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Summary: The file tree structure in Version is prebuilt and the range of each file is known. On the Get() code path, we do binary search in FindFile() by comparing target key with each file's largest key and also check the range for each L0 file. With some pre-calculated knowledge, each key comparision that has been done can serve as a hint to narrow down further searches: (1) If a key falls within a L0 file's range, we can safely skip the next file if its range does not overlap with the current one. (2) If a key falls within a file's range in level L0 - Ln-1, we should only need to binary search in the next level for files that overlap with the current one. (1) will be able to skip some files depending one the key distribution. (2) can greatly reduce the range of binary search, especially for bottom levels, given that one file most likely only overlaps with N files from the level below (where N is max_bytes_for_level_multiplier). So on level L, we will only look at ~N files instead of N^L files. Some inital results: measured with 500M key DB, when write is light (10k/s = 1.2M/s), this improves QPS ~7% on top of blocked bloom. When write is heavier (80k/s = 9.6M/s), it gives us ~13% improvement. Test Plan: make all check Reviewers: haobo, igor, dhruba, sdong, yhchiang Reviewed By: haobo CC: leveldb Differential Revision: https://reviews.facebook.net/D17205
rocksdb: A persistent key-value store for flash storage
Authors: * The Facebook Database Engineering Team
* Build on earlier work on leveldb by Sanjay Ghemawat
(sanjay@google.com) and Jeff Dean (jeff@google.com)
This code is a library that forms the core building block for a fast
key value server, especially suited for storing data on flash drives.
It has an Log-Structured-Merge-Database (LSM) design with flexible tradeoffs
between Write-Amplification-Factor(WAF), Read-Amplification-Factor (RAF)
and Space-Amplification-Factor(SAF). It has multi-threaded compactions,
making it specially suitable for storing multiple terabytes of data in a
single database.
The core of this code has been derived from open-source leveldb.
The code under this directory implements a system for maintaining a
persistent key/value store.
See doc/index.html and github wiki (https://github.com/facebook/rocksdb/wiki)
for more explanation.
The public interface is in include/*. Callers should not include or
rely on the details of any other header files in this package. Those
internal APIs may be changed without warning.
Guide to header files:
include/rocksdb/db.h
Main interface to the DB: Start here
include/rocksdb/options.h
Control over the behavior of an entire database, and also
control over the behavior of individual reads and writes.
include/rocksdb/comparator.h
Abstraction for user-specified comparison function. If you want
just bytewise comparison of keys, you can use the default comparator,
but clients can write their own comparator implementations if they
want custom ordering (e.g. to handle different character
encodings, etc.)
include/rocksdb/iterator.h
Interface for iterating over data. You can get an iterator
from a DB object.
include/rocksdb/write_batch.h
Interface for atomically applying multiple updates to a database.
include/rocksdb/slice.h
A simple module for maintaining a pointer and a length into some
other byte array.
include/rocksdb/status.h
Status is returned from many of the public interfaces and is used
to report success and various kinds of errors.
include/rocksdb/env.h
Abstraction of the OS environment. A posix implementation of
this interface is in util/env_posix.cc
include/rocksdb/table_builder.h
Lower-level modules that most clients probably won't use directly
include/rocksdb/cache.h
An API for the block cache.
include/rocksdb/compaction_filter.h
An API for a application filter invoked on every compaction.
include/rocksdb/filter_policy.h
An API for configuring a bloom filter.
include/rocksdb/memtablerep.h
An API for implementing a memtable.
include/rocksdb/statistics.h
An API to retrieve various database statistics.
include/rocksdb/transaction_log.h
An API to retrieve transaction logs from a database.
Design discussions are conducted in https://www.facebook.com/groups/rocksdb.dev/