PowerFlex Native Asynchronous Replication RPO with Oracle

PowerFlex software-defined storage platform provides a reliable, high-performance foundation for mission-critical applications like Oracle databases. In many of these deployments, replication and disaster recovery have become a common practice for protecting critical data and ensuring application uptime. In this blog, I will be discussing strategies for replicating mission-critical Oracle databases using Dell EMC PowerFlex software-defined storage.

The Role of Replication and Disaster Recovery in Enterprise Applications

Customers require Disaster Recovery and Replication capabilities to meet mission-critical business requirements where SLAs require the highest uptime. Customers also want the ability to quickly recover from physical or logical disasters to ensure business continuity in the event of disaster and be able to bring up the applications in minimal time without impact to data. Replication means that the same data is available at multiple locations. For Oracle database environments, it is important to have local and remote replicas of application data which are suitable for testing, development, reporting, and disaster recovery and many other operations. Replication improves the performance and protects the availability of Oracle database application because the data exists in another location. Advantages of having multiple copies of data being present across geographies is that, critical business applications will continue to function if the local Oracle database server experiences a failure.

Replication enables customers in various scenarios such as:

• Disaster Recovery for applications ensuring business continuity
• Distributing with one type of use case such as analytics
• Offloading for mission-critical workloads such as BI, Analytics, Data Warehousing, ERP, MRP, and so on
• Data Migration
• Disaster Recovery testing
  
  

PowerFlex Software-Defined Storage – Flexibility Unleashed

PowerFlex is a software-defined storage platform designed to significantly reduce operational and infrastructure complexity, empowering organizations to move faster by delivering flexibility, elasticity, and simplicity with predictable performance and resiliency at scale. The PowerFlex family provides a foundation that combines compute as well as high performance storage resources in a managed unified fabric.

PowerFlex is designed to provide extreme performance and massive scalability up to 1000s of nodes. It can be deployed as a disaggregated storage / compute (two-layer), HCI (single-layer), or a mixed architecture. PowerFlex inclusively supports applications ranging from bare-metal workloads and virtualized machines to cloud-native containerized apps. It is widely used for large-scale mission-critical applications like Oracle database. For information about best practices for deploying Oracle RAC on PowerFlex, see Oracle RAC on PowerFlex rack.

PowerFlex also offers several enterprise-class native capabilities to protect critical data at various levels:

1. Storage Disk layer: PowerFlex storage distributed data layout scheme is designed to maximize protection and optimize performance. A single volume is divided into chunks. These chunks will be striped on physical disks throughout the cluster, in a balanced and random manner. Each chunk has a total of two copies for redundancy.
2. Fault Sets: By implementing Fault sets, we can ensure the persistent data availability at all time. PowerFlex (previously VxFlex OS) will mirror data for a Fault Set on SDSs that are outside the Fault Set. Thus, availability is assured even if all the servers within one Fault Set fail simultaneously. Fault Sets are subgroup of SDSs installed on host servers within a Protection Domain.

PowerFlex replication overview

PowerFlex software consists of a few important components - Meta Data Manager (MDM), Storage Data Server (SDS), Storage Data Client (SDC) and Storage Data Replicator (SDR). MDM manages the PowerFlex system as a whole, which includes metadata, devices mapping, volumes, snapshots, system capacity, errors and failures, system rebuild and rebalance tasks. SDS is the software component that enables a node to contribute its local storage to the aggregated PowerFlex pool. SDC is a lightweight device driver that exposes PowerFlex volumes as block devices to the applications and hosts. SDR handles the replication activities. PowerFlex has a unique feature called Protection Domain. A Protection Domain is a logical entity that contains a group of SDSs. Each SDS belongs to only one Protection Domain.

Figure 1. PowerFlex asynchronous replication between two systems

Replication occurs between two PowerFlex systems designated as peer systems. These peer systems are connected using LAN or WAN and are physically separated for protection purposes. Replication is defined in scope of a protection domain. All objects which participate in replication are contained in the protection domain, including volumes in Replication Consistency Group (RCG). Journal capacity from storage pools in the protection domain is shared among RCGs in the protection domain.

The SDR handles replication activities and manages I/O of replicated logical volumes. The SDR is deployed on the same server as SDS. Only I/Os from replicated volumes flows through SDR.

Replication Data Flow

Figure 2. PowerFlex replication I/O flow between two systems 

• At the source, application I/O are passed from SDS to SDR.
• Application I/O are stored in the source journal space before it is sent to target. SDR packages I/O in bundles and sends them to the target journal space.
• Once the I/O are sent to target journal and get placed in target journal space, they are cleared from source.
• Once I/O are applied to target volumes, they are cleared from destination journal.
• For replicated volumes, SDS communicates to other SDS via SDR. For non-replicated volumes, SDS communicates directly with other SDS.

For detailed information about Architecture Overview, see Dell EMC PowerFlex: Introduction to Replication White Paper.

It is important to note that this approach to replication allows PowerFlex to support replication at extreme scales. As the number of nodes contributing storage are scaled, so are the SDR instances. As a result, this replication mechanism can scale effortlessly from 4 to 1000s of nodes while delivering RPOs as low as 30 seconds and meeting IO and throughput requirements.

Oracle Databases on PowerFlex

The following illustration demonstrates that the volumes participating in replication are grouped to form the Replication Consistency Group (RCG). RCG acts as the logical container for the volumes.

Figure 3. PowerFlex replication with Oracle database

Depending on the scenario, we can create multiple RCGs for each volume pair or combine multiple volume pairs in a single RCG.

In the above Oracle setup, PowerFlex System-1 is the source and PowerFlex System-2 is the destination. For replication to occur between the source and target, the following criteria must be met:

• A volume pair must be created in both source and target.
• Size of volumes in both source and target should be same. However, the volumes can be in different storage pools.
• Volumes are in read-write access mode on the source and read-only access mode in secondary. This is done to maintain data integrity and consistency between two peer systems.

The PowerFlex replication is designed to recover from as low as a 30 seconds RPOs minimizing the data-loss if there is a disaster recovery. During creation of RCG, users can specify RPO starting from 30 seconds to maximum of 60 minutes.

All the operations performed on source will be replicated to destination within the RPO. To ensure RPO compliance, PowerFlex replicates at least twice for every RPO period. For example, setting RