[dm-devel] [Bcache v15 09/16] bcache: Documentation, and changes to generic code
Kent Overstreet
koverstreet at google.com
Mon Jul 23 23:50:52 UTC 2012
Signed-off-by: Kent Overstreet <koverstreet at google.com>
---
Documentation/ABI/testing/sysfs-block-bcache | 156 ++++++++++++++++
Documentation/bcache.txt | 255 ++++++++++++++++++++++++++
drivers/md/Kconfig | 2 +
drivers/md/Makefile | 1 +
drivers/md/bcache/Kconfig | 41 ++++
drivers/md/bcache/Makefile | 14 ++
include/linux/cgroup_subsys.h | 6 +
include/linux/sched.h | 4 +
kernel/fork.c | 4 +
9 files changed, 483 insertions(+), 0 deletions(-)
create mode 100644 Documentation/ABI/testing/sysfs-block-bcache
create mode 100644 Documentation/bcache.txt
create mode 100644 drivers/md/bcache/Kconfig
create mode 100644 drivers/md/bcache/Makefile
diff --git a/Documentation/ABI/testing/sysfs-block-bcache b/Documentation/ABI/testing/sysfs-block-bcache
new file mode 100644
index 0000000..9e4bbc5
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-block-bcache
@@ -0,0 +1,156 @@
+What: /sys/block/<disk>/bcache/unregister
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ A write to this file causes the backing device or cache to be
+ unregistered. If a backing device had dirty data in the cache,
+ writeback mode is automatically disabled and all dirty data is
+ flushed before the device is unregistered. Caches unregister
+ all associated backing devices before unregistering themselves.
+
+What: /sys/block/<disk>/bcache/clear_stats
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ Writing to this file resets all the statistics for the device.
+
+What: /sys/block/<disk>/bcache/cache
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a backing device that has cache, a symlink to
+ the bcache/ dir of that cache.
+
+What: /sys/block/<disk>/bcache/cache_hits
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: integer number of full cache hits,
+ counted per bio. A partial cache hit counts as a miss.
+
+What: /sys/block/<disk>/bcache/cache_misses
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: integer number of cache misses.
+
+What: /sys/block/<disk>/bcache/cache_hit_ratio
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: cache hits as a percentage.
+
+What: /sys/block/<disk>/bcache/sequential_cutoff
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: Threshold past which sequential IO will
+ skip the cache. Read and written as bytes in human readable
+ units (i.e. echo 10M > sequntial_cutoff).
+
+What: /sys/block/<disk>/bcache/bypassed
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ Sum of all reads and writes that have bypassed the cache (due
+ to the sequential cutoff). Expressed as bytes in human
+ readable units.
+
+What: /sys/block/<disk>/bcache/writeback
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: When on, writeback caching is enabled and
+ writes will be buffered in the cache. When off, caching is in
+ writethrough mode; reads and writes will be added to the
+ cache but no write buffering will take place.
+
+What: /sys/block/<disk>/bcache/writeback_running
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: when off, dirty data will not be written
+ from the cache to the backing device. The cache will still be
+ used to buffer writes until it is mostly full, at which point
+ writes transparently revert to writethrough mode. Intended only
+ for benchmarking/testing.
+
+What: /sys/block/<disk>/bcache/writeback_delay
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: In writeback mode, when dirty data is
+ written to the cache and the cache held no dirty data for that
+ backing device, writeback from cache to backing device starts
+ after this delay, expressed as an integer number of seconds.
+
+What: /sys/block/<disk>/bcache/writeback_percent
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For backing devices: If nonzero, writeback from cache to
+ backing device only takes place when more than this percentage
+ of the cache is used, allowing more write coalescing to take
+ place and reducing total number of writes sent to the backing
+ device. Integer between 0 and 40.
+
+What: /sys/block/<disk>/bcache/synchronous
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a cache, a boolean that allows synchronous mode to be
+ switched on and off. In synchronous mode all writes are ordered
+ such that the cache can reliably recover from unclean shutdown;
+ if disabled bcache will not generally wait for writes to
+ complete but if the cache is not shut down cleanly all data
+ will be discarded from the cache. Should not be turned off with
+ writeback caching enabled.
+
+What: /sys/block/<disk>/bcache/discard
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a cache, a boolean allowing discard/TRIM to be turned off
+ or back on if the device supports it.
