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Diffstat (limited to 'Documentation/admin-guide/bcache.rst')
| -rw-r--r-- | Documentation/admin-guide/bcache.rst | 49 |
1 files changed, 22 insertions, 27 deletions
diff --git a/Documentation/admin-guide/bcache.rst b/Documentation/admin-guide/bcache.rst index c0ce64d75bbf..f71f349553e4 100644 --- a/Documentation/admin-guide/bcache.rst +++ b/Documentation/admin-guide/bcache.rst @@ -5,17 +5,19 @@ 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: +The bcache wiki can be found at: + https://bcache.evilpiepirate.org - - http://bcache.evilpiepirate.org - - http://evilpiepirate.org/git/linux-bcache.git - - http://evilpiepirate.org/git/bcache-tools.git +This is the git repository of bcache-tools: + https://git.kernel.org/pub/scm/linux/kernel/git/colyli/bcache-tools.git/ + +The latest bcache kernel code can be found from mainline Linux kernel: + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ It's designed around the performance characteristics of SSDs - it only 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. +designed to avoid random writes at all costs. Both writethrough and writeback caching are supported. Writeback defaults to off, but can be switched on and off arbitrarily at runtime. Bcache goes to @@ -41,17 +43,21 @@ 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 +You'll need bcache util 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 + bcache make -B /dev/sdb + bcache make -C /dev/sdc -make-bcache has the ability to format multiple devices at the same time - if +`bcache make` 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 make -B /dev/sda /dev/sdb -C /dev/sdc + +If your bcache-tools is not updated to latest version and does not have the +unified `bcache` utility, you may use the legacy `make-bcache` utility to format +bcache device with same -B and -C parameters. bcache-tools now ships udev rules, and bcache devices are known to the kernel immediately. Without udev, you can manually register devices like this:: @@ -188,7 +194,7 @@ 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`. +--data-offset when you originally formatted bcache with `bcache make`. For example:: @@ -197,7 +203,7 @@ For example:: 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. +cache device without losing data. E) Wiping a cache device @@ -210,7 +216,7 @@ E) Wiping a cache device After you boot back with bcache enabled, you recreate the cache and attach it:: - host:~# make-bcache -C /dev/sdh2 + host:~# bcache make -C /dev/sdh2 UUID: 7be7e175-8f4c-4f99-94b2-9c904d227045 Set UUID: 5bc072a8-ab17-446d-9744-e247949913c1 version: 0 @@ -318,7 +324,7 @@ want for getting the best possible numbers when benchmarking. 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 + width using `bcache make --data-offset`. If you intend to expand your disk array in the future, then multiply a series of primes by your raid stripe size to get the disk multiples that you would like. @@ -501,9 +507,6 @@ cache_miss_collisions 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 ~~~~~~~~~~~~~~~~~ @@ -614,19 +617,11 @@ bucket_size 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. + purposes (i.e. testing how different size caches affect your hit rate). io_errors Number of errors that have occurred, decayed by io_error_halflife. |