- misc. LCD driver documentation (cfag12864b, ks0108).
backlight/
- directory with info on controlling backlights in flat panel displays
-bcache.txt
- - Block-layer cache on fast SSDs to improve slow (raid) I/O performance.
block/
- info on the Block I/O (BIO) layer.
blockdev/
--- /dev/null
+============================
+A block layer cache (bcache)
+============================
+
+Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be
+nice if you could use them as cache... Hence bcache.
+
+Wiki and git repositories are at:
+
+ - http://bcache.evilpiepirate.org
+ - http://evilpiepirate.org/git/linux-bcache.git
+ - http://evilpiepirate.org/git/bcache-tools.git
+
+It's designed around the performance characteristics of SSDs - it only allocates
+in erase block sized buckets, and it uses a hybrid btree/log to track cached
+extents (which can be anywhere from a single sector to the bucket size). It's
+designed to avoid random writes at all costs; it fills up an erase block
+sequentially, then issues a discard before reusing it.
+
+Both writethrough and writeback caching are supported. Writeback defaults to
+off, but can be switched on and off arbitrarily at runtime. Bcache goes to
+great lengths to protect your data - it reliably handles unclean shutdown. (It
+doesn't even have a notion of a clean shutdown; bcache simply doesn't return
+writes as completed until they're on stable storage).
+
+Writeback caching can use most of the cache for buffering writes - writing
+dirty data to the backing device is always done sequentially, scanning from the
+start to the end of the index.
+
+Since random IO is what SSDs excel at, there generally won't be much benefit
+to 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 the
+average is above the cutoff it will skip all IO from that task - instead of
+caching the first 512k after every seek. Backups and large file copies should
+thus entirely bypass the cache.
+
+In the event of a data IO error on the flash it will try to recover by reading
+from disk or invalidating cache entries. For unrecoverable errors (meta data
+or dirty data), caching is automatically disabled; if dirty data was present
+in the cache it first disables writeback caching and waits for all dirty data
+to be flushed.
+
+Getting started:
+You'll need make-bcache from the bcache-tools repository. Both the cache device
+and backing device must be formatted before use::
+
+ make-bcache -B /dev/sdb
+ make-bcache -C /dev/sdc
+
+make-bcache has the ability to format multiple devices at the same time - if
+you format your backing devices and cache device at the same time, you won't
+have to manually attach::
+
+ make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
+
+bcache-tools now ships udev rules, and bcache devices are known to the kernel
+immediately. 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 can
+now format it and use it as normal. But the first time using a new bcache
+device, 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 your
+slow devices as bcache backing devices without a cache, and you can choose to add
+a 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 yet
+but will allow for mirroring of metadata and dirty data in the future. Your new
+cache set shows up as /sys/fs/bcache/<UUID>
+
+Attaching
+---------
+
+After your cache device and backing device are registered, the backing device
+must be attached to your cache set to enable caching. Attaching a backing
+device 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 all
+your 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 - particularly
+important if you have writeback caching turned on.
+
+If you're booting up and your cache device is gone and never coming back, you
+can 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 a
+partition, 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 the
+cache, don't expect the filesystem to be recoverable - you will have massive
+filesystem corruption, though ext4's fsck does work miracles.
+
+Error Handling
+--------------
+
+Bcache tries to transparently handle IO errors to/from the cache device without
+affecting normal operation; if it sees too many errors (the threshold is
+configurable, and defaults to 0) it shuts down the cache device and switches all
+the 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 need
+to 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. However
+if it's absent, or registration fails for some reason, you can still
+start 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 boot
+or 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 dmesg
+output if available so that we may assist.
+
+
+D) Recovering data without bcache:
+
+If bcache is not available in the kernel, a filesystem on the backing
+device is still available at an 8KiB offset. So either via a loopdev
+of the backing device created with --offset 8K, or any value defined by
+--data-offset when you originally formatted bcache with `make-bcache`.
+
+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 the
+cache device without loosing 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:~# make-bcache -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 backing
+and 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 can
+fdisk run and re-register a changed partition table, which won't work
+if 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 to
+be reasonable for typical desktop and server workloads, but they're not what you
+want 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 `make-bcache --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 as
+versions that decay over the past day, hour and 5 minutes; they're also
+aggregated 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.
+
+cache_bypass_hits, cache_bypass_misses
+ Hits and misses for IO that is intended to skip the cache are still counted,
+ but broken out here.
