Messages in this thread Patch in this message | | | Date | Fri, 30 Apr 2010 16:12:44 -0800 | From | Kent Overstreet <> | Subject | [PATCH 1/3] Bcache: version 4 |
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Documentation/bcache.txt | 56 ++++++++++++++++++++++++++++++++++++++++++++++ block/Kconfig | 15 ++++++++++++ 2 files changed, 71 insertions(+), 0 deletions(-)
diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt new file mode 100644 index 0000000..dee1514 --- /dev/null +++ b/Documentation/bcache.txt @@ -0,0 +1,56 @@ +Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be +nice if you could use them as cache... Hence bcache. + +It's designed around the performance characteristics of SSDs - it only allocates +in erase block sized buckets, and it uses a bare minimum btree to track cached +extants (which can be anywhere from a single sector to the bucket size). It's +also designed to be very lazy, and use garbage collection to clean stale +pointers. + +Cache devices are used as a pool, and hold data for all the devices that are +being cached. The cache devices store the UUIDs of devices they have, allowing +caches to safely be persistent across reboots. + +Caching can be transparently enabled and disabled for devices while they are in +use. All configuration is done via sysfs. To use our SSD sde to cache our +raid md1: + + make-bcache /dev/sdc + echo "/dev/sdc" > /sys/kernel/bcache/register_cache + echo "<UUID> /dev/md1 " > /sys/kernel/bcache/register_dev + +And that's it. + +To script the UUID lookup, you could do: + echo "`find /dev/disk/by-uuid/ -lname "*md1"|cut -d/ -f5` /dev/md"\ + > /sys/kernel/bcache/register_dev + +Of course, if you were already referencing your devices by UUID, you could do: + echo "$UUID /dev/disk/by-uiid/$UUID"\ + > /sys/kernel/bcache/register_dev + +There are a number of other files in sysfs, some that provide statistics, +others that allow tweaking of heuristics. Directories are also created +for both cache devices and devices that are being cached, for per device +statistics and device removal. + +Statistics: cache_hits, cache_misses, cache_hit_ratio +These should be fairly obvious, they're simple counters. + +Cache hit heuristics: cache_priority_seek contributes to the new bucket +priority once per cache hit; this lets us bias in favor of random IO. +The file cache_priority_hit is scaled by the size of the cache hit, so +we can give a 128k cache hit a higher weighting than a 4k cache hit. + +When new data is added to the cache, the initial priority is taken from +cache_priority_initial. Every so often, we must rescale the priorities of +all the in use buckets, so that the priority of stale data gradually goes to +zero: this happens every N sectors, taken from cache_priority_rescale. The +rescaling is currently hard coded at priority *= 7/8. + +For cache devices, there are a few more files. Most should be obvious; +min_priority shows the priority of the bucket that will next be pulled off +the heap, and tree_depth shows the current btree height. + +Writing to the unregister file in a device's directory will trigger the +closing of that device. diff --git a/block/Kconfig b/block/Kconfig index f9e89f4..0e5dbf2 100644 --- a/block/Kconfig +++ b/block/Kconfig @@ -100,6 +100,21 @@ config DEBUG_BLK_CGROUP in the blk group which can be used by cfq for tracing various group related activity. +config BLK_CACHE + tristate "Block device as cache" + select SLOW_WORK + default m + ---help--- + Allows a block device to be used as cache for other devices; uses + a btree for indexing and the layout is optimized for SSDs. + + Caches are persistent, and store the UUID of devices they cache. + Hence, to open a device as cache, use + "echo /dev/foo" > /sys/kernel/bcache/register_cache + And to enable caching for a device + "echo <UUID> /dev/foo" > /sys/kernel/bcache/register_dev + See Documentation/bcache.txt for details. + endif # BLOCK config BLOCK_COMPAT
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