Oracle Dataguard Log Gap Test

A Data Guard Archive Gap Sequence is a set of archived redo logs that could not be transmitted to the standby site. As a result of an archive gap, the standby database will lag behind the primary database. Usually, a Data Guard archive gap sequence is created as a result of a network outage and when the connection between the primary and the standby site is lost. The archive gap sequence can also occur during the creation of a standby database. If the redo logs that are to be archived are missed out or if a certain sequence of logs are alone missed out, then, it indicates that the primary database and the standby databases are not up-to-date. This may cause critical data loss when database failures occur. To avoid such critical failures, it is essential to monitor the redo logs, redo log gap and the status of the redo log. The Oracle Dataguard Log Gap test helps administrators in this regard!

For each destination to which the redo log files are to be archived, this test reports the count of redo log sequences received and the log sequences applied. The number of log sequences that could not be transmitted to the destination are also reported along with the status of the redo logs.

Target of the test : An Oracle server on which Data Guard feature is enabled

Agent deploying the test : An internal/external agent

Outputs of the test : One set of results for each destination to which the redo log files are to be archived from the target Oracle Database server being monitored.

Configurable parameters for the test
  1. TEST PERIOD - How often should the test be executed
  2. Host – The host for which the test is to be configured
  3. Port - The port on which the server is listening

  4. User – In order to monitor an Oracle database server, a special database user account has to be created in every Oracle database instance that requires monitoring. A Click here hyperlink is available in the test configuration page, using which a new oracle database user can be created. Alternatively, you can manually create the special database user. When doing so, ensure that this user is vested with the select_catalog_role and create session privileges.

    The sample script we recommend for user creation (in Oracle database server versions before 12c) for eG monitoring is:

    create user oraeg identified by oraeg

    create role oratest;

    grant create session to oratest;

    grant select_catalog_role to oratest;

    grant oratest to oraeg;

    The sample script we recommend for user creation (in Oracle database server 12c) for eG monitoring is:

    alter session set container=<Oracle_service_name>;

    create user <user_name>identified by <user_password> container=current default tablespace <name_of_default_tablespace> temporary tablespace <name_of_temporary_tablespace>;

    Grant create session to <user_name>;                                

    Grant select_catalog_role to <user_name>;

    The name of this user has to be specified here.

  5. Password – Password of the specified database user

    This login information is required to query Oracle’s internal dynamic views, so as to fetch the current status / health of the various database components.

  6. Confirm password – Confirm the password by retyping it here.
  7. ISPASSIVE – If the value chosen is yes, then the Oracle server under consideration is a passive server in an Oracle cluster. No alerts will be generated if the server is not running. Measures will be reported as “Not applicable" by the agent if the server is not up.
  8. includepath - This test reports a set of results for each datafile on the target Oracle database server. This means that every datafile is a descriptor of this test. By default, while displaying the descriptors of this test, the eG monitoring console does not prefix the datafile names with the full path to the datafiles. This is why, the INCLUDE PATH flag is set to No by default. If you want the data file names to be prefixed by the full path to the data files, then, set the INCLUDE PATH flag to Yes.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Log sequence archived

Indicates the number of log sequences received at this archived redo log destination.

Number

 

Log sequence applied

Indicates the number of log sequences applied to this archived redo log destination.

Number

 

Log gap

Indicates the number of log sequences that could not be transmitted to this archived redo log destination.

Number

An archive gap can occur whenever the primary database archives a log locally, but the log is not received at the standby site. Every minute, the primary database polls its standby databases to see if there are gaps in the sequence of archived redo log files.

If too many gaps are detected in the sequence of the archived redo log files, then, it implies that the primary and the standby databases are not up-to-date. A consistent increase in the value of this measure reduces the efficiency of the database consistently.

Log gap status

Indicates the current status of the redo logs in this archived redo log destination.

 

The values reported by this measure and its numeric equivalents are mentioned in the table below:

Measure Value Numeric Value
No gap 0
Resolvable gap 1
Log switch gap 2
Unresolvable gap 3
Locally unresolvable gap 4

Note:

By default, this measure reports the current status of the redo logs in this archived redo log destination. The graph of this measure however, is represented using the numeric equivalents only - 0 to 4.