Common Wait Events---buffer busy waits

buffer busy waits
The buffer busy waits event occurs when a session wants to access a data block in the buffer cache that is currently in use by some other session. The other session is either reading the same data block

into the buffer cache from the datafile, or it is modifying the one in the buffer cache. 

当一个session试图访问buffer cache中其他session正在使用的同一数据块时就会发生buffer busy waits事件，这些session正在从数据文件中读这些块到buffer cache中，或正在修改buffer cache中的这些块。

In order to guarantee that the reader session has a coherent image of the block with either all of the changes or none of the changes, the session modifying the block marks the block header with a flag

letting other sessions know that a change is taking place and to wait until the complete change is applied.

为了保证这些读session对这些数据块的一致性读，修改数据块的session会在数据块的头部作个标记，以告诉其他session当前数据块正在被修改，等待修改完成并commit后再才读取。

Prior to Oracle Database 10g Release 1, buffer busy waits event was posted by the session when it had to wait for the other session to read the same data block into the buffer cache. However, starting

with Oracle Database 10g Release 1, such waits are now posted as read by other session event. The buffer busy waits event denotes the waits by a session for data block change completion by some

other session.

Oracle Database 10g Release 1 has another event titled buffer busy. Do not confuse this event with

buffer busy waits. The buffer busy event is posted by sessions accessing cached metadata in a

database using Automatic Storage Management (ASM).

Oracle  10g Release 1中，还有另外一个buffer busy的等待事件，不要和buffer  busy waits事件混肴，在数据库中使用ASM访问缓存中的元数据时就会出现buffer busy事件、

分别看一个这两个事件的的参数含义：

SQL> SELECT * FROM V$EVENT_NAME e WHERE e.NAME LIKE 'buffer busy%';     EVENT#  EVENT_ID    NAME                 PARAMETER1             PARAMETER2                 PARAMETER3        WAIT_CLASS_ID WAIT_CLASS#      WAIT_CLASS----------  ----------  -----------------    --------------       ----------------             ------            ------------- -------------    ----------        64  2161531084  buffer busy waits    file#                  block#                      class#           3875070507      4              Concurrency       790  2251737163  buffer busy          group#                 obj#                        block#           1893977003      0              Other 

buffer busy waits：三个参数分别代表：文件号、数据块号、wait class号

buffer busy           ：三个参数分别代表：组号 、对象号、数据块号