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diff --git a/README.md b/README.md new file mode 100644 index 0000000..3c4d14d --- /dev/null +++ b/README.md @@ -0,0 +1,230 @@ +**LevelDB is a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values.** + +[](https://github.com/google/leveldb/actions/workflows/build.yml) + +Authors: Sanjay Ghemawat ([email protected]) and Jeff Dean ([email protected]) + +# Features + + * Keys and values are arbitrary byte arrays. + * Data is stored sorted by key. + * Callers can provide a custom comparison function to override the sort order. + * The basic operations are `Put(key,value)`, `Get(key)`, `Delete(key)`. + * Multiple changes can be made in one atomic batch. + * Users can create a transient snapshot to get a consistent view of data. + * Forward and backward iteration is supported over the data. + * Data is automatically compressed using the [Snappy compression library](https://google.github.io/snappy/). + * External activity (file system operations etc.) is relayed through a virtual interface so users can customize the operating system interactions. + +# Documentation + + [LevelDB library documentation](https://github.com/google/leveldb/blob/master/doc/index.md) is online and bundled with the source code. + +# Limitations + + * This is not a SQL database. It does not have a relational data model, it does not support SQL queries, and it has no support for indexes. + * Only a single process (possibly multi-threaded) can access a particular database at a time. + * There is no client-server support builtin to the library. An application that needs such support will have to wrap their own server around the library. + +# Getting the Source + +```bash +git clone --recurse-submodules https://github.com/google/leveldb.git +``` + +# Building + +This project supports [CMake](https://cmake.org/) out of the box. + +### Build for POSIX + +Quick start: + +```bash +mkdir -p build && cd build +cmake -DCMAKE_BUILD_TYPE=Release .. && cmake --build . +``` + +### Building for Windows + +First generate the Visual Studio 2017 project/solution files: + +```cmd +mkdir build +cd build +cmake -G "Visual Studio 15" .. +``` +The default default will build for x86. For 64-bit run: + +```cmd +cmake -G "Visual Studio 15 Win64" .. +``` + +To compile the Windows solution from the command-line: + +```cmd +devenv /build Debug leveldb.sln +``` + +or open leveldb.sln in Visual Studio and build from within. + +Please see the CMake documentation and `CMakeLists.txt` for more advanced usage. + +# Contributing to the leveldb Project + +The leveldb project welcomes contributions. leveldb's primary goal is to be +a reliable and fast key/value store. Changes that are in line with the +features/limitations outlined above, and meet the requirements below, +will be considered. + +Contribution requirements: + +1. **Tested platforms only**. We _generally_ will only accept changes for + platforms that are compiled and tested. This means POSIX (for Linux and + macOS) or Windows. Very small changes will sometimes be accepted, but + consider that more of an exception than the rule. + +2. **Stable API**. We strive very hard to maintain a stable API. Changes that + require changes for projects using leveldb _might_ be rejected without + sufficient benefit to the project. + +3. **Tests**: All changes must be accompanied by a new (or changed) test, or + a sufficient explanation as to why a new (or changed) test is not required. + +4. **Consistent Style**: This project conforms to the + [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html). + To ensure your changes are properly formatted please run: + + ``` + clang-format -i --style=file <file> + ``` + +## Submitting a Pull Request + +Before any pull request will be accepted the author must first sign a +Contributor License Agreement (CLA) at https://cla.developers.google.com/. + +In order to keep the commit timeline linear +[squash](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Squashing-Commits) +your changes down to a single commit and [rebase](https://git-scm.com/docs/git-rebase) +on google/leveldb/master. This keeps the commit timeline linear and more easily sync'ed +with the internal repository at Google. More information at GitHub's +[About Git rebase](https://help.github.com/articles/about-git-rebase/) page. + +# Performance + +Here is a performance report (with explanations) from the run of the +included db_bench program. The results are somewhat noisy, but should +be enough to get a ballpark