Return a key suitable for lookup in a MemTable. 可以看到，MemTable 获取的 end_ - start_，也就是说前面的变长 size 也会作为 MemTable Key * 的一部分 */

Extracts the file name from a path pointing to a file. The returned Slice points to |filename|'s data buffer, so it is only valid while |filename| is

Convert an index iterator value (i.e., an encoded BlockHandle) into an iterator over the contents of the corresponding block.

REQUIRES: Writer list must be non-empty REQUIRES: First writer must have a non-null batch

Initialize a builder with the files from *base and other info from *vset * 构造函数接收一个 VersionSet 和 Base Version */

Switch to a fresh database with the next option configuration to test. Return false if there are no more configurations to test.

Callback from leveldb_writebatch_iterate()

Callback from leveldb_writebatch_iterate()