- Options：主要用於控制DB的一些操作。
- ReadOptions：用於控制讀操作屬性。
- WriteOptions：用於控制寫操作屬性。
一、Options
// Options to control the behavior of a database (passed to DB::Open)
<!用於控制DB的一些特性>
struct LEVELDB_EXPORT Options {
// Create an Options object with default values for all fields.
Options();
// -------------------
// Parameters that affect behavior
// Comparator used to define the order of keys in the table.
// Default: a comparator that uses lexicographic byte-wise ordering
//
// REQUIRES: The client must ensure that the comparator supplied
// here has the same name and orders keys *exactly* the same as the
// comparator provided to previous open calls on the same DB.
<!key的排序方式，默認使用字典字節序排序，\
一個數據庫的排序方式確定之後不要再改變>
const Comparator* comparator;
// If true, the database will be created if it is missing.
<!如果是true，數據庫打開時，若數據庫丟失會重新創建一份>
bool create_if_missing = false;
// If true, an error is raised if the database already exists.
<!如果是true，數據庫打開時如果存在則報錯。>
bool error_if_exists = false;
// If true, the implementation will do aggressive checking of the
// data it is processing and will stop early if it detects any
// errors. This may have unforeseen ramifications: for example, a
// corruption of one DB entry may cause a large number of entries to
// become unreadable or for the entire DB to become unopenable.
<!如果爲true，在數據處理過程中會嚴格檢查數據，檢查到任何錯誤都會提前停止，\
這可能會到來無法預料的後果，例如一個條目的損害可能導致大量的條目無法讀取\
或者整個數據庫無法打開了，所以這裏應該是建議false>
bool paranoid_checks = false;
// Use the specified object to interact with the environment,
// e.g. to read/write files, schedule background work, etc.
// Default: Env::Default()
<!使用指定的對象與環境交互，比如讀寫文件，安排後臺工作等\
默認使用Env默認值。一些複雜的操作用戶可自己指定而不依賴於默認環境。>
Env* env;
// Any internal progress/error information generated by the db will
// be written to info_log if it is non-null, or to a file stored
// in the same directory as the DB contents if info_log is null.
<!如果日誌不爲空，則將db產生的處理和錯誤日誌寫與指定文件，
如果爲空，則在同一目錄下創建一個文件作爲db內容寫入>
Logger* info_log = nullptr;
// -------------------
// Parameters that affect performance
// Amount of data to build up in memory (backed by an unsorted log
// on disk) before converting to a sorted on-disk file.
//
// Larger values increase performance, especially during bulk loads.
// Up to two write buffers may be held in memory at the same time,
// so you may wish to adjust this parameter to control memory usage.
// Also, a larger write buffer will result in a longer recovery time
// the next time the database is opened.
<!這個大小應該是memtable和immutable大小。
在大容量負載期間，增大這個值可以提升性能，
最多可保持兩份寫緩衝區在內存中，通過調節此值
可控制內存使用大小，唯一不好的是如果寫緩衝區太大，
下次打開數據庫時會耗時更久。>
size_t write_buffer_size = 4 * 1024 * 1024;
// Number of open files that can be used by the DB. You may need to
// increase this if your database has a large working set (budget
// one open file per 2MB of working set).
<!DB可打開的最大文件數，如果系統工作集很大，可考慮增大此值，\
每個打開的文件大概會佔用2MB的工作集大小>
int max_open_files = 1000;
// Control over blocks (user data is stored in a set of blocks, and
// a block is the unit of reading from disk).
// If non-null, use the specified cache for blocks.
// If null, leveldb will automatically create and use an 8MB internal cache.
<!block是DB的控制塊，是與磁盤交互的最小單元，用戶數據存儲在這個控制塊中。
用戶可指定一個cache，如果不指定，系統默認創建一個8MB的內部cache>
Cache* block_cache = nullptr;
// Approximate size of user data packed per block. Note that the
// block size specified here corresponds to uncompressed data. The
// actual size of the unit read from disk may be smaller if
// compression is enabled. This parameter can be changed dynamically.
<!每個block塊的大小，這裏指的是爲壓縮數據大小。如果開啓了數據壓縮，
則一個從磁盤讀取的塊大小可能是小於未壓縮之前大小的。此參數可動態改變>
size_t block_size = 4 * 1024;
// Number of keys between restart points for delta encoding of keys.
