Oracle Tips by Burleson

Oracle PGA

PGA stands for Process Global Area. This is memory reserved for each process that uses Oracle. It contains the context area. Oracle sets this area’s size based on the values of the initialization parameters:

OPEN_LINKS. The number of database links allowed open per user.
DB_FILES. The number of database files allowed for the database (up to the value of MAX_DATAFILES).
LOG_FILES. The maximum number of redo log file groups (up to the value of MAX_LOGFILES).

The PGA also contains session-related information if MTS is not used. In environments where MTS is used, the session information is placed in the SHARED_POOL region of the SGA if the LARGE POOL (in Oracle8,8i and 9i) is not configured. This session information consists of the user’s private SQL area and other session-specific data. The PGA will always hold the user’s stack information. The section of the shared or LARGE POOL allocated for the user is called the UGA, which stands for Users Global Area.

Another contributor to the memory footprint of each process is the size of the SORT_AREA_SIZE and SORT_AREA_RETAINED_SIZE parameters. When a process executes a sort memory, the size of SORT_AREA_SIZE is allocated to the user. If SORT_AREA_RETAINED_SIZE is also set, then this amount of memory is reduced to that value and allowed to grow to SORT_AREA_SIZE. If the sort requires more space than is specified in SORT_AREA_SIZE, the sort is broken into SORT_AREA_SIZED chunks, which are swapped out to disk as needed.

Some System-Specific Notes

Each specific operating system contains certain items that a Remote DBA needs to be aware of and take into account when installing Oracle. This section attempts to consolidate these system-specific notes in one location.

UNIX-Specific Notes

There are many different versions of UNIX, including Solaris, AIX, SV4, and the new kid on the block, Linux (while Linux has been around since the early 90's it is just now coming to the fore in business uses, hence the new kid label.) This subsection provides general guidelines and some specific suggestions for those systems I have experience with.

Datafiles 

On the install for the general-use database, the MAXDATAFILES parameter defaults to a value of 100. On older versions of UNIX there may be a kernel-based limit of 60 open files per process. This can be overcome by altering the OPEN_MAX value in the limits.h file. Under some versions of UNIX this may be different; to make the determination, look under configurable kernel parameters. Another UNIX-specific limit is on the total number of file extents.

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