Which is the correct description of a pinned buffer in the database buffer cache?
A. The buffer is currently being accessed
B. The buffer is empty and has not been used
C. The contents of the buffer have changed and must be flushed to the disk by the DBWn process
D. The buffer is a candidate for immediate aging out and its contents are synchronized with the block contents on the disk
buffer cache中的buffer根据不同的状态可以分为以下4种类型：
free buffer/unused buffer：buffer cache初始化时或者在执行alter system flush buffer_cache以后的buffer,该buffer中没有存放任何内容。
clean buffer：server process从数据文件中读入的block且还没有被其他进程所修改或者又dbwr将dirt buffer刚刚写入到数据文件中的buffer,该buffer中的内容与数据文件中的block一致。
dirt buffer：该buffer的内容与数据文件中block的内容不一致。
pinned buffer：当前的buffer被某个进程正在读取或者正在修改时。