+
+cache_miss_collisions
+ Counts instances where data was going to be inserted into the cache from a
+ cache miss, but raced with a write and data was already present (usually 0
+ since the synchronization for cache misses was rewritten)
+
+cache_readaheads
+ Count of times readahead occurred.
+
+Sysfs - cache set
+~~~~~~~~~~~~~~~~~
+
+Available at /sys/fs/bcache/<cset-uuid>
+
+average_key_size
+ Average data per key in the btree.
+
+bdev<0..n>
+ Symlink to each of the attached backing devices.
+
+block_size
+ Block size of the cache devices.
+
+btree_cache_size
+ Amount of memory currently used by the btree cache
+
+bucket_size
+ Size of buckets
+
+cache<0..n>
+ Symlink to each of the cache devices comprising this cache set.
+
+cache_available_percent
+ Percentage of cache device which doesn't contain dirty data, and could
+ potentially be used for writeback. This doesn't mean this space isn't used
+ for clean cached data; the unused statistic (in priority_stats) is typically
+ much lower.
+
+clear_stats
+ Clears the statistics associated with this cache
+
+dirty_data
+ Amount of dirty data is in the cache (updated when garbage collection runs).
+
+flash_vol_create
+ Echoing a size to this file (in human readable units, k/M/G) creates a thinly
+ provisioned volume backed by the cache set.
+
+io_error_halflife, io_error_limit
+ These determines how many errors we accept before disabling the cache.
+ Each error is decayed by the half life (in # ios). If the decaying count
+ reaches io_error_limit dirty data is written out and the cache is disabled.
+
+journal_delay_ms
+ Journal writes will delay for up to this many milliseconds, unless a cache
+ flush happens sooner. Defaults to 100.
+
+root_usage_percent
+ Percentage of the root btree node in use. If this gets too high the node
+ will split, increasing the tree depth.
+
+stop
+ Write to this file to shut down the cache set - waits until all attached
+ backing devices have been shut down.
+
+tree_depth
+ Depth of the btree (A single node btree has depth 0).
+
+unregister
+ Detaches all backing devices and closes the cache devices; if dirty data is
+ present it will disable writeback caching and wait for it to be flushed.
+
+Sysfs - cache set internal
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This directory also exposes timings for a number of internal operations, with
+separate files for average duration, average frequency, last occurrence and max
+duration: garbage collection, btree read, btree node sorts and btree splits.
+
+active_journal_entries
+ Number of journal entries that are newer than the index.
+
+btree_nodes
+ Total nodes in the btree.
+
+btree_used_percent
+ Average fraction of btree in use.
+
+bset_tree_stats
+ Statistics about the auxiliary search trees
+
+btree_cache_max_chain
+ Longest chain in the btree node cache's hash table
+
+cache_read_races
+ Counts instances where while data was being read from the cache, the bucket
+ was reused and invalidated - i.e. where the pointer was stale after the read
+ completed. When this occurs the data is reread from the backing device.
+
+trigger_gc
+ Writing to this file forces garbage collection to run.
+
+Sysfs - Cache device
+~~~~~~~~~~~~~~~~~~~~
+
+Available at /sys/block/<cdev>/bcache
+
+block_size
+ Minimum granularity of writes - should match hardware sector size.
+
+btree_written
+ Sum of all btree writes, in (kilo/mega/giga) bytes
+
+bucket_size
+ Size of buckets
+
+cache_replacement_policy
+ One of either lru, fifo or random.
+
+discard
+ Boolean; if on a discard/TRIM will be issued to each bucket before it is
+ reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
+ slow).
+
+freelist_percent
+ Size of the freelist as a percentage of nbuckets. Can be written to to
+ increase the number of buckets kept on the freelist, which lets you
+ artificially reduce the size of the cache at runtime. Mostly for testing
+ purposes (i.e. testing how different size caches affect your hit rate), but
+ since buckets are discarded when they move on to the freelist will also make
+ the SSD's garbage collection easier by effectively giving it more reserved
+ space.
+
+io_errors
+ Number of errors that have occurred, decayed by io_error_halflife.
+
+metadata_written
+ Sum of all non data writes (btree writes and all other metadata).
+
+nbuckets
+ Total buckets in this cache
+
+priority_stats
+ Statistics about how recently data in the cache has been accessed.
+ This can reveal your working set size. Unused is the percentage of
+ the cache that doesn't contain any data. Metadata is bcache's
+ metadata overhead. Average is the average priority of cache buckets.