performance estimate. + +## Setup + +We use a database with a million entries. Each entry has a 16 byte +key, and a 100 byte value. Values used by the benchmark compress to +about half their original size. + + LevelDB: version 1.1 + Date: Sun May 1 12:11:26 2011 + CPU: 4 x Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz + CPUCache: 4096 KB + Keys: 16 bytes each + Values: 100 bytes each (50 bytes after compression) + Entries: 1000000 + Raw Size: 110.6 MB (estimated) + File Size: 62.9 MB (estimated) + +## Write performance + +The "fill" benchmarks create a brand new database, in either +sequential, or random order. The "fillsync" benchmark flushes data +from the operating system to the disk after every operation; the other +write operations leave the data sitting in the operating system buffer +cache for a while. The "overwrite" benchmark does random writes that +update existing keys in the database. + + fillseq : 1.765 micros/op; 62.7 MB/s + fillsync : 268.409 micros/op; 0.4 MB/s (10000 ops) + fillrandom : 2.460 micros/op; 45.0 MB/s + overwrite : 2.380 micros/op; 46.5 MB/s + +Each "op" above corresponds to a write of a single key/value pair. +I.e., a random write benchmark goes at approximately 400,000 writes per second. + +Each "fillsync" operation costs much less (0.3 millisecond) +than a disk seek (typically 10 milliseconds). We suspect that this is +because the hard disk itself is buffering the update in its memory and +responding before the data has been written to the platter. This may +or may not be safe based on whether or not the hard disk has enough +power to save its memory in the event of a power failure. + +## Read performance + +We list the performance of reading sequentially in both the forward +and reverse direction, and also the performance of a random lookup. +Note that the database created by the benchmark is quite small. +Therefore the report characterizes the performance of leveldb when the +working set fits in memory. The cost of reading a piece of data that +is not present in the operating system buffer cache will be dominated +by the one or two disk seeks needed to fetch the data from disk. +Write performance will be mostly unaffected by whether or not the +working set fits in memory. + + readrandom : 16.677 micros/op; (approximately 60,000 reads per second) + readseq : 0.476 micros/op; 232.3 MB/s + readreverse : 0.724 micros/op; 152.9 MB/s + +LevelDB compacts its underlying storage data in the background to +improve read performance. The results listed above were done +immediately after a lot of random writes. The results after +compactions (which are usually triggered automatically) are better. + + readrandom : 11.602 micros/op; (approximately 85,000 reads per second) + readseq : 0.423 micros/op; 261.8 MB/s + readreverse : 0.663 micros/op; 166.9 MB/s + +Some of the high cost of reads comes from repeated decompression of blocks +read from disk. If we supply enough cache to the leveldb so it can hold the +uncompressed blocks in memory, the read performance improves again: + + readrandom : 9.775 micros/op; (approximately 100,000 reads per second before compaction) + readrandom : 5.215 micros/op; (approximately 190,000 reads per second after compaction) + +## Repository contents + +See [doc/index.md](doc/index.md) for more explanation. See +[doc/impl.md](doc/impl.md) for a brief overview of the implementation. + +The public interface is in include/leveldb/*.h. 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/leveldb/db.h**: Main interface to the DB: Start here. + +* **include/leveldb/options.h**: Control over the behavior of an entire database, +and also control over the behavior of individual reads and writes. + +* **include/leveldb/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/leveldb/iterator.h**: Interface for iterating over data. You can get +an iterator from a DB object. + +* **include/leveldb/write_batch.h**: Interface for atomically applying multiple +updates to a database. + +* **include/leveldb/slice.h**: A simple module for maintaining a pointer and a +length into some other byte array. + +* **include/leveldb/status.h**: Status is returned from many of the public interfaces +and is used to report success and various kinds of errors. + +* **include/leveldb/env.h**: +Abstraction of the OS environment. A posix implementation of this interface is +in util/env_posix.cc. + +* **include/leveldb/table.h, include/leveldb/table_builder.h**: Lower-level modules that most +clients probably won't use directly. |