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DSS Sizing and Tuning of Oracle8 for Windows NT on CompaqServers Page 5
SGA. When a shadow thread must write to the database, it writes into the SGA only. At a later
time the DBWR writes this "dirty" data out to disk.
Parallel execution begins with a shadow thread executing a SQL statement that contains
operations, some of which may be performed in parallel. This becomes the shadow parallel
coordinator. It dispatches the execution of a statement to several parallel server threads and
coordinates the results from the parallel server threads to send the results back to the user. The
number of parallel server threads assigned to a single operation is called the "degree of
parallelism".
The redo log contains a history of all committed transactions for the database to perform an
instance recovery. A minimum of two redo logs is required and more may be used. When a redo
log fills, a log switch occurs. At log switch time all new redo information goes to the next redo
log file in line. If the system is running in ARCHIVELOG mode, which is recommended, the
previous log file is copied out to an archive log file.
With a recent backup, the redo log files, and the redo log archive files, the database can be
recovered if needed. The control and configuration files are used to store information of the state
and layout of the database as well as system tunables. A more in-depth discussion of the Oracle8
Server architecture can be found in the Oracle8 Server Concepts Manual.
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