K/V存儲數據庫：Lightning MDB

LMDB是一個超快速，超緊湊的鍵 - 值存儲數據庫由Symas為OpenLDAP項目開發。它使用內存映射文件，所以它擁有一個純內存數據庫的讀取性能，同時還提供標準的基于磁盤的數據庫持久性，并且只限于虛擬地址空間的大小（但不限于物理RAM的大小）。

LMDB是一個微小的數據庫，但有一些強大的功能：

• 有序Map接口 (鍵總是排序，支持范圍查詢)
• 完整的事務支持，支持完整的 ACID semantics with MVCC.
• Reader/writer transactions: readers don't block writers and writers don't block readers. Writers are fully serialized, so writes are always deadlock-free.
• Read transactions are extremely cheap, and can be performed using no mallocs or any other blocking calls.
• Supports multi-thread and multi-process concurrency, environments may be opened by multiple processes on the same host.
• Multiple sub-databases may be created with transactions covering all sub-databases.
• Memory-mapped, allowing for zero-copy lookup and iteration.
• Maintenance-free, no external process or background cleanup/compaction required.
• No application-level caching. LMDB fully exploits the operating system's buffer cache.
• 32KB of object code and 6KLOC of C.
• Licensed under the OpenLDAP Public License
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這是一個只讀優化設計和執行讀取比其他數據庫引擎快好幾倍，幾個數量級，在很多情況下更快。這不是一個寫優化設計;注重隨機寫入工作負載的其他數據庫的設計可能更適合。

特性比較

一些流行的嵌入式鍵/值存儲比較

	LMDB	BDB	LevelDB	Kyoto TreeDB
ACID Transactions	x	x
Nested Transactions	x	x
Multiple Namespaces	x	x
Sorted Keys	x	x	x	x
Sorted Duplicate Keys	x	x
Multi-thread concurrency	x	x	x	x
Multi-process concurrency	x	x
No cache tuning: zero-config	x
Instantaneous crash recovery	x
Zero-copy reads	x
Zero-copy writes	x
Atomic hot backup	x




項目主頁：http://www.baiduhome.net/lib/view/home/1389838839258