+
+What: /sys/block/<disk>/bcache/bucket_size
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a cache, bucket size in human readable units, as set at
+ cache creation time; should match the erase block size of the
+ SSD for optimal performance.
+
+What: /sys/block/<disk>/bcache/nbuckets
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a cache, the number of usable buckets.
+
+What: /sys/block/<disk>/bcache/tree_depth
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a cache, height of the btree excluding leaf nodes (i.e. a
+ one node tree will have a depth of 0).
+
+What: /sys/block/<disk>/bcache/btree_cache_size
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ Number of btree buckets/nodes that are currently cached in
+ memory; cache dynamically grows and shrinks in response to
+ memory pressure from the rest of the system.
+
+What: /sys/block/<disk>/bcache/written
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a cache, total amount of data in human readable units
+ written to the cache, excluding all metadata.
+
+What: /sys/block/<disk>/bcache/btree_written
+Date: November 2010
+Contact: Kent Overstreet <kent.overstreet at gmail.com>
+Description:
+ For a cache, sum of all btree writes in human readable units.
diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt
new file mode 100644
index 0000000..270c734
--- /dev/null
+++ b/Documentation/bcache.txt
@@ -0,0 +1,255 @@
+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.
+
+Userspace tools and a wiki are at:
+ git://evilpiepirate.org/~kent/bcache-tools.git
+ http://bcache.evilpiepirate.org
+
+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
+extants (which can be anywhere from a single sector to the bucket size). It's
+designed to avoid random writes at all costs; it fills up an erase block
+sequentially, then issues a discard before reusing it.
+
+Both writethrough and writeback caching are supported. Writeback defaults to
+off, but can be switched on and off arbitrarily at runtime. Bcache goes to
+great lengths to protect your data - it reliably handles unclean shutdown. (It
+doesn't even have a notion of a clean shutdown; bcache simply doesn't return
+writes as completed until they're on stable storage).
+
+Writeback caching can use most of the cache for buffering writes - writing
+dirty data to the backing device is always done sequentially, scanning from the
+start to the end of the index.
+
+Since random IO is what SSDs excel at, there generally won't be much benefit
+to caching large sequential IO. Bcache detects sequential IO and skips it;
+it also keeps a rolling average of the IO sizes per task, and as long as the
+average is above the cutoff it will skip all IO from that task - instead of
+caching the first 512k after every seek. Backups and large file copies should
+thus entirely bypass the cache.
+
+In the event of a data IO error on the flash it will try to recover by reading
+from disk or invalidating cache entries. For unrecoverable errors (meta data
+or dirty data), caching is automatically disabled; if dirty data was present
+in the cache it first disables writeback caching and waits for all dirty data
+to be flushed.
+
+Getting started:
+You'll need make-bcache from the bcache-tools repository. Both the cache device
+and backing device must be formatted before use.
+ make-bcache -B /dev/sdb
+ make-bcache -C -w2k -b1M -j64 /dev/sdc
+
+To make bcache devices known to the kernel, echo them to /sys/fs/bcache/register:
+ echo /dev/sdb > /sys/fs/bcache/register
+ echo /dev/sdc > /sys/fs/bcache/register
+
+To register your bcache devices automatically, you could add something like
+this to an init script:
+ echo /dev/sd* > /sys/fs/bcache/register_quiet
+
+It'll look for bcache superblocks and ignore everything that doesn't have one.
+
+When you register a backing device, you'll get a new /dev/bcache# device:
+ mkfs.ext4 /dev/bcache0
+ mount /dev/bcache0 /mnt
+
+Cache devices are managed as sets; multiple caches per set isn't supported yet
+but will allow for mirroring of metadata and dirty data in the future. Your new
+cache set shows up as /sys/fs/bcache/<UUID>
+
+To enable caching, you need to attach the backing device to the cache set by
+specifying the UUID:
+ echo <UUID> > /sys/block/sdb/bcache/attach
+
+The cache set with that UUID need not be registered to attach to it - the UUID
+will be saved to the backing device's superblock and it'll start being cached
+when the cache set does show up.
+
+This only has to be done once. The next time you reboot, just reregister all
+your bcache devices. If a backing device has data in a cache somewhere, the
+/dev/bcache# device won't be created until the cache shows up - particularly
+important if you have writeback caching turned on.
