1 Oracle Database Architecture on Windows

1.1.1 Thread-Based Architecture

The internal process architecture of Oracle Database is thread-based. Threads are objects within a process that run program instructions. Threads allow concurrent operations within a process so that a process can run different parts of its program simultaneously on different processors. A thread-based architecture provides the following advantages:

• Faster context switching

• Simpler System Global Area allocation routine, because it does not require use of shared memory

• Faster spawning of new connections, because threads are created more quickly than processes

• Decreased memory usage, because threads share more data structures than processes

Internally, the code to implement the thread model is compact and separate from the main body of Oracle Database code. Exception handlers and routines track and de-allocate resources. They add robustness, with no downtime because of resource leaks or an ill-behaved program.

Oracle Database is not a typical Windows process. On Windows, an Oracle Database instance (threads and memory structures) is a Windows service: a background process registered with the operating system. The service is started by Windows and requires no user interaction to start. This enables the database to open automatically at startup.

When running multiple Oracle Database instances on Windows, each instance runs its own Windows service with multiple component threads. Each thread may be required for the database to be available, or it may be optional and specific to certain platforms. The background processes read and write from various datafiles, depending on your configuration. Oracle Database architecture on Windows is illustrated in Figure 1-1. Examples of optional and required threads on Windows are listed in Table 1-1.

Figure 1-1 Oracle Database architecture on Windows

Description of the illustration ntqrf003.gif

SQL> select * from v$bgprocess where paddr <> '00'; 

Oracle Database for Windows is supplied as a set of executables and dynamic link libraries (DLLs). Executable images can be modified using ORASTACK to change the size of the stack used by the threads of the Oracle Database process. (Oracle recommends you use this tool only under the guidance of Oracle Support Services.)

1.1.2 File I/O Enhancements

Oracle Database supports 64-bit file I/O to allow use of files larger than 4 gigabytes (GB). In addition, physical and logical raw files are supported as data, log, and control files to support Oracle Real Application Clusters on Windows and for those cases where performance needs to be maximized.

All Oracle Database file I/O routines support 64-bit file offsets, meaning there are no 2 GB or 4 GB file size limitations when it comes to data, log, or control files, as there are on some other platforms. In fact, the limitations that are in place are generic Oracle Database limitations across all platforms. These limits include 4 million database blocks for each file, 16KB maximum block size, and 64K files for each database. If these values are multiplied, then maximum file size for a database file on Windows is 64 GB, and maximum total database size supported (with 16KB database blocks) is 4 petabytes.

1.1.3 Raw File Support

Windows supports raw files, similar to UNIX. Using raw files for database or log files can have a slight performance gain. Raw files are unformatted disk partitions that can be used as one large file. Raw files have the benefit of no file system overhead, because they are unformatted partitions. However, standard Windows commands do not support manipulating or backing up raw files. As a result, raw files are generally used only by very high-end installations and by Oracle Real Application Clusters, where they are required.

To Oracle Database, raw files are no different from other Oracle Database files. They are treated in the same way by Oracle Database as any other file and can be backed up and restored through Recovery Manager or OCOPY.

1.2.1 Support for Very Large Memory (VLM) Configurations

Oracle Database for Windows supports Very Large Memory (VLM) configurations in Windows 2000 and Windows XP, which allows Oracle Database to access more than the 4 gigabyte (GB) of RAM traditionally available to Windows applications.

Specifically, Oracle Database uses Address Windowing Extensions (AWE) built into Windows 2000 and Windows XP to access more than 4 GB of RAM.

The requirements for taking advantage of this support are:

1. The computer on which Oracle Database is installed must have more than 4 GB of memory.

2. The operating system must be configured to take advantage of Physical Address Extensions (PAE) by adding the /PAE switch in boot.ini. See Microsoft Knowledge Base article Q268363 for instructions on modifying boot.ini to enable PAE.

3. It is advisable (though not necessary) to enable 4GT support by adding the /3GB parameter in boot.ini. See Microsoft Knowledge Base article Q171793 for additional requirements and instructions on modifying boot.ini to enable 4GT.

