Redo file recovery

This article aims at walking you through some of the common recovery scenarios after a loss of Online Redolog. If a media failure has affected the online redo logs of a database, then the. If the hardware problem is temporary, then correct it. The log writer process accesses the previously unavailable online redo log files as if the problem never existed.

If the hardware problem is permanent, then drop the damaged member and add a new member by using the following procedure. For example, to drop redo log member from group 2, issue:. For example, to add new redo log member to group 2, issue:. If all members of an online redo log group with INACTIVE status are damaged, then the procedure depends on whether you can fix the media problem that damaged the inactive redo log group.

If the failure is LGWR can reuse the redo log group when required. If a single disk fails, then only one member of a group becomes unavailable to LGWR and other members remain accessible to LGWR, so the instance can continue to function. For instance, if you have two groups of multiplexed redo log members a duplexed redo log , store each member on a separate disk and specify your archiving destination to a fifth disk.

To decrease congestion while writing data blocks and redo records, datafiles should be put on distinct disks than redo log files. Consider whether you will archive the redo log when determining the size of the redo log files. Redo log files should be sized so that a whole group may be archived to a single unit of offline storage media such as a tape or disk with as little space on the medium as possible. In this instance, it is preferable to significantly reduce the size of the redo log files so that two log groups may be stored on each tape.

The size of all members of the same multiplexed redo log group must be the same. Members of various organisations might be of varying sizes. Changing the file size across groups, on the other hand, is pointless. If checkpoints are not configured to occur between log switches, make all groups the same size to ensure that they occur at regular intervals.

Testing multiple configurations is the best technique to discover the proper amount of redo log files for a database instance. The optimal design includes as few groups as possible while still allowing LGWR to write redo log information. A Database instance may only require two groups in specific instances. In some cases, an extra group may be required by a database instance to ensure that a recycled group is always available to LGWR.

Examining the contents of the LGWR trace file and the database alert log during testing is the simplest approach to examine whether the current redo log configuration is adequate. Add groups if messages show that LGWR regularly has to wait for a group because a checkpoint has not been completed or a group has not been archived.

Before you set up or change the configuration of an instance redo log, you can decide the parameters that will restrict the number of redo log files. The following parameters limit the amount of redo log files that may be added to a database:.

You can compel all enabled redo log threads to swap their current logs at regular intervals. Setting this parameter allows you to determine how long the lag might be in seconds time. If any of the following circumstances are satisfied, the instance will change the log:. It also specifies a time limit in seconds for the current log of the primary database. This is not the precise log switch time because the expected archiving time is also taken into account.

When LGWR stops writing to one redo log group and begins writing to another, then a log switch occurs. By default, a log switch occurs automatically when the current redo log file group is full.

You can force a log switch to deactivate the currently active group and make it available for redo log maintenance procedures. You may configure the database to utilise checksums to validate blocks in redo log files.

The checksum is used by Oracle Database to detect corruption in a redo log block. When a redo log block is read from an archived log during recovery or written to an archive log file, the database verifies it. If corruption is detected, an error is generated and recorded in the alert log. If the system detects corruption in a redo log block while attempting to archive it, it attempts to read the block from another member of the group.

To see the names of all of the members of a group, you can use a query similar as showcased below:. For further information on Oracle Redo Logs , you can visit here. If you want an easier alternative to transfer data from your Oracle database without having to use Oracle Redo Logs described above, you can use an automated data integration solution such as Hevo.

Hevo Data , a No-code Data Pipeline, helps you transfer data from a source of your choice in a fully automated and secure manner without having to write the code repeatedly. Therefore, the redo log also protects rollback data. When you recover the database using redo data, the database reads the change vectors in the redo records and applies the changes to the relevant blocks. Whenever a transaction is committed, LGWR writes the transaction redo records from the redo log buffer of the SGA to a redo log file, and assigns a system change number SCN to identify the redo records for each committed transaction.

Only when all redo records associated with a given transaction are safely on disk in the online logs is the user process notified that the transaction has been committed. Redo records can also be written to a redo log file before the corresponding transaction is committed. If the redo log buffer fills, or another transaction commits, LGWR flushes all of the redo log entries in the redo log buffer to a redo log file, even though some redo records may not be committed. If necessary, the database can roll back these changes.

The redo log of a database consists of two or more redo log files. See "Managing Archived Redo Logs" for more information. LGWR writes to redo log files in a circular fashion. When the current redo log file fills, LGWR begins writing to the next available redo log file. When the last available redo log file is filled, LGWR returns to the first redo log file and writes to it, starting the cycle again. Figure illustrates the circular writing of the redo log file. The numbers next to each line indicate the sequence in which LGWR writes to each redo log file.

Filled redo log files are available to LGWR for reuse depending on whether archiving is enabled. Oracle Database uses only one redo log files at a time to store redo records written from the redo log buffer.

The redo log file that LGWR is actively writing to is called the current redo log file.