+ Next is a list of quantiles with the priority threshold of each.
+
+written
+ Sum of all data that has been written to the cache; comparison with
+ btree_written gives the amount of write inflation in bcache.
mono
java
ras
+ bcache
pm/index
thunderbolt
LSM/index
+++ /dev/null
-============================
-A block layer cache (bcache)
-============================
-
-Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be
-nice if you could use them as cache... Hence bcache.
-
-Wiki and git repositories are at:
-
- - http://bcache.evilpiepirate.org
- - http://evilpiepirate.org/git/linux-bcache.git
- - http://evilpiepirate.org/git/bcache-tools.git
-
-It's designed around the performance characteristics of SSDs - it only allocates
-in erase block sized buckets, and it uses a hybrid btree/log to track cached
-extents (which can be anywhere from a single sector to the bucket size). It's
-designed to avoid random writes at all costs; it fills up an erase block
-sequentially, then issues a discard before reusing it.
-
-Both writethrough and writeback caching are supported. Writeback defaults to
-off, but can be switched on and off arbitrarily at runtime. Bcache goes to
-great lengths to protect your data - it reliably handles unclean shutdown. (It
-doesn't even have a notion of a clean shutdown; bcache simply doesn't return
-writes as completed until they're on stable storage).
-
-Writeback caching can use most of the cache for buffering writes - writing
-dirty data to the backing device is always done sequentially, scanning from the
-start to the end of the index.
-
-Since random IO is what SSDs excel at, there generally won't be much benefit
-to 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 the
-average is above the cutoff it will skip all IO from that task - instead of
-caching the first 512k after every seek. Backups and large file copies should
-thus entirely bypass the cache.
-
-In the event of a data IO error on the flash it will try to recover by reading
-from disk or invalidating cache entries. For unrecoverable errors (meta data
-or dirty data), caching is automatically disabled; if dirty data was present
-in the cache it first disables writeback caching and waits for all dirty data
-to be flushed.
-
-Getting started:
-You'll need make-bcache from the bcache-tools repository. Both the cache device
-and backing device must be formatted before use::
-
- make-bcache -B /dev/sdb
- make-bcache -C /dev/sdc
-
-make-bcache has the ability to format multiple devices at the same time - if
-you format your backing devices and cache device at the same time, you won't
-have to manually attach::
-
- make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
-
-bcache-tools now ships udev rules, and bcache devices are known to the kernel
-immediately. 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 can
-now format it and use it as normal. But the first time using a new bcache
-device, 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 your
-slow devices as bcache backing devices without a cache, and you can choose to add
-a 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 yet
-but will allow for mirroring of metadata and dirty data in the future. Your new
-cache set shows up as /sys/fs/bcache/<UUID>
-
-Attaching
----------
-
-After your cache device and backing device are registered, the backing device
-must be attached to your cache set to enable caching. Attaching a backing
-device 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 all
-your 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 - particularly
-important if you have writeback caching turned on.
-
-If you're booting up and your cache device is gone and never coming back, you
-can 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 a
-partition, 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 the
-cache, don't expect the filesystem to be recoverable - you will have massive
-filesystem corruption, though ext4's fsck does work miracles.
-
-Error Handling
---------------
-
-Bcache tries to transparently handle IO errors to/from the cache device without
-affecting normal operation; if it sees too many errors (the threshold is
-configurable, and defaults to 0) it shuts down the cache device and switches all
-the 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 need
-to 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. However
-if it's absent, or registration fails for some reason, you can still
-start 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 boot
-or 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 dmesg
-output if available so that we may assist.
-
-
-D) Recovering data without bcache:
-
-If bcache is not available in the kernel, a filesystem on the backing
-device is still available at an 8KiB offset. So either via a loopdev
-of the backing device created with --offset 8K, or any value defined by
---data-offset when you originally formatted bcache with `make-bcache`.
-
-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 the
-cache device without loosing 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:~# make-bcache -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 backing
-and 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 can
-fdisk run and re-register a changed partition table, which won't work
-if 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 to
-be reasonable for typical desktop and server workloads, but they're not what you
-want 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 `make-bcache --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 as
-versions that decay over the past day, hour and 5 minutes; they're also
-aggregated 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.
-
-cache_bypass_hits, cache_bypass_misses
- Hits and misses for IO that is intended to skip the cache are still counted,
- but broken out here.