4. The user account under which Oracle Database runs (typically the LocalSystem account), must have the "Lock memory pages" Windows 2000 and Windows XP privilege.

5. USE_INDIRECT_DATA_BUFFERS=TRUE must be present in the initialization parameter file for the database instance that will use VLM support. If this parameter is not set, then Oracle Database 10g Release 1 (10.1) behaves in exactly the same way as previous releases.

6. Initialization parameters DB_BLOCK_BUFFERS and DB_BLOCK_SIZE must be set to values you have chosen for Oracle Database.

   
   

   Note: The total number of bytes of database buffers (that is, DB_BLOCK_BUFFERS multiplied by DB_BLOCK_SIZE) is no longer limited to 3 GB. Dynamic SGA and multiple block size are not supported with VLM. When VLM is enabled, the following new buffer cache parameters are not supported: DB_CACHE_SIZE DB_2K_CACHE_SIZE DB_4K_CACHE_SIZE DB_8K_CACHE_SIZE DB_16K_CACHE_SIZE DB_32K_CACHE_SIZE To select the block size for the instance, use the initialization parameter DB_BLOCK_SIZE. The buffer cache size is set by the initialization parameter DB_BLOCK_BUFFERS.

7. Registry parameter AWE_WINDOW_MEMORY must be created and set in the appropriate key for your Oracle home. This parameter is specified in bytes and has a default value of 1 GB. AWE_WINDOW_MEMORY tells Oracle Database how much of its 3 GB address space to reserve for mapping in database buffers.

   This memory comes from the 3 GB virtual address space in Oracle Database, so its value must be less than 3 GB. Setting this parameter to a large value has the effect of using more of the address space for buffers and using less AWE memory for buffers. However, since accessing AWE buffers is somewhat slower than accessing virtual address space buffers, Oracle recommends that you tune these parameters to be as large as possible without adversely limiting database operations.

   In general, the higher AWE_WINDOW_MEMORY is set, the fewer connections and memory allocations will be possible for Oracle Database. The lower AWE_WINDOW_MEMORY is set, the lower the performance.

8. Once this parameter is set, Oracle Database can be started and will function exactly the same as before except that more database buffers are available to the instance. In addition, disk I/O may be reduced because more Oracle Database data blocks can be cached in the System Global Area (SGA).

   
   

   Note: Registry parameter VLM_BUFFER_MEMORY, which enabled VLM configurations in Windows NT in Oracle8i release 2 (8.1.6), is not supported in Oracle Database 10g Release 1 (10.1).

VLM Instance Tuning

VLM configurations improve database performance by caching more database buffers in memory. This reduces disk I/O compared to configurations without VLM. VLM support in Oracle Database 10g Release 1 (10.1) has been re-written to integrate very closely with Windows. Compared to Oracle8i release 2 (8.1.6), VLM users should see better performance with the newer implementation.

Tuning for VLM is no different than tuning for configurations without VLM. It is an iterative task that begins by selecting appropriate DB_BLOCK_SIZE and DB_BLOCK_BUFFERS initialization parameters for the application being supported.

AWE_WINDOW_MEMORY, a new registry parameter specific to VLM, tells Oracle Database how much of its address space to reserve for mapping in database buffers. It defaults to a value of 1 GB, which should be suitable for most installations. If DB_BLOCK_SIZE is large, however, the default AWE_WINDOW_MEMORY value of 1 GB may not be sufficient to start the database.

Increasing the value of AWE_WINDOW_MEMORY will improve performance, but it will also limit the amount of memory available for other Oracle Database threads (like foreground threads). Clients may see "out of memory" errors if this value is set too large. As a general guideline, increase the AWE_WINDOW_MEMORY registry value by 20 percent.

For example, if DB_BLOCK_SIZE is set to 8 KB, AWE_WINDOW_MEMORY is set to 1 GB, and the number of LRU latches is set to 32 (16 processor computer), then database startup fails with out of memory errors 27102 and 34. Increasing the value of AWE_WINDOW_MEMORY to 1.2 GB fixes the problem.