// This parameter can be changed dynamically. Most clients should
// leave this parameter alone.
<!block重啓點之間的key的個數，至於什麼是重啓點可參考（leveldb 二）中的說明>
int block_restart_interval = 16;
// Leveldb will write up to this amount of bytes to a file before
// switching to a new one.
// Most clients should leave this parameter alone. However if your
// filesystem is more efficient with larger files, you could
// consider increasing the value. The downside will be longer
// compactions and hence longer latency/performance hiccups.
// Another reason to increase this parameter might be when you are
// initially populating a large database.
<!落地磁盤文件最大大小，超過此大小重新一個新文件寫。
若文件系統在大文件方面表現好，可增大此值。值的增大會帶來以下問題：
1、更長的壓縮耗時，2、更長的性能中斷。增大此值的一個原因可能是
一開始就要填充一個大的數據庫>
size_t max_file_size = 2 * 1024 * 1024;
// Compress blocks using the specified compression algorithm. This
// parameter can be changed dynamically.
//
// Default: kSnappyCompression, which gives lightweight but fast
// compression.
//
// Typical speeds of kSnappyCompression on an Intel(R) Core(TM)2 2.4GHz:
// ~200-500MB/s compression
// ~400-800MB/s decompression
// Note that these speeds are significantly faster than most
// persistent storage speeds, and therefore it is typically never
// worth switching to kNoCompression. Even if the input data is
// incompressible, the kSnappyCompression implementation will
// efficiently detect that and will switch to uncompressed mode.
<!壓縮block的方式。壓縮速率指標如上，作者是是建議壓縮的，即使輸入數據不可壓縮
壓縮算法也可以自己檢測出來而切換到不壓縮模式>
CompressionType compression = kSnappyCompression;
// EXPERIMENTAL: If true, append to existing MANIFEST and log files
// when a database is opened. This can significantly speed up open.
//
// Default: currently false, but may become true later.
<!如果爲true，直接使用文件追加的方式，可使打開DB更快，
作者默認默認false>
bool reuse_logs = false;
// If non-null, use the specified filter policy to reduce disk reads.
// Many applications will benefit from passing the result of
// NewBloomFilterPolicy() here.
<!指定過濾策略來減少磁盤的讀取，作者推薦使用NewBloomFilterPolicy（）>
const FilterPolicy* filter_policy = nullptr;
};
二、ReadOptions
控制讀方式選項
// Options that control read operations
struct LEVELDB_EXPORT ReadOptions {
ReadOptions() = default;
// If true, all data read from underlying storage will be
// verified against corresponding checksums.
<!如果爲ture，所有讀取數據都會校驗>
bool verify_checksums = false;
// Should the data read for this iteration be cached in memory?
// Callers may wish to set this field to false for bulk scans.
<!從迭代器讀取的數據是否要緩存在內存中，
數據批量掃描可能希望爲false>
bool fill_cache = true;
// If "snapshot" is non-null, read as of the supplied snapshot
// (which must belong to the DB that is being read and which must
// not have been released). If "snapshot" is null, use an implicit
// snapshot of the state at the beginning of this read operation.
<!快照，有快照就讀取快照數據，沒快照就正常讀取>
const Snapshot* snapshot = nullptr;
};
三、WriteOptions
// Options that control write operations
struct LEVELDB_EXPORT WriteOptions {
WriteOptions() = default;
// If true, the write will be flushed from the operating system
// buffer cache (by calling WritableFile::Sync()) before the write
// is considered complete. If this flag is true, writes will be
// slower.
//
// If this flag is false, and the machine crashes, some recent
// writes may be lost. Note that if it is just the process that
// crashes (i.e., the machine does not reboot), no writes will be
// lost even if sync==false.
//
// In other words, a DB write with sync==false has similar
// crash semantics as the "write()" system call. A DB write
// with sync==true has similar crash semantics to a "write()"
// system call followed by "fsync()".
<!是否寫同步，同步寫是忙於異步寫的，但不會造成數據丟失，
如果是異步寫，只有在機器重啓的情況下才會造成數據丟失，
其它情況這不會丟失>
bool sync = false;
};
