Day 8: ASM Database Clones and Flex Disk Group

December 17, 2020

In keeping with today's storage-based replication technologies, such as 'mirror splitting' within a storage array, or 'snapshot replication' in a file system, and ASM with Flex Disk Groups in Oracle Grid Infrastructure, 18c and 19c provides the ability for creating near instantaneous copies of databases. These quick database copies are typically leveraged as Development and Test environments. They can also be used to create a read-only master for an Exadata snapshot copy (when used with Exadata). Greatest advantages of using this 'ASM Database Clone' feature are:In keeping with today's storage-based replication technologies, such as 'mirror splitting' within a storage array, or 'snapshot replication' in a file system, and ASM with Flex Disk Groups in Oracle Grid Infrastructure, 18c and 19c provides the ability for creating near instantaneous copies of databases. These quick database copies are typically leveraged as Development and Test environments. They can also be used to create a read-only master for an Exadata snapshot copy (when used with Exadata). Greatest advantages of using this 'ASM Database Clone' feature are:

  • Instantaneous capability to provisioning databases no matter the size
  • Limited consumption of physical storage to provision copies of databases

Prerequisites and requirements exist to leverage this database cloning innovation. First, it is supported only in Oracle ASM flex and extended disk groups. The feature is supported only with Oracle Database 18c, version 18.1 or higher. The disk group compatibility attributes, COMPATIBLE.ASM and COMPATIBLE.RDBMS needs to be set to 18.0 or higher. Lastly, the source database (parent) must be a pluggable database, and the database clone (child) is created as a pluggable database in the same container database.When an ASM database clone is made, all the files associated with the database are split together to provide an independent database. The following diagram represents the splitting of the files for database 'DB3' providing a separate and independent database 'DB3a'.

To utilize the ASM Database Cloning feature, we must first prepare a Mirror Copy. During this step, Oracle ASM allocates space for the additional copies of data. This process involves creating the cloned files and linking them with the source files. Note: that the data is not copied in this step; the copying is done during re-mirroring (shown later). Remirroring occurs during the prepare phase of rebalance, which is initiated as part of this step.

SQL> show pdbs

 

CON_ID CON_NAME                 OPEN MODE  RESTRICTED

———- —————————— ———-            ———-

2 PDB$SEED                   READ ONLY   NO

3 DB3                              READ WRITE  NO

….

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