Oracle内存结构
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下一篇 2011-09-22 00:08:40
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From Expert Oracle Architecture:
the PGA is private to a process. It is the set of variables that an Oracle dedicated or shared server needs to have independent of a session. The PGA is a 'heap' of memory in which other structures may be allocated. The UGA on the other hand, is also a heap of memory in which various session-specific structures may be defined. The UGA is allocated from the PGA when you use the dedicated server mode to connect to Oracle, and from the SGA in MTS mode. This implies that when using MTS, you must size your SGA to have enough UGA space in it to cater for every possible user that will ever connect to your database concurrently. So, the SGA of a database running MTS is generally much larger than the SGA for a
similarly configured, dedicated server mode only, database.
Every Oracle instance has one big memory structure collectively referred to as the SGA, the System Global Area. This is a large, shared memory structure that every Oracle process will access at one point or another. It will vary from a couple of MBs on small test systems to hundreds of MBs on medium to large systems, and into many GBs in size for really big systems.
The SGA is broken up into various pools. They are:
Java pool – The Java pool is a fixed amount of memory allocated for the JVM running in the database.
Large pool – The large pool is used by the MTS for session memory, by Parallel Execution for message buffers, and by RMAN Backup for disk I/O buffers.
Shared pool – The shared pool contains shared cursors, stored procedures, state objects, dictionary caches, and many dozens of other bits of data.
The 'Null' pool – This one doesn't really have a name. It is the memory dedicated to block buffers (cached database blocks), the redo log buffer and a 'fixed SGA' area.
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