A persistent single-threaded engine, backed by a read-optimized B+ tree.
It is disabled by default. It can be enabled in CMake using the ENGINE_TREE3
option.
Configuration must specify a path
to a PMDK persistent pool, which can be a file (on a DAX filesystem),
a DAX device, or a PMDK poolset file.
- path -- Path to the database file
- type: string
- force_create -- If 0, pmemkv opens file specified by 'path', otherwise it creates it
- type: uint64_t
- default value: 0
- size -- Only needed when force_create is not 0, specifies size of the database [in bytes]
- type: uint64_t
- min value: 8388608 (8MB)
Internally, tree3
uses a hybrid fingerprinted B+ tree implementation. Rather than keeping
inner and leaf nodes of the tree in persistent memory, tree3
uses a hybrid structure where
inner nodes are kept in DRAM and leaf nodes only are kept in persistent memory. Though tree3
has to recover all inner nodes when the engine is started, searches are performed in
DRAM except for a final read from persistent memory.
Leaf nodes in tree3
contain multiple key-value pairs, indexed using 1-byte fingerprints
(Pearson hashes) that speed locating
a given key. Leaf modifications are accelerated using
zero-copy updates.
Libpmemobj-cpp package is required.
A persistent, single-threaded and sorted engine, backed by a B+ tree.
It is disabled by default. It can be enabled in CMake using the ENGINE_STREE
option.
- path -- Path to the database file
- type: string
- force_create -- If 0, pmemkv opens file specified by 'path', otherwise it creates it
- type: uint64_t
- default value: 0
- size -- Only needed when force_create is not 0, specifies size of the database [in bytes]
- type: uint64_t
(TBD)
Libpmemobj-cpp package is required.
This engine is using a sub engine from the list above to cache requests to external Redis or Memcached server.
It is disabled by default. It can be enabled in CMake using the ENGINE_CACHING
option.
Caching engine itself requires server connection settings. Part of the config required for the sub engine should be relevant to chosen engine.
- host -- Server's IP
- type: string
- port -- Server's port
- type: int64_t
- attempts -- Number of connection attempts
- type: int64_t
- ttl -- Time to live [in seconds]
- type: int64_t
- default value: 0
- remote_type -- Server's type (Redis or Memcached)
- type: string
- remote_user -- Connection's user
- type: string
- remote_pwd -- User's password
- type: string
- remote_url -- Remote (server's) URL
- type: string
- subengine -- Config object for sub engine with its required settings
- type: object
(TBD)
Memcached and libacl (see here for installation guide) packages are required.
pmse
tree3
has a lot in common with pmse
-- both implementations rely on PMDK internally, although
they expose different APIs externally. Both pmse
and tree3
are based on a B+ tree
implementation. The biggest difference is that the pmse
tree keeps inner and leaf nodes in persistent memory,
where tree3
keeps inner nodes in DRAM and leaf nodes in
persistent memory. (This means that tree3
has to recover
all inner nodes when the engine is started)
FPTree
This research paper describes a hybrid DRAM/NVM tree design (similar
to the tree3
storage engine) but this paper doesn't provide any code, and
omits certain important implementation details.
Beyond providing a clean-room implementation, the design of tree3
differs from FPTree in several important areas:
-
tree3
is written using PMDK C++ bindings, which exerts influence on its design and implementation.tree3
uses generic PMDK transactions (i.e.transaction::run()
closures), there is no need for micro-logging structures as described in the FPTree paper to make internal delete and split operations safe.tree3
also adjusts sizes of data structures (to fit PMDK primitive types) for best cache-line optimization. -
FPTree does not specify a hash method implementation, where
tree3
uses a Pearson hash (RFC 3074). -
Within its persistent leaves, FPTree uses an array of key hashes with a separate visibility bitmap to track what hash slots are occupied.
tree3
takes a different approach and uses key hashes themselves to track visibility. This relies on a specially modified Pearson hash function, where a hash value of zero always indicates the slot is unused. This optimization eliminates the cost of using and maintaining visibility bitmaps as well as cramming more hashes into a single cache-line, and affects the implementation of every primitive operation in the tree. -
tree3
caches key hashes in DRAM (in addition to storing these as part of the persistent leaf). This speeds leaf operations, especially with slower media, for what seems like an acceptable rise in DRAM usage. -
Within its persistent leaves,
tree3
combines hash, key and value into a single slot type (KVSlot
). This leads to improved leaf split performance and reduced write amplification, since splitting can be performed by swapping pointers to slots without copying any key or value data stored in the slots.KVSlot
internally stores key and value to a single persistent buffer, which minimizes the number of persistent allocations and improves storage efficiency with larger keys and values.
cpp_map
Use of PMDK C++ bindings by tree3
was lifted from this example program.
Many thanks to @tomaszkapela
for providing a great example to follow!