2012-10-18 11gR2 concepts page 326 - 367

 

Shared Server Processes
The dispatcher process receives requests from connected clients and puts them into a
request queue in the large pool (see "Large Pool" on page 14-21). The first available
shared server process takes the request from the queue and processes it. Afterward,
the shared server place the result into the dispatcher response queue. The dispatcher
process is monitors this queue and transmits the result to the client.

 

Manageability Monitor Processes (MMON and MMNL) -- 这两个比较不那么常见的进程
The manageability monitor process (MMON) performs many tasks related to the
Automatic Workload Repository (AWR). For example, MMON writes when a metric
violates its threshold value, taking snapshots, and capturing statistics value for
recently modified SQL objects.
The manageability monitor lite process (MMNL) writes statistics from the Active
Session History (ASH) buffer in the SGA to disk. MMNL writes to disk when the ASH
buffer is full.

MMNL负责把ASH从内存写到磁盘。

 

Recoverer Process (RECO) -- 在分布式数据库中很常用
In a distributed database, the recoverer process (RECO) automatically resolves
failures in distributed transactions. The RECO process of a node automatically
connects to other databases involved in an in-doubt distributed transaction. When
RECO reestablishes a connection between the databases, it automatically resolves all
in-doubt transactions, removing from each database's pending transaction table any
rows that correspond to the resolved transactions.

 

Space Management Coordinator Process (SMCO)
The SMCO process coordinates the execution of various space management related
tasks, such as proactive space allocation and space reclamation. SMCO dynamically
spawns slave processes (Wnnn) to implement the task.

 

I/O Slave Processes -- 负责异步IO
I/O slave processes (Innn) simulate asynchronous I/O for systems and devices that
do not support it. In asynchronous I/O, there is no timing requirement for
transmission, enabling other processes to start before the transmission has finished.

 

To simulate asynchronous I/O, one process oversees several slave processes. The
invoker process assigns work to each of the slave processes, who wait for each write
to complete and report back to the invoker when done. In true asynchronous I/O the
operating system waits for the I/O to complete and reports back to the process, while
in simulated asynchronous I/O the slaves wait and report back to the invoker.

 

 

The database supports different types of I/O slaves, including the following:
 -- I/O slaves for Recovery Manager (RMAN) -- RMAN的SLAVE进程
When using RMAN to back up or restore data, you can make use of I/O slaves for
both disk and tape devices.
 -- Database writer slaves -- 数据库写进程的SLAVE进程
If it is not practical to use multiple database writer processes, such as when the
computer has one CPU, then the database can distribute I/O over multiple slave
processes. DBWR is the only process that scans the buffer cache LRU list for blocks
to be written to disk. However, I/O slaves perform the I/O for these blocks.

Dedicated Server Architecture -- 专有服务进程
In the dedicated server architecture, the user and server processes communicate using
different mechanisms:
 -- If the client process and the dedicated server process run on the same computer,
then the program interface uses the host operating system's interprocess
communication mechanism to perform its job.
 -- If the client process and the dedicated server process run on different computers,
then the program interface provides the communication mechanisms (such as the
network software and Oracle Net Services) between the programs.

 

Data Corruption -- 数据损坏
In addition to Data Guard and Recovery Manager, Oracle Database supports the
following forms of protection against data corruption:
 -- Oracle Hardware Assisted Resilient Data (HARD)
The HARD Initiative is a joint initiative between Oracle and hardware vendors to
prevent data corruptions from being written to disk. Under the HARD Initiative,
Oracle works with selected system and storage vendors to build operating system
and storage components that can detect corruption early and prevent corrupted
data from being written to disk.
 -- Lost write protection -- 丢失的"写“的保护
A data lost write occurs when an I/O subsystem acknowledges the
completion of the block write when the write did not occur. You can configure the
database so that is records buffer cache block reads in the redo log. Lost write
detection is most effective when used with Data Guard.
 -- Data block corruption detection -- 数据坏块的探测
A block corruption is a data block that is not in a recognized Oracle format, or
whose contents are not internally consistent. Several database components and
utilities, including RMAN, can detect a corrupt block and record it in
V$DATABASE_BLOCK_CORRUPTION. If the environment uses a real-time standby
database, then RMAN can automatically repair corrupt blocks.
 -- Data Recovery Advisor
Data Recovery Advisor is an Oracle tool that automatically diagnoses data
failures, determines and presents appropriate repair options, and executes repairs
at the user's request.