+
+If you're booting up and your cache device is gone and never coming back, you
+can force run the backing device:
+ echo 1 > /sys/block/sdb/bcache/running
+
+The backing device will still use that cache set if it shows up in the future,
+but all the cached data will be invalidated. If there was dirty data in the
+cache, don't expect the filesystem to be recoverable - you will have massive
+filesystem corruption, though ext4's fsck does work miracles.
+
+
+Other sysfs files for the backing device:
+
+ bypassed
+ Sum of all IO, reads and writes, than have 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_miss_collisions
+ Count of times a read completes but the data is already in the cache and
+ is therefore redundant. This is usually caused by readahead while a
+ read to the same location occurs.
+
+ cache_readaheads
+ Count of times readahead occured.
+
+ clear_stats
+ Writing to this file resets all the statistics.
+
+ flush_delay_ms
+ flush_delay_ms_sync
+ Optional delay for btree writes to allow for more coalescing of updates to
+ the index. Default to 0.
+
+ label
+ Name of underlying device.
+
+ readahead
+ Size of readahead that should be performed. Defaults to 0. If set to e.g.
+ 1M, it will round cache miss reads up to that size, but without overlapping
+ existing cache entries.
+
+ running
+ 1 if bcache is running.
+
+ sequential_cutoff
+ A sequential IO will bypass the cache once it passes this threshhold; the
+ most recent 128 IOs are tracked so sequential IO can be detected even when
+ it isn't all done at once.
+
+ sequential_cutoff_average
+ If the weighted average from a client is higher than this cutoff we bypass
+ all IO.
+
+ unregister
+ Writing to this file disables caching on that device
+
+ writeback
+ Boolean, if off only writethrough caching is done
+
+ 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
+ To allow for more buffering of random writes, writeback only proceeds when
+ more than this percentage of the cache is unavailable. Defaults to 0.
+
+ 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.
+
+For the cache set:
+ active_journal_entries
+ Number of journal entries that are newer than the index.
+
+ average_key_size
+ Average data per key in the btree.
+
+ average_seconds_between_gc
+ How often garbage collection is occuring.
+
+ block_size
+ Block size of the virtual device.
+
+ btree_avg_keys_written
+ Average number of keys per write to the btree when a node wasn't being
+ rewritten - indicates how much coalescing is taking place.
+
+
+ btree_cache_size
+ Number of btree buckets currently cached in memory
+
+ btree_nodes
+ Total nodes in the btree.
+
+ btree_used_percent
+ Average fraction of btree in use.
+
+ bucket_size
+ Size of Buckets
+
+ bypassed
+ Sum of all IO, reads and writes, than have bypassed the cache
+
+ cache_available_percent
+ Percentage of cache device free.
+
+ clear_stats
+ Clears the statistics associated with this cache
+
+ dirty_data
+ How much dirty data is in the cache.
+
+ gc_ms_max
+ Longest garbage collection.
+
+ internal/bset_tree_stats
+ internal/btree_cache_max_chain
+ Internal. Statistics about the bset tree and chain length. Likely to be
+ hidden soon.
+
+ 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.
+
+ root_usage_percent
+ Percentage of the root btree node in use. If this gets too high the node
+ will split, increasing the tree depth.
+
+ seconds_since_gc
+ When was the last garbage collection.
+
+ synchronous
+ Boolean; when on all writes to the cache are strictly ordered such that it
+ can recover from unclean shutdown. If off it will not generally wait for
+ writes to complete, but the entire cache contents will be invalidated on
+ unclean shutdown. Not recommended that it be turned off when writeback is
+ on.
+
+ tree_depth
+ Depth of the btree.
+
+ trigger_gc
+ Force garbage collection to run now.
+
+ unregister
+ Closes the cache device and all devices being cached; if dirty data is
+ present it will disable writeback caching and wait for it to be flushed.
+
+
+For each cache within a cache set:
+ btree_written
+ Sum of all btree writes, in (kilo/mega/giga) bytes
+
+ discard
+ Boolean; if on a discard/TRIM will be issued to each bucket before it is
+ reused. Defaults to on if supported.
+
+ io_errors
+ Number of errors that have occured, decayed by io_error_halflife.
+
+ metadata_written
+ Total Metadata written (btree + other meta data).
+
+ 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.
+
+ written
+ Sum of all data that has been written to the cache; comparison with
+ btree_written gives the amount of write inflation in bcache.