-
-cache_miss_collisions
- Counts instances where data was going to be inserted into the cache from a
- cache miss, but raced with a write and data was already present (usually 0
- since the synchronization for cache misses was rewritten)
-
-cache_readaheads
- Count of times readahead occurred.
-
-Sysfs - cache set
-~~~~~~~~~~~~~~~~~
-
-Available at /sys/fs/bcache/<cset-uuid>
-
-average_key_size
- Average data per key in the btree.
-
-bdev<0..n>
- Symlink to each of the attached backing devices.
-
-block_size
- Block size of the cache devices.
-
-btree_cache_size
- Amount of memory currently used by the btree cache
-
-bucket_size
- Size of buckets
-
-cache<0..n>
- Symlink to each of the cache devices comprising this cache set.
-
-cache_available_percent
- Percentage of cache device which doesn't contain dirty data, and could
- potentially be used for writeback. This doesn't mean this space isn't used
- for clean cached data; the unused statistic (in priority_stats) is typically
- much lower.
-
-clear_stats
- Clears the statistics associated with this cache
-
-dirty_data
- Amount of dirty data is in the cache (updated when garbage collection runs).
-
-flash_vol_create
- Echoing a size to this file (in human readable units, k/M/G) creates a thinly
- provisioned volume backed by the cache set.
-
-io_error_halflife, io_error_limit
- These determines how many errors we accept before disabling the cache.
- Each error is decayed by the half life (in # ios). If the decaying count
- reaches io_error_limit dirty data is written out and the cache is disabled.
-
-journal_delay_ms
- Journal writes will delay for up to this many milliseconds, unless a cache
- flush happens sooner. Defaults to 100.
-
-root_usage_percent
- Percentage of the root btree node in use. If this gets too high the node
- will split, increasing the tree depth.
-
-stop
- Write to this file to shut down the cache set - waits until all attached
- backing devices have been shut down.
-
-tree_depth
- Depth of the btree (A single node btree has depth 0).
-
-unregister
- Detaches all backing devices and closes the cache devices; if dirty data is
- present it will disable writeback caching and wait for it to be flushed.
-
-Sysfs - cache set internal
-~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-This directory also exposes timings for a number of internal operations, with
-separate files for average duration, average frequency, last occurrence and max
-duration: garbage collection, btree read, btree node sorts and btree splits.
-
-active_journal_entries
- Number of journal entries that are newer than the index.
-
-btree_nodes
- Total nodes in the btree.
-
-btree_used_percent
- Average fraction of btree in use.
-
-bset_tree_stats
- Statistics about the auxiliary search trees
-
-btree_cache_max_chain
- Longest chain in the btree node cache's hash table
-
-cache_read_races
- Counts instances where while data was being read from the cache, the bucket
- was reused and invalidated - i.e. where the pointer was stale after the read
- completed. When this occurs the data is reread from the backing device.
-
-trigger_gc
- Writing to this file forces garbage collection to run.
-
-Sysfs - Cache device
-~~~~~~~~~~~~~~~~~~~~
-
-Available at /sys/block/<cdev>/bcache
-
-block_size
- Minimum granularity of writes - should match hardware sector size.
-
-btree_written
- Sum of all btree writes, in (kilo/mega/giga) bytes
-
-bucket_size
- Size of buckets
-
-cache_replacement_policy
- One of either lru, fifo or random.
-
-discard
- Boolean; if on a discard/TRIM will be issued to each bucket before it is
- reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
- slow).
-
-freelist_percent
- Size of the freelist as a percentage of nbuckets. Can be written to to
- increase the number of buckets kept on the freelist, which lets you
- artificially reduce the size of the cache at runtime. Mostly for testing
- purposes (i.e. testing how different size caches affect your hit rate), but
- since buckets are discarded when they move on to the freelist will also make
- the SSD's garbage collection easier by effectively giving it more reserved
- space.
-
-io_errors
- Number of errors that have occurred, decayed by io_error_halflife.
-
-metadata_written
- Sum of all non data writes (btree writes and all other metadata).
-
-nbuckets
- Total buckets in this cache
-
-priority_stats
- Statistics about how recently data in the cache has been accessed.
- This can reveal your working set size. Unused is the percentage of
- the cache that doesn't contain any data. Metadata is bcache's
- metadata overhead. Average is the average priority of cache buckets.
- Next is a list of quantiles with the priority threshold of each.
-
-written
- Sum of all data that has been written to the cache; comparison with
- btree_written gives the amount of write inflation in bcache.
projects / linux.git / commitdiff