A volatile concurrent engine that accepts an unlimited amount of data, but never returns anything.
PutandRemovealways returnsOKGetalways returnsNOT_FOUNDAllandEachnever trigger callbacks
No required configuration parameters. JSON configuration is never parsed.
Internally, blackhole does not use a persistent pool or any durable structures. The intended
use this engine is to profile and tune high-level bindings, and similar cases when persistence
should be intentionally skipped.
A persistent single-threaded engine, backed by a read-optimized B+ tree.
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" : "my-pool" }
Configuration may optionally specify a size in bytes.
If omitted the default value of 1073741824 bytes (1GB) is applied.
This value cannot be smaller than 8388608 (8MB).
{ "path" : "my-pool", "size" : 1073741824 }
Internally, kvtree uses a hybrid fingerprinted B+ tree implementation. Rather than keeping
inner and leaf nodes of the tree in persistent memory, kvtree uses a hybrid structure where
inner nodes are kept in DRAM and leaf nodes only are kept in persistent memory. Though kvtree
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 kvtree 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.
pmse
kvtree has a lot in common with pmse
-- both implementations rely on PMDK internally, although
they expose different APIs externally. Both pmse and kvtree are based on a B+ tree
implementation. The biggest difference is that the pmse
tree keeps inner and leaf nodes in persistent memory,
where kvtree keeps inner nodes in DRAM and leaf nodes in
persistent memory. (This means that kvtree 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 kvtree 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 kvtree
differs from FPTree in several important areas:
-
kvtreeis written using PMDK C++ bindings, which exerts influence on its design and implementation.kvtreeuses generic PMDK transactions (ie.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.kvtreealso adjusts sizes of data structures (to fit PMDK primitive types) for best cache-line optimization. -
FPTree does not specify a hash method implementation, where
kvtreeuses 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.
kvtreetakes 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. -
kvtreecaches 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,
kvtreecombines 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.KVSlotinternally 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 kvtree was lifted from this example program.
Many thanks to @tomaszkapela
for providing a great example to follow!
A volatile single-threaded engine, backed by memkind.
Configuration must specify a path to a local directory where temporary files will be created.
For best performance this directory should reside on a DAX-enabled filesystem.
{ "path" : "my-directory" }
Configuration may optionally specify a size in bytes.
If omitted the default value of 1073741824 bytes (1GB) is applied.
This value cannot be smaller than 8388608 (8MB).
{ "path" : "my-path", "size" : 1073741824 }
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A volatile concurrent engine, backed by memkind.
(same as the vmap engine)
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