RMAN通道channel

Channel可以看作是一个IO的进程，所以多通道的方式一般是提供RMAN的backup的并行度，对于多IO支持的设备，效果明显。 


在执行我们执行backup,restore,delete等有关物理文件的磁盘IO的命令是，我们可以用allocate channel来给他们分配通道，这里也可以执行多个allocate channel来分配多个通道，这个allocate channel必须在run的命令块里包含。例如 
run 
{ 
allocate channel c1 maxpiecesize=100m device type disk format '/backup/%T_%t_%U'; 
backup incremental level 0 database tag 'level_0_datafile'; 
} 


rman执行这些命令的时候，至少需要一个通道，多个通道的话就会对应多个进程来进行IO的操作。直白一点的话，多一个通道，可以看做是多了一个IO的进程来进行操作。 


我们也可以在run里不allocate通道，这时就没有手动的指定通道，那么就会用默认的通道，也就是预定义的通道了。 
预定义的通道，可以通过show all来查看。 


下列预定义配置命令均可以分配通道： 


* CONFIGURE DEVICE TYPE ... PARALLELISM 
* CONFIGURE DEFAULT DEVICE TYPE 
* CONFIGURE CHANNEL DEVICE TYPE 
* CONFIGURE CHANNEL n DEVICE TYPE 


以下是RMAN文档中对channel的描述 


CHANNEL : 


A connection between RMAN and the target database. Each allocated channel starts a new Oracle server session; the session then performs backup, restore, and recovery operations. The type of channel determines whether the Oracle server process will attempt to read or write and whether it will work through a third-party media manager. If the channel is of type: 
·DISK, the server process reads backups from or write backups to disk 
·sbt, the server process reads backups from or write backups to a third-party media manager 
Channels are always able to read and write datafiles to and from disk, no matter what their type.