so you want to disable that:: 	# echo 0 > /sys/block/bcache0/bcache/sequential_cutoff    To set it back to the default (4 mb), do:: 	# echo 4M > /sys/block/bcache0/bcache/sequential_cutoff  - Traffic's still going to the spindle/still getting cache misses    In the real world, SSDs don't always keep up with disks - particularly with   slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So   you want to avoid being bottlenecked by the SSD and having it slow everything   down.    To avoid that bcache tracks latency to the cache device, and gradually   throttles traffic if the latency exceeds a threshold (it does this by   cranking down the sequential bypass).    You can disable this if you need to by setting the thresholds to 0:: 	# echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us	# echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us    The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.  - Still getting cache misses, of the same data    One last issue that sometimes trips people up is actually an old bug, due to   the way cache coherency is handled for cache misses. If a btree node is full,   a cache miss won't be able to insert a key for the new data and the data   won't be written to the cache.    In practice this isn't an issue because as soon as a write comes along it'll   cause the btree node to be split, and you need almost no write traffic for   this to not show up enough to be noticeable (especially since bcache's btree   nodes are huge and index large regions of the device). But when you're   benchmarking, if you're trying to warm the cache by reading a bunch of data   and there's no other traffic - that can be a problem.    Solution: warm the cache by doing writes, or use the testing branch (there's   a fix for the issue there).  Sysfs - backing device---------------------- Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and(if attached) /sys/fs/bcache/<cset-uuid>/bdev* attach  Echo the UUID of a cache set to this file to enable caching. cache_mode  Can be one of either writethrough, writeback, writearound or none. clear_stats  Writing to this file resets the running total stats (not the day/hour/5 minute  decaying versions). detach  Write to this file to detach from a cache set. If there is dirty data in the  cache, it will be flushed first. dirty_data  Amount of dirty data for this backing device in the cache. Continuously  updated unlike the cache set's version, but may be slightly off. label  Name of underlying device. readahead  Size of readahead that should be performed.  Defaults to 0.  If set to e.g.  1M, it will round cache miss reads up to that size, but without overlapping  existing cache entries. running  1 if bcache is running (i.e. whether the /dev/bcache device exists, whether  it's in passthrough mode or caching). sequential_cutoff  A sequential IO will bypass the cache once it passes this threshold; the  most recent 128 IOs are tracked so sequential IO can be detected even when  it isn't all done at once. sequential_merge  If non zero, bcache keeps a list of the last 128 requests submitted to compare  against all new requests to determine which new requests are sequential  continuations of previous requests for the purpose of determining sequential  cutoff. This is necessary if the sequential cutoff value is greater than the  maximum acceptable sequential size for any single request. state  The backing device can be in one of four different states:   no cache: Has never been attached to a cache set.   clean: Part of a cache set, and there is no cached dirty data.   dirty: Part of a cache set, and there is cached dirty data.   inconsistent: The backing device was forcibly run by the user when there was  dirty data cached but the cache set was unavailable; whatever data was on the  backing device has likely been corrupted. stop  Write to this file to shut down the bcache device and close the backing  device. writeback_delay  When dirty data is written to the cache and it previously did not contain  any, waits some number of seconds before initiating writeback. Defaults to  30. writeback_percent  If nonzero, bcache tries to keep around this percentage of the cache dirty by  throttling background writeback and using a PD controller to smoothly adjust  the rate. writeback_rate  Rate in sectors per second - if writeback_percent is nonzero, background  writeback is throttled to this rate. Continuously adjusted by bcache but may  also be set by the user. writeback_running  If off, writeback of dirty data will not take place at all. Dirty data will  still be added to the cache until it is mostly full; only meant for  benchmarking. Defaults to on. Sysfs - backing device stats~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There are directories with these numbers for a running total, as well asversions that decay over the past day, hour and 5 minutes; they're alsoaggregated in the cache set directory as well. bypassed  Amount of IO (both reads and writes) that has bypassed the cache cache_hits, cache_misses, cache_hit_ratio  Hits and misses are counted per individual IO as bcache sees them; a  partial hit is counted as a miss. 

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