Logging Volume Considerations for Oracle

Tips for Continuous Availability with Extended Oracle RAC and EMC VPLEX Metro

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Over the years we have presented at shows like IOUG COLLABORATE, EMC World, and Oracle OpenWorld on how VPLEX Metro and Extended Oracle RAC can together provide a zero downtime architecture. Those 60 minutes are important as we have to cover the foundation of how to architect this continuous uptime solution. Let’s explore a technical tip and show where you can learn more about this Oracle solution.


Logging Volume Considerations

 

Benefits of reading this configuration tip include:

 

  • Increased performance for applications
  • More granular monitoring capabilities
  • Faster sync restores after a failed WAN connection

 

The content for this tip can be found on the ECN VPLEX community in a blog called, “logging volume Considerations.” A logging volume is dedicated capacity for tracking any blocks written to a cluster. To use an Oracle analogy the logging volume for VPLEX is similar to the online redo logs for a database. A logging volume is a required prerequisite to creating a distributed device(s) and a remote device. The default configuration is a one-to-many relationship meaning many distributed devices using the same logging volume.

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In the above picture we show the logging volume is used for tracking written blocks. There are several components of the VPLEX Metro architecture illustrated in the picture and providing a definition of the components is important to our understanding:


Storage Volumes shown at the bottom of the picture next to the VMAX and VNX are LUNs. These LUNs are presented to the back-end ports for VPLEX and therefor visible and available for use.Extents ~ are created from the storage volumes. The general recommendation is to have one extent per storage volume but if necessary multiple extents per storage volume is supported. For example, if you plan to use VPLEX for a database requiring 1TB of capacity then create one extent of the same size (1TB).


Devices are created from extents. Multiple extents can be used for create one device. In the configuration of devices the administrator specifies RAID type. For example, RAID 0 (no mirroring of devices together), RAID 1 (mirroring of devices), and RAID-C (concatenating devices). As DBAs we write scripts used to concatenating files together and so we are familiar with the concept. From the storage perspective RAID-C is the ability to create devices that span multiple extents. One tip to mention is to avoid creating a RAID 0 and RAID C devices within the same virtual volume. Having a homogenous RAID 1 or RAID C configuration for the virtual volume improves responsiveness and reduces complexity.

 

In a VPLEX Metro configuration devices are referred two as distributed devices meaning they are mirrored across to VPLEX clusters. As you might have guessed using VPLEX Metro requires the distributed devices are configured using RAID 1.

Virtual Volumes are built from devices. It is the virtual volume that is presented to the Oracle database server. As virtual volume(s) appear as normal storage to the Oracle database the VPLEX Metro configuration is transparent and requires no DBA complexity or management overhead. In a VPLEX Metro configuration the virtual volume is referred to as a distributed virtual volume.

Recommendations for building a dedicated logging volume:

Oracle DBAs working in a physical (non-virtualized) infrastructure like dedicating one server to one database. This is because we can guarantee the database using a dedicated server will not have to compete for server resources. Most of the time this is 1-to-1 architecture is for production only but the benefits are: consistent performance and more granular monitoring. Building a dedicated logging volume for your production database using VPLEX offers similar advantages. Below are some of the guidelines in building the logging volume.The best practices for creating logging volumes can be found in the paper, “Vblock Data Protection Best Practices for EMC VPLEX with Vblock Systems.

  • Create one logging volume for each cluster
  • Use RAID 1 for logging volumes
  • Configure at lease 1GB (preferably more) of logging volume space for every 16TB of distributed device space.

 

Planning for resynchronization in the case of a failure


Most likely your company is using VPLEX Metro with Extended Oracle RAC to create an continuous availability architecture in which the loss of a storage array or data center does not impact availability of your enterprise applications. Architecting for an unplanned outage the infrastructure team should consider dependencies related to recovery. In this case the logging volumes will be subject to high levels of I/O when resychronizing the local and remote devices. Having a dedicated logging volume for your production database(s) means resychronizing of I/O will be for your database and not other applications translating into faster recovery. When a database is sharing the same logging volume with other applications resychronization involves the database and all the other applications thus lengthening the time to have the devices reach a sychronuous state. Our objective is to avoid this situation by having a dedicated logging volume for the database.

 

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Can you create more than one logging volume to use with the same device? The answer is yes, as this enables the business to grow the logging volume capacity with the growth of the database. The part to be mindful of is the default behavior meaning “that if no dedicated logging volume is specified, a logging volume is automatically selected from any available logging volume that has sufficient space for the requested entries. If no available logging volume exists, an error message is returned.” The quote was taken from the blog, “Logging Volume Considerations.”Checklist of guidelines in this blog:

 

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  • Match the database capacity requirement to the extent size
    • If the capacity required is 1TB then create a 1TB extent
  • VPLEX Local devices can use RAID 0, 1, and C
    • If possible standardize on one RAID type for improved responsiveness and reduced complexity
  • VPLEX Metro uses distributed devices that must be configured as RAID 1
  • Building logging volumes
    • Create one logging volume for each cluster
    • Use RAID 1 for logging volumes
    • Configure at least 1GB of logging volume space for every 16TB

 

For more Oracle Extended RAC with VPLEX Metro I recommend:

 

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