There are four reasons that a session cannot pin a buffer in the buffer cache, and a separate wait event exists for each reason:
- "buffer busy waits": A session cannot pin the buffer in the buffer cache because another session has the buffer pinned.
- "read by other session": A session cannot pin the buffer in the buffer cache because another session is reading the buffer from disk.
- "gc buffer busy acquire": A session cannot pin the buffer in the buffer cache because another session is reading the buffer from the cache of another instance.
- "gc buffer busy release": A session cannot pin the buffer in the buffer cache because another session on another instance is taking the buffer from this cache into its own cache so it can pin it.
Prior to release 10.1, all four reasons were covered by "buffer busy waits." In release 10.1, the "gc buffer busy" wait event covered both the "gc buffer busy acquire" and "gc buffer busy release" wait events.
buffer busy waits
因为另一个会话已经pin住了buffer，当前会话不能pin此buffer时产生此等待，直到该buffer可用。
可以看出该等待是不跨实例的，因为无论是“另一个会话”从哪个实例读取的buffer，都应该是gc buffer busy acquire或者gc buffer busy release。
read by other session
当另一个会话从磁盘读取数据到buffer时，当前会话无法pin该buffer时，则会产生此等待。在10.1之前，该等待归属于buffer busy wait。
该等待是可以跨实例也可以不跨实例的，因为等待其他会话从磁盘读数据到buffer可能在多实例环境中发生。
gc buffer busy acquire
当其他实例的一个会话在读取它本地的buffer时，当前实例的会话无法pin该buffer会产生此等待。
In 11.1 and earlier version, this type of waits are grouped as “gc buffer busy” waits.
starting with Oracle database 11g Release 2, the “gc buffer busy” waits are categorized into two new wait categories:
• gc buffer busy acquire
• gc buffer busy release
gc buffer busy release
当其他实例的一个会话在读取本地（当前实例）的buffer到它的实例的cache中时，当前实例的会话无法pin该buffer，则会产生此等待。
gc buffer busy acquire和gc buffer busy release的区别就在两个会话申请的buffer在哪个实例中。
自己画的图，羞耻