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Although secondary storage (usually magnetic disks) is significantly larger than main memory, it s also significantly slower. A disk I/O involves either moving a data block from disk to memory (a disk read) or writing a data block to disk from memory (a disk write). Typically, it takes about 10 40 milliseconds (0.01 0.04 seconds) to perform a single disk I/O. Suppose your update transaction involves 25 I/Os you could spend up to 1 second just waiting to read or write data. In that same second, your CPUs could have performed millions of instructions the update takes a negligible amount of time compared to the disk reads and disk writes. If you already have the necessary data in Oracle s memory, the retrieval time would be much faster, as memory read/writes take only a few nanoseconds. This is why avoiding or minimizing disk I/Os plays such a big role in providing high performance in Oracle databases.
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All computers use memory, which actually consists of a hierarchy of different levels of memory. The heart of this hierarchy is main memory, which contains all the instruction executions and data manipulations. All main memories are random access memory (RAM), which means that you can read any byte in memory in the same amount of time. Typically, you can access main memory data in the 10 100 nanosecond range. An important part of the information Oracle stores in the RAM allocated to it is the program code that is executing currently or that has been executed recently. If a new user process needs to use the same code, it s available in memory in a compiled form, making the processing time a whole lot
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faster. The memory areas also hold information about which users are locking a certain table, thereby helping different sessions communicate effectively. Most important, perhaps, the memory areas help in processing data that s stored in permanent disk storage. Oracle doesn t make changes directly to the data on disk: data is always read from the disks, held in memory, and changed there before being transferred back to disk. It s common to use the term buffers to refer to units of memory. Memory buffers are page-sized areas of memory into which Oracle transfers the contents of the disk blocks. If the database wants to read (select) or update data, it copies the relevant blocks from disk to the memory buffers. After it makes any necessary changes, Oracle transfers the contents of the memory buffers to disk. Oracle uses two kinds of memory structures, one shared and the other process-specific. The system global area is the part of total memory that all server processes (including background processes) share. The process-specific part of the memory is known as the program global area (PGA), or process-private memory. The following sections examine these two components of Oracle s memory in more detail.
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The SGA is the most important memory component in an Oracle instance. In large OLTP databases, especially, the SGA is a much larger and more important memory area than the PGA. In data warehousing environments, on the other hand, the PGA can be the more important Oracle memory area, because it critically influences the efficiency of large data sorts and hashes, which are commonly part of analytic computations in data warehouses. The SGA s purpose is to speed up query performance and to enable a high amount of concurrent database activity. Because processing in memory is much faster than disk I/O, the size of the SGA is one of the more important configuration issues when you re tuning the database for optimal performance. When you start an instance in Oracle, the instance takes a certain amount of memory from the operating system s RAM the amount is based on the size of the SGA component in the initialization file. When the instance is shut down, the memory used by the SGA goes back to the host system. The SGA isn t a homogeneous entity; rather, it s a combination of several memory structures. The following are the main components of the SGA: Database buffer cache: Holds copies of data blocks read from datafiles. Shared pool: Contains the library cache for storing SQL and PL/SQL parsed code in order to share it among users. It also contains the data dictionary cache, which holds key data dictionary information. Redo log buffer: Contains the information necessary to reconstruct changes made to the database by DML operations. This information is then recorded in the redo logs by the log writer. Java pool: Represents the heap space for instantiating your java objects. Large pool: Stores large memory allocations, such as RMAN backup buffers. Streams pool: Supports the Oracle Streams feature. When you start the Oracle instance, Oracle allocates memory as needed until it reaches the size set in the MEMORY_TARGET initialization parameter, which sets the limit for the total memory allocation. If your total memory allocation is already at the MEMORY_TARGET limit, you can t dynamically increase memory to any memory component without decreasing some other component s memory allocation. Oracle does allow you to exchange the memory from one dynamically sizable memory component to another. For example, you can increase the memory assigned to the buffer cache by taking it from the shared pool. If you have certain jobs run only at specified times of the day, you can write a simple script that runs before the job executes and modifies the allocation of memory among the various
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