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 10f122a..d977b45 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -185,6 +185,8 @@ config MD_FAULTY
In unsure, say N.
+source "drivers/md/bcache/Kconfig"
+
config BLK_DEV_DM
tristate "Device mapper support"
---help---
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 8b2e0df..0d4b86b 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -26,6 +26,7 @@ obj-$(CONFIG_MD_RAID10) += raid10.o
obj-$(CONFIG_MD_RAID456) += raid456.o
obj-$(CONFIG_MD_MULTIPATH) += multipath.o
obj-$(CONFIG_MD_FAULTY) += faulty.o
+obj-$(CONFIG_BCACHE) += bcache/
obj-$(CONFIG_BLK_DEV_MD) += md-mod.o
obj-$(CONFIG_BLK_DEV_DM) += dm-mod.o
obj-$(CONFIG_DM_BUFIO) += dm-bufio.o
diff --git a/drivers/md/bcache/Kconfig b/drivers/md/bcache/Kconfig
new file mode 100644
index 0000000..9acd870
--- /dev/null
+++ b/drivers/md/bcache/Kconfig
@@ -0,0 +1,41 @@
+
+config BCACHE
+ tristate "Block device as cache"
+ select CLOSURES
+ ---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.
+
+ See Documentation/bcache.txt for details.
+
+config BCACHE_DEBUG
+ bool "Bcache debugging"
+ depends on BCACHE
+ ---help---
+ Don't select this option unless you're a developer
+
+ Enables extra debugging tools (primarily a fuzz tester)
+
+config BCACHE_EDEBUG
+ bool "Extended runtime checks"
+ depends on BCACHE
+ ---help---
+ Don't select this option unless you're a developer
+
+ Enables extra runtime checks which significantly affect performance
+
+config BCACHE_LATENCY_DEBUG
+ bool "Latency tracing for bcache"
+ depends on BCACHE
+ ---help---
+ Hacky latency tracing that has nevertheless been useful in the past:
+ adds a global variable accessible via /sys/fs/bcache/latency_warn_ms,
+ which defaults to 0. If nonzero, any timed operation that takes longer
+ emits a printk.
+
+config CGROUP_BCACHE
+ bool "Cgroup controls for bcache"
+ depends on BCACHE && BLK_CGROUP
+ ---help---
+ TODO
+
diff --git a/drivers/md/bcache/Makefile b/drivers/md/bcache/Makefile
new file mode 100644
index 0000000..0e5305d
--- /dev/null
+++ b/drivers/md/bcache/Makefile
@@ -0,0 +1,14 @@
+
+obj-$(CONFIG_BCACHE) += bcache.o
+
+bcache-y := alloc.o btree.o bset.o io.o journal.o\
+ writeback.o movinggc.o request.o super.o debug.o util.o trace.o stats.o
+
+CFLAGS_alloc.o += -std=gnu99
+CFLAGS_btree.o += -std=gnu99
+CFLAGS_bset.o += -std=gnu99
+CFLAGS_journal.o += -std=gnu99
+CFLAGS_movinggc.o += -std=gnu99
+CFLAGS_request.o += -std=gnu99 -Iblock
+CFLAGS_super.o += -std=gnu99
+CFLAGS_debug.o += -std=gnu99
diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index 0bd390c..d698634 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -72,3 +72,9 @@ SUBSYS(net_prio)
#endif
/* */
+
+#ifdef CONFIG_CGROUP_BCACHE
+SUBSYS(bcache)
+#endif
+
+/* */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4a1f493..1741596 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1583,6 +1583,10 @@ struct task_struct {
struct uprobe_task *utask;
int uprobe_srcu_id;
#endif
+#if defined(CONFIG_BCACHE) || defined(CONFIG_BCACHE_MODULE)
+ unsigned int sequential_io;
+ unsigned int sequential_io_avg;
+#endif
};
/* Future-safe accessor for struct task_struct's cpus_allowed. */
diff --git a/kernel/fork.c b/kernel/fork.c
index f00e319..d47494b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1314,6 +1314,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
p->memcg_batch.do_batch = 0;
p->memcg_batch.memcg = NULL;
#endif
+#ifdef CONFIG_BCACHE
+ p->sequential_io = 0;
+ p->sequential_io_avg = 0;
+#endif
/* Perform scheduler related setup. Assign this task to a CPU. */
sched_fork(p);
--
1.7.7.3
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