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