@@ -57,7 +57,133 @@ endif::rootpath[]
5757
5858=== 打开一个数据库
5959
60+ 使用 `static Status Open(const Options &options, const std::string &name, DB **dbptr)` 可以打开一个数据库， options填入数据库打开选项，name指定数据库名，DB给出一个数据库指针(原先指向nullptr)，因为调用之后会把创建的数据库句柄赋值给对应的指针。
6061
62+ - Options 用来控制数据库行为的一些参数在调用 `DB::Open` 的时候使用。
63+
64+ [source,c++]
65+ ----
66+ struct LEVELDB_EXPORT Options {
67+ // Create an Options object with default values for all fields.
68+ Options();
69+
70+ // -------------------
71+ // Parameters that affect behavior
72+
73+ // Comparator used to define the order of keys in the table.
74+ // Default: a comparator that uses lexicographic byte-wise ordering
75+ //
76+ // REQUIRES: The client must ensure that the comparator supplied
77+ // here has the same name and orders keys *exactly* the same as the
78+ // comparator provided to previous open calls on the same DB.
79+ const Comparator *comparator;
80+
81+ // If true, the database will be created if it is missing.
82+ bool create_if_missing = false;
83+
84+ // If true, an error is raised if the database already exists.
85+ bool error_if_exists = false;
86+
87+ // If true, the implementation will do aggressive checking of the
88+ // data it is processing and will stop early if it detects any
89+ // errors. This may have unforeseen ramifications: for example, a
90+ // corruption of one DB entry may cause a large number of entries to
91+ // become unreadable or for the entire DB to become unopenable.
92+ bool paranoid_checks = false;
93+
94+ // Use the specified object to interact with the environment,
95+ // e.g. to read/write files, schedule background work, etc.
96+ // Default: Env::Default()
97+ Env *env;
98+
99+ // Any internal progress/error information generated by the db will
100+ // be written to info_log if it is non-null, or to a file stored
101+ // in the same directory as the DB contents if info_log is null.
102+ Logger *info_log = nullptr;
103+
104+ // -------------------
105+ // Parameters that affect performance
106+
107+ // Amount of data to build up in memory (backed by an unsorted log
108+ // on disk) before converting to a sorted on-disk file.
109+ //
110+ // Larger values increase performance, especially during bulk loads.
111+ // Up to two write buffers may be held in memory at the same time,
112+ // so you may wish to adjust this parameter to control memory usage.
113+ // Also, a larger write buffer will result in a longer recovery time
114+ // the next time the database is opened.
115+ size_t write_buffer_size = 4 * 1024 * 1024;
116+
117+ // Number of open files that can be used by the DB. You may need to
118+ // increase this if your database has a large working set (budget
119+ // one open file per 2MB of working set).
120+ int max_open_files = 1000;
121+
122+ // Control over blocks (user data is stored in a set of blocks, and
123+ // a block is the unit of reading from disk).
124+
125+ // If non-null, use the specified cache for blocks.
126+ // If null, leveldb will automatically create and use an 8MB internal cache.
127+ Cache *block_cache = nullptr;
128+
129+ // Approximate size of user data packed per block. Note that the
130+ // block size specified here corresponds to uncompressed data. The
131+ // actual size of the unit read from disk may be smaller if
132+ // compression is enabled. This parameter can be changed dynamically.
133+ size_t block_size = 4 * 1024;
134+
135+ // Number of keys between restart points for delta encoding of keys.
136+ // This parameter can be changed dynamically. Most clients should
137+ // leave this parameter alone.
138+ int block_restart_interval = 16;
139+
140+ // Leveldb will write up to this amount of bytes to a file before
141+ // switching to a new one.
142+ // Most clients should leave this parameter alone. However if your
143+ // filesystem is more efficient with larger files, you could
144+ // consider increasing the value. The downside will be longer
145+ // compactions and hence longer latency/performance hiccups.
146+ // Another reason to increase this parameter might be when you are
147+ // initially populating a large database.
148+ size_t max_file_size = 2 * 1024 * 1024;
149+
150+ // Compress blocks using the specified compression algorithm. This
151+ // parameter can be changed dynamically.
152+ //
153+ // Default: kSnappyCompression, which gives lightweight but fast
154+ // compression.
155+ //
156+ // Typical speeds of kSnappyCompression on an Intel(R) Core(TM)2 2.4GHz:
157+ // ~200-500MB/s compression
158+ // ~400-800MB/s decompression
159+ // Note that these speeds are significantly faster than most
160+ // persistent storage speeds, and therefore it is typically never
161+ // worth switching to kNoCompression. Even if the input data is
162+ // incompressible, the kSnappyCompression implementation will
163+ // efficiently detect that and will switch to uncompressed mode.
164+ CompressionType compression = kSnappyCompression;
165+
166+ // EXPERIMENTAL: If true, append to existing MANIFEST and log files
167+ // when a database is opened. This can significantly speed up open.
168+ //
169+ // Default: currently false, but may become true later.
170+ bool reuse_logs = false;
171+
172+ // If non-null, use the specified filter policy to reduce disk reads.
173+ // Many applications will benefit from passing the result of
174+ // NewBloomFilterPolicy() here.
175+ const FilterPolicy *filter_policy = nullptr;
176+ };
177+ ----
178+
179+
180+ // Open the database with the specified "name".
181+ // Stores a pointer to a heap-allocated database in *dbptr and returns
182+ // OK on success.
183+ // Stores nullptr in *dbptr and returns a non-OK status on error.
184+ // Caller should delete *dbptr when it is no longer needed.
185+ static Status Open(const Options &options, const std::string &name,
186+ DB **dbptr);
61187A leveldb database has a name which corresponds to a file system directory. All
62188of the contents of database are stored in this directory. The following example
63189shows how to open a database, creating it if necessary:
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