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Preserving Oracle VDB Archive Logs for Snapshot Provisioning (KBA7524)




KBA# 7524


Snapshots of Oracle databases taken on a Delphix engine are assigned a consistency state, according to whether redo recovery would be required when provisioning from them.  The states are listed below.

CONSISTENT Does not require any media recovery during provision.
INCONSISTENT Requires media recovery from archived logs during provision to bring all of the datafiles to the same SCN.
CRASH_CONSISTENT Requires the recovery of just the online log during provision.  Archived logs are not required.

The consistency state of a snapshot can be seen via the Delphix engine admin command-line interface (CLI).  Navigate to 'snapshot' and select the snapshot of interest.  Listing the snapshot properties shows the consistency state.

delphix-engine> snapshot
delphix-engine snapshot> select '@2021-04-20T10:30:05.986Z'
delphix-engine snapshot ''@2021-04-20T10:30:05.986Z''> ls
    type: OracleSnapshot
    name: '@2021-04-20T10:30:05.986Z'
    consistency: CRASH_CONSISTENT
    container: vdbm1
    creationTime: 2021-04-20T10:30:05.986Z
        type: OracleTimeflowPoint
        location: 10142873
        timeflow: 'DB_REFRESH@2021-03-29T13:45:51'
    fromPhysicalStandbyVdb: false
        type: OracleTimeflowPoint
        location: 10142885
        timeflow: 'DB_REFRESH@2021-03-29T13:45:51'
        timestamp: 2021-04-20T10:29:18.000Z
    missingNonLoggedData: false
    redoLogSizeInBytes: 1073741824
    reference: ORACLE_SNAPSHOT-450
    retention: 0
        type: OracleSnapshotRuntime
        missingLogs: (empty)
        provisionable: true
    temporary: false
    timeflow: 'DB_REFRESH@2021-03-29T13:45:51'
    timezone: America/New_York,EDT-0400

When a dSource snapshot is taken, it is often the case that the source database will be changing, as the SnapSync job copies the datafiles.  Delphix will also copy the archived redo logs that cover the duration of the snapshot.  The snapshot would therefore be labelled INCONSISTENT, and media recovery would be required when provisioning from it, in order to bring all of the datafiles to the same SCN.  The Delphix engine will retain the archived redo logs required to provision from dSource snapshots.

Oracle LiveSource snapshots need no recovery and are labelled as CONSISTENT.

With one exception, snapshots of VDBs are CRASH_CONSISTENT, meaning that the recovery required in order to provision from them can be performed from the online log and does not require any archived logs.  The exception is for snapshots of a vCDB.  These are INCONSISTENT.  In order to provision from a vPDB/vCDB snapshot, archived log data is required from the start SCN of the vCDB snapshot to the end SCN of the vPDB snapshot.  This can have implications for archived log management.

If you are managing the retention of your vCDB archive logs via the database itself, for instance using RMAN, it is possible that the archived logs required to provision from vPDB/vCDB snapshots could unintentionally be removed, leaving the snapshots in an unprovisionable state.

Applicable Delphix Versions

Click here to view the versions of the Delphix engine to which this article applies
Major Release All Sub Releases






In order to avoid unintentionally deleting archived logs that are required to provision from vPDB/vCDB snapshots, it is recommended to use retention policies for the vPDB and vCDB on the Delphix engine to manage the log retention for you.  This will retain any logs required to provision from the snapshots.

For more information on policies, see Policies for Scheduled Jobs.



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