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layout: manual Content-Style: 'text/css' title: _MP(PMEMKV, 7) collection: libpmemkv header: PMEMKV secondary_title: pmemkv ...

NAME
DESCRIPTION
ENGINES
BINDINGS
SEE ALSO

NAME

pmemkv - Key/Value Datastore for Persistent Memory

DESCRIPTION

pmemkv is a key-value datastore framework optimized for persistent memory. It provides native C API and C++ headers. Support for other languages is described in the BINDINGS section below.

It has multiple storage engines, each optimized for a different use case. They differ in implementation and capabilities:

  • persistence - this is a trade-off between data preservation and performance; persistent engines retain their content and are power fail/crash safe, but are slower; volatile engines are faster, but keep their content only until the database is closed (or application crashes; power fail occurs)

  • concurrency - engines provide a varying degree of write scalability in multi-threaded workloads. Concurrent engines support non-blocking retrievals and, on average, highly scalable updates. For details see the description of individual engines.

  • keys' ordering - "sorted" engines support querying above/below the given key

Persistent engines usually use libpmemobj++ and PMDK to access NVDIMMs. They can work with files on DAX filesystem (fsdax) or DAX device.

For description of pmemkv core API see libpmemkv(3). For description of pmemkv configuration API see libpmemkv_config(3).

ENGINES

Engine Name Description Persistent? Concurrent? Sorted?
cmap Concurrent hash map Yes Yes No
vcmap Volatile concurrent hash map No Yes No
vsmap Volatile sorted hash map No No Yes
blackhole Accepts everything, returns nothing No Yes No

The most mature and recommended engine to use for persistent use-cases is cmap. It provides good performance results and stability.

Each engine can be manually turned on and off at build time, using CMake options. All engines listed here are enabled and ready to use.

cmap

A persistent concurrent engine, backed by a hashmap that allows calling get, put, and remove concurrently from multiple threads and ensures good scalability. Rest of the methods (e.g. range query methods) are not thread-safe and should not be called from more than one thread. Data stored using this engine is persistent and guaranteed to be consistent in case of any kind of interruption (crash / power loss / etc).

Internally this engine uses persistent concurrent hashmap and persistent string from libpmemobj-cpp library (for details see https://github.com/pmem/libpmemobj-cpp). Persistent string is used as a type of a key and a value. Engine's functions should not be called within libpmemobj transactions (improper call by user will result thrown exception). libpmemobj-cpp packages are required.

This engine requires the following config parameters (see libpmemkv_config(3) for details how to set them):

  • path -- Path to a database file or to a poolset file (see poolset(5) for details). Not that when using poolset file, size should be 0
    • 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)
  • oid -- Pointer to oid (for details see libpmemobj(7)) which points to engine data. If oid is null, engine will allocate new data, otherwise it will use existing one.
    • type: object

The following table shows three possible combinations of parameters (where '-' means 'cannot be set'):

# path force_create size oid
1 set 0 - -
2 set 1 set -
3 - - - set

A database file or a poolset file can also be created using pmempool utility (see pmempool-create(1)). When using pmempool create, "pmemkv" should be passed as layout. Only PMEMOBJ pools are supported.

vcmap

A volatile concurrent engine, backed by memkind. Data written using this engine is lost after database is closed.

This engine is built on top of tbb::concurrent_hash_map data structure and uses PMEM C++ allocator to allocate memory. std::basic_string is used as a type of a key and a value. Memkind, TBB and libpmemobj-cpp packages are required.

This engine requires the following config parameters (see libpmemkv_config(3) for details how to set them):

  • path -- Path to an existing directory
    • type: string
  • size -- Specifies size of the database [in bytes]
    • type: uint64_t
    • min value: 8388608 (8MB)

vsmap

A volatile single-threaded sorted engine, backed by memkind. Data written using this engine is lost after database is closed.

This engine is built on top of std::map and uses PMEM C++ allocator to allocate memory. std::basic_string is used as a type of a key and a value. Memkind and libpmemobj-cpp packages are required.

This engine requires the following config parameters (see libpmemkv_config(3) for details how to set them):

  • path -- Path to an existing directory
    • type: string
  • size -- Specifies size of the database [in bytes]
    • type: uint64_t
    • min value: 8388608 (8MB)

blackhole

A volatile engine that accepts an unlimited amount of data, but never returns anything. Internally, blackhole does not use a persistent pool or any durable structure. The intended use of this engine is to profile and tune high-level bindings, and similar cases when persistence should be intentionally skipped. No additional packages are required. No supported configuration parameters.

Experimental engines

There are also more engines in various states of development, for details see https://github.com/pmem/pmemkv/blob/master/ENGINES-experimental.md. Three of them (tree3, stree and csmap) requires the config parameters like cmap and similarly to cmap should not be used within libpmemobj transaction(s).

BINDINGS

Bindings for other languages are available on GitHub. Currently they support only subset of native API.

Existing bindings:

SEE ALSO

libpmemkv(3), libpmemkv_config(3), pmempool(1), libpmemobj(7) and https://pmem.io