Having a large cache in a VLM configuration may also slow down database writer (DBWR) threads. Having more DBWR threads will distribute work required to identify and write buffers to disk and will distribute I/O loads among threads. Initialization parameter DB_WRITER_PROCESSES enables you to configure multiple database writer threads.

A large cache can also introduce contention on the LRU (least recently used) latch. On symmetric multiprocessor (SMP) systems, Oracle Database sets the number of LRU latches to a value equal to one half the number of processors on the system. You can reduce contention on such configurations by increasing the number of LRU latches to twice (or four times) the number of processors on the system.

1.2.2 4 GB RAM Tuning (4GT)

The following Windows operating systems include a feature called 4 GB RAM Tuning (4GT):

• Windows Server 2003

• Windows 2000 Advanced Server

• Windows 2000 Datacenter Server

• Windows NT 4.0 Enterprise Edition

This feature allows memory-intensive applications running on Oracle Database Enterprise Edition to access up to 3 GB of memory, as opposed to the standard 2 GB in previous operating system versions. 4GT provides a tremendous benefit: 50 percent more memory is available for database use, increasing SGA sizes or connection counts.

1.2.3 Large User Populations

Several features allow Oracle Database to support an increasingly large number of database connections on Windows:

• Oracle Database Shared Server Process, which limits the number of threads needed in the Oracle Database process, supports over 10,000 simultaneous connections to a single database instance.

• Oracle Net multiplexing and connection pooling features allow a large configuration to connect more users to a single database instance.

• Oracle Real Application Clusters raises connection counts dramatically by allowing multiple server computers to access the same database files, increasing the number of user connections by tens of thousands, as well as increasing throughput.

  
  

  Note: Oracle Real Application Clusters is not supported on Windows XP.

1.3.1 Oracle PKI Integration with Windows

Oracle Advanced Security includes Oracle PKI (public key infrastructure) integration for authentication and single sign-on. You can integrate Oracle-based applications with the PKI authentication and encryption framework, using Oracle Wallet Manager.

1.3.2 Oracle Services for Microsoft Transaction Server

Microsoft Transaction Server (MTS) is used in the middle tier as an application server for COM/COM+ objects and transactions in distributed environments. In Windows 2000 and Windows XP it is part of COM+. Oracle Services for Microsoft Transaction Server allows Oracle Database to be used as a resource manager in Microsoft Transaction Server-coordinated transactions, providing strong integration between Oracle solutions and Microsoft Transaction Server. Oracle Services for Microsoft Transaction Server can operate with Oracle Database running on any operating system.

Oracle Database takes advantage of a native implementation and also stores recovery information in Oracle Database itself. Oracle Services for Microsoft Transaction Server allows development in all industry-wide data access interfaces, including Oracle Objects for OLE (OO4O), Oracle Call Interface (OCI), ActiveX Data Objects (ADO), OLE DB, and Open Database Connectivity (ODBC). The Oracle APIs, OO4O and OCI, offer greatest efficiency.

1.3.3 Oracle Fail Safe Integration with Windows

Oracle Fail Safe ensures that Oracle Database (and also other Oracle and third-party applications) can be configured and managed for high availability on Windows clusters. An instance runs on only one node at a time.

A cluster is a group of independent computing systems that operates as a single virtual system, eliminating individual host systems as points of failure. Oracle Fail Safe works with Microsoft Cluster Server to ensure that if a failure occurs on one cluster system, then workloads running on that system fail over quickly and automatically to a surviving system. Oracle Database combined with Oracle Fail Safe on a Windows cluster ensures protection from both hardware and software failures.

For well-configured solutions, Oracle Fail Safe ensures a surviving system to be operational in less than a minute, even for heavily-used databases.

1.3.4 Oracle Real Application Clusters Integration with Windows

Oracle Real Application Clusters are integrated with Microsoft Cluster Server clusters deployed on all Windows operating systems that support clustering. This enhances high availability features of Oracle Real Application Clusters by offering:

• Optional automatic restarts of a failed instance or listener in a cluster

• Detection and resolution of instance hangs

• Elimination of connect-time failover TCP/IP timeout delays for new connection requests

• Use of user-written scripts after database state (online/offline) changes