Oracle等待事件之dbfilescatteredread-创新互联

db file scattered read

创新互联专注于企业成都全网营销、网站重做改版、营口网站定制设计、自适应品牌网站建设、H5技术、成都做商城网站、集团公司官网建设、外贸网站建设、高端网站制作、响应式网页设计等建站业务，价格优惠性价比高，为营口等各大城市提供网站开发制作服务。

官网解释：

This event signifies that the user process is reading buffers into the SGA buffer cache and is waiting for a physical I/Ocall to return. A db file scattered read issues a scattered read to read the data into multiple discontinuous memory locations. A scattered read is usually a multiblock read. It can occur for a fast full scan (of an index) in addition to a full table scan.

The db file scattered read wait event identifies that a full scan is occurring. When performing a full scan into the buffer cache, the blocks read are read into memory locations that are not physically adjacent to each other. Such reads are called scattered read calls, because the blocks are scattered throughout memory. This is why the corresponding wait event is called 'db file scattered read'. multiblock (up to DB_FILE_MULTIBLOCK_READ_COUNT blocks) reads due to full scans into the buffer cache show up as waits for 'db file scattered read'.

Check the following V$SESSION_WAIT parameter columns:

• P1: The absolute file number

• P2: The block being read

• P3: The number of blocks (should be greater than 1)

on a healthy system, physical read waits should be the biggest waits after the idle waits. However, also consider whether there are direct read waits (signifying full table scans with parallel query) or db file scattered read waits on an operational (OLTP) system that should be doing small indexed accesses.

Other things that could indicate excessive I/O load on the system include the following:

• Poor buffer cache hit ratio

• These wait events accruing most of the wait time for a user experiencing poor response time

解释：db file scattered read 等待事件：是由于多数据块读操作产生的，当我们检索数据时从磁盘上读取数据到内存中，一次I/0读取多个数据块，而数据块在内存中是分散分布并不是连续的，当数据块读取到内存中的这个过程就会产生“db file scattered read” 事件。

    多个数据块读场景：

      （1） FTS（Full Table Scans) 全表扫描

      （2） IFFS（Index Fast Full Scans) 索引快速全扫描： 把索引链切割成很多份，多块并行读取。

另外有需要云服务器可以了解下创新互联cdcxhl.cn，海内外云服务器15元起步，三天无理由+7*72小时售后在线，公司持有idc许可证，提供“云服务器、裸金属服务器、高防服务器、香港服务器、美国服务器、虚拟主机、免备案服务器”等云主机租用服务以及企业上云的综合解决方案，具有“安全稳定、简单易用、服务可用性高、性价比高”等特点与优势，专为企业上云打造定制，能够满足用户丰富、多元化的应用场景需求。

当前标题：Oracle等待事件之dbfilescatteredread-创新互联
分享链接：http://www.cdkjz.cn/article/doedep.html