[PATCH 1/2] Bcache: Version 7 - Writeback

[Kent Overstreet]

Bcache is a patch for caching arbitrary block devices with other block devices
- it's designed for SSDs. It's meant to be suitable for use in any situation,
easy to flip on and not requiring any consideration for i.e. backups - by
default sequential IO bypasses the cache. It also uses a btree (hybrid
log/btree, really) for the index; it never does random writes and allocates in
erase block sized buckets - it's designed for trim, and to work well with
cheaper MLC drives.

Some rough numbers:
bonnie++, seeks per second - 90% reads, 10% a read then a write:
WD EARS Green hd: 269
Corsair Nova: 10886
Bcache: 15302

I've been stabilizing writeback for around almost two months now; it should be
considered alpha quality, but it's been beaten on quite a bit and there's no
known data corruption bugs. I wouldn't suggest running it on your desktop just
yet, but outside testing would be very welcome.

Writeback is working well; in writeback mode, it'll use most of your SSD for
buffering writes and write everything out sequentially, skipping what's been
overwritten. If you want good RAID5/6 performance, when bcache is stable you
won't need to buy a raid card with a battery backup - you'll get better
performance with bcache and Linux software raid. Probably cheaper, too.

It updates the index synchronously (unless you tell it not to), and it doesn't
start the update to the index until after data's been written to the cache;
the cache should always be consistent in the event of power failure, and once
bcache returns a write as completed it won't get lost if the power goes out.
Recovering from unclean shutdown has not been heavily tested though, it's
likely to still be buggy.

Code wise: The hooks in __generic_make_request were merely ugly for
writethrough caching - for writeback it's become a hack far too ugly to live.
I very much want the ability to turn on caching for a block device while it's
in use, without prior planning - i.e. without require the user to already be
using device mapper for everything - but that's going to have to be done a
different way. The current code is functional, but I'm probably going to have
to port it to device mapper to get something sane before I submit it for
inclusion, and hopefully I'll eventually have the time to move some dm
functionality into the generic block layer or somesuch.

Error handling is unfinished, though I don't think there's a ton of left work
now. Memory allocation is going to take more work, to make it deadlock free
under memory pressure. There's still some needed cleanup (locking especially),
but the code should be fairly sane in design. Quite a bit of work has gone
into garbage collection; it doesn't do incremental garbage collection yet but
locking shouldn't need any more work to make that happen.

Further off, there's some new plans to use bcache's index to implement
overcommitted/lazily allocated storage; by using the same index there'll be
approximately 0 extra runtime overhead, and I think it'll work out pretty
nicely. There's also been a surprising amount of interest in tiered storage,
and with overcommited storage done that should be very little extra work.
There's slightly more in the wiki.

Main git repository:
git://evilpiepirate.org/~kent/linux-bcache.git
Userspace tools:
git://evilpiepirate.org/~kent/bcache-tools.git
Wiki
http://bcache.evilpiepirate.org

diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt
new file mode 100644
index 0000000..bcd5b41
--- /dev/null
+++ b/Documentation/bcache.txt
@@ -0,0 +1,75 @@
+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 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; all available cache devices are used for all
+the devices that are being cached.  The cache devices store the UUIDs of
+devices they have, allowing caches to safely persist across reboots.  There's
+space allocated for 256 UUIDs right after the superblock - which means for now
+that there's a hard limit of 256 devices being cached.
+
+Currently only writethrough caching is supported; data is transparently added
+to the cache on writes but the write is not returned as completed until it has
+reached the underlying storage. Writeback caching will be supported when
+journalling is implemented.
+
+To protect against stale data, the entire cache is invalidated if it wasn't
+cleanly shutdown, and if caching is turned on or off for a device while it is
+opened read/write, all data for that device is invalidated.
+
+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/sde
+  echo "/dev/sde" > /sys/kernel/bcache/register_cache
+  echo "<UUID> /dev/md1" > /sys/kernel/bcache/register_dev
+
+And that's it.
+
+If md1 was a raid 1 or 10, that's probably all you want to do; there's no point
+in caching multiple copies of the same data. However, if you have a raid 5 or
+6, caching the raw devices will allow the p and q blocks to be cached, which
+will help your random write performance:
+  echo "<UUID> /dev/sda1" > /sys/kernel/bcache/register_dev
+  echo "<UUID> /dev/sda2" > /sys/kernel/bcache/register_dev
+  etc.
+
+To script the UUID lookup, you could do something like:
+  echo "`blkid /dev/md1 -s UUID -o value` /dev/md1"\
+	  > /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 9be0b56..a6ae422 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -77,6 +77,19 @@ config BLK_DEV_INTEGRITY
 	T10/SCSI Data Integrity Field or the T13/ATA External Path
 	Protection.  If in doubt, say N.
 
+config BLK_CACHE
+	tristate "Block device as cache"
+	---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/bar" > /sys/kernel/bcache/register_dev
+	See Documentation/bcache.txt for details.
+
 endif # BLOCK
 
 config BLOCK_COMPAT
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