Oracle Data File (3)

1. Online log files

1. Each Oracle database has at least two online redo log file groups. Each redo log group contains one or more redo log members, the individual redo log file members of these groups actually form an image of each other. The size of these online redo log files is fixed and used cyclically.
2. Switching from one log file group to another is called Log switching.
3. Database buffer cache is the place where temporary database blocks are stored. This is a structure in Oracle SGA. The read block is stored in the cache, so that you do not have to read it again physically. The buffer cache is a Performance Tuning Device first. Its purpose is to make the very slow physical I/O Process look faster. When you modify a block (update a row on the block), these modifications are completed in the memory and written to the block in the buffer cache. In addition, sufficient information required to redo these changes will be saved in the redo log buffer, which is another SGA data structure. When you submit (COMMIT) changes, these changes will become permanent. Oracle does not access all the blocks modified in SGA and writes them to the disk. Instead, it only writes the contents of the redo log buffer to the online redo log. As long as the modified block is still in the buffer cache and not on the disk, when the database fails, we will need to redo the log content online.
4. How to confirm the number and size of online log files

• Peak Load: you may expect that the system does not have to wait for a checkpoint to establish for unfinished messages, and does not encounter any bottlenecks during peak traffic.
• A large number of users modify the same block: if a large number of users want to modify the same block, you may want to redo a large log file.
• Average recovery time: if the recovery must be completed as quickly as possible, even if a large number of users want to modify the same block, they may prefer to use smaller redo log files.

Ii. Archiving redo logs

1. Oracle databases can run in two modes: archivelog mode and noachivelog mode.

Iii. Password File

1. The password file is an optional file that allows remote sysdba or administrator access to the database.
2. When oracle is started in the local system, the network is not needed, and Oracle performs Authentication through the Operating System
3. The password file stores a list of user names and passwords. These user names and passwords correspond to users who can remotely authenticate sysdba over the network. Oracle must use this file to authenticate users, rather than the list of normal passwords stored in the database.
4. Set remote_login_passwordlife as the parameter value.

• The default value is none, indicating that the password file does not exist.
• Shared (multiple databases can use the same password file)
• Exclusive (only one database uses a given password file)

1. Use the orapwd command to create and enter the initial password file

Iv. Flash Back log files

1. The flashback log file is a new feature introduced in Oracle10g to support the flashback database command. The flashback log contains the pre-Modified Image of the data block, which can be used to restore the database to the status before the time point.
2. Delete the user recovery mode accidentally before introducing the flash back Database Function

• DBA wants to close the database
• DBA restores a complete tape drive for a long time
• DBAs need to restore all generated archived redo logs because there is no backup on the system.
• DBA rolls the database in front and stops at the time point before the error drop USER command.
• To open a database with the resetlogs Option

1. Flash back processing mode

• DBA closes Database
• DBA can start and load the database. It can use SCN, Oracle clock or timestamp (wall clock time) to issue a flash back database command. The time can be accurate to one or two seconds.
• DBA opens the database with the resetlogs Option

1. To use the flashback feature, the database must adopt the archivelog mode and be configured to support the flashbacj database command.
2. The Flash recovery area is a new location where Oracle manages multiple files related to database backup and recovery.

• Copy of data files on the disk
• Incremental Database Backup
• Redo log (archive redo log)
• Control File and Control
• File backup