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656 lines
24 KiB
656 lines
24 KiB
============================ |
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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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The bcache wiki can be found at: |
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https://bcache.evilpiepirate.org |
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This is the git repository of bcache-tools: |
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https://git.kernel.org/pub/scm/linux/kernel/git/colyli/bcache-tools.git/ |
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The latest bcache kernel code can be found from mainline Linux kernel: |
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https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.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 bcache util 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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bcache make -B /dev/sdb |
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bcache make -C /dev/sdc |
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`bcache make` 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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bcache make -B /dev/sda /dev/sdb -C /dev/sdc |
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If your bcache-tools is not updated to latest version and does not have the |
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unified `bcache` utility, you may use the legacy `make-bcache` utility to format |
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bcache device with same -B and -C parameters. |
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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: [email protected] |
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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 `bcache make`. |
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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:~# bcache make -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 `bcache make --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. |
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dirty: Part of a cache set, and there is cached dirty data. |
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inconsistent: The backing device was forcibly run by the user when there was |
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dirty data cached but the cache set was unavailable; whatever data was on the |
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backing device has likely been corrupted. |
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stop |
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Write to this file to shut down the bcache device and close the backing |
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device. |
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writeback_delay |
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When dirty data is written to the cache and it previously did not contain |
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any, waits some number of seconds before initiating writeback. Defaults to |
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30. |
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writeback_percent |
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If nonzero, bcache tries to keep around this percentage of the cache dirty by |
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throttling background writeback and using a PD controller to smoothly adjust |
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the rate. |
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writeback_rate |
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Rate in sectors per second - if writeback_percent is nonzero, background |
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writeback is throttled to this rate. Continuously adjusted by bcache but may |
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also be set by the user. |
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writeback_running |
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If off, writeback of dirty data will not take place at all. Dirty data will |
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still be added to the cache until it is mostly full; only meant for |
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benchmarking. Defaults to on. |
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Sysfs - backing device stats |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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There are directories with these numbers for a running total, as well as |
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versions that decay over the past day, hour and 5 minutes; they're also |
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aggregated in the cache set directory as well. |
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bypassed |
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Amount of IO (both reads and writes) that has bypassed the cache |
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cache_hits, cache_misses, cache_hit_ratio |
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Hits and misses are counted per individual IO as bcache sees them; a |
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partial hit is counted as a miss. |
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cache_bypass_hits, cache_bypass_misses |
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Hits and misses for IO that is intended to skip the cache are still counted, |
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but broken out here. |
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cache_miss_collisions |
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Counts instances where data was going to be inserted into the cache from a |
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cache miss, but raced with a write and data was already present (usually 0 |
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since the synchronization for cache misses was rewritten) |
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cache_readaheads |
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Count of times readahead occurred. |
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Sysfs - cache set |
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~~~~~~~~~~~~~~~~~ |
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Available at /sys/fs/bcache/<cset-uuid> |
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average_key_size |
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Average data per key in the btree. |
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bdev<0..n> |
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Symlink to each of the attached backing devices. |
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block_size |
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Block size of the cache devices. |
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btree_cache_size |
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Amount of memory currently used by the btree cache |
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bucket_size |
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Size of buckets |
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cache<0..n> |
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Symlink to each of the cache devices comprising this cache set. |
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cache_available_percent |
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Percentage of cache device which doesn't contain dirty data, and could |
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potentially be used for writeback. This doesn't mean this space isn't used |
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for clean cached data; the unused statistic (in priority_stats) is typically |
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much lower. |
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clear_stats |
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Clears the statistics associated with this cache |
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dirty_data |
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Amount of dirty data is in the cache (updated when garbage collection runs). |
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flash_vol_create |
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Echoing a size to this file (in human readable units, k/M/G) creates a thinly |
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provisioned volume backed by the cache set. |
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io_error_halflife, io_error_limit |
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These determines how many errors we accept before disabling the cache. |
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Each error is decayed by the half life (in # ios). If the decaying count |
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reaches io_error_limit dirty data is written out and the cache is disabled. |
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journal_delay_ms |
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Journal writes will delay for up to this many milliseconds, unless a cache |
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flush happens sooner. Defaults to 100. |
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root_usage_percent |
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Percentage of the root btree node in use. If this gets too high the node |
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will split, increasing the tree depth. |
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stop |
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Write to this file to shut down the cache set - waits until all attached |
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backing devices have been shut down. |
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tree_depth |
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Depth of the btree (A single node btree has depth 0). |
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unregister |
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Detaches all backing devices and closes the cache devices; if dirty data is |
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present it will disable writeback caching and wait for it to be flushed. |
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Sysfs - cache set internal |
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~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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This directory also exposes timings for a number of internal operations, with |
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separate files for average duration, average frequency, last occurrence and max |
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duration: garbage collection, btree read, btree node sorts and btree splits. |
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active_journal_entries |
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Number of journal entries that are newer than the index. |
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btree_nodes |
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Total nodes in the btree. |
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btree_used_percent |
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Average fraction of btree in use. |
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bset_tree_stats |
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Statistics about the auxiliary search trees |
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btree_cache_max_chain |
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Longest chain in the btree node cache's hash table |
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cache_read_races |
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Counts instances where while data was being read from the cache, the bucket |
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was reused and invalidated - i.e. where the pointer was stale after the read |
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completed. When this occurs the data is reread from the backing device. |
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trigger_gc |
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Writing to this file forces garbage collection to run. |
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Sysfs - Cache device |
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~~~~~~~~~~~~~~~~~~~~ |
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Available at /sys/block/<cdev>/bcache |
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block_size |
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Minimum granularity of writes - should match hardware sector size. |
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btree_written |
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Sum of all btree writes, in (kilo/mega/giga) bytes |
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bucket_size |
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Size of buckets |
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cache_replacement_policy |
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One of either lru, fifo or random. |
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discard |
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Boolean; if on a discard/TRIM will be issued to each bucket before it is |
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reused. Defaults to off, since SATA TRIM is an unqueued command (and thus |
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slow). |
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freelist_percent |
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Size of the freelist as a percentage of nbuckets. Can be written to to |
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increase the number of buckets kept on the freelist, which lets you |
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artificially reduce the size of the cache at runtime. Mostly for testing |
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purposes (i.e. testing how different size caches affect your hit rate), but |
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since buckets are discarded when they move on to the freelist will also make |
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the SSD's garbage collection easier by effectively giving it more reserved |
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space. |
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io_errors |
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Number of errors that have occurred, decayed by io_error_halflife. |
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metadata_written |
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Sum of all non data writes (btree writes and all other metadata). |
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nbuckets |
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Total buckets in this cache |
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priority_stats |
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Statistics about how recently data in the cache has been accessed. |
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This can reveal your working set size. Unused is the percentage of |
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the cache that doesn't contain any data. Metadata is bcache's |
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metadata overhead. Average is the average priority of cache buckets. |
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Next is a list of quantiles with the priority threshold of each. |
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written |
|
Sum of all data that has been written to the cache; comparison with |
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btree_written gives the amount of write inflation in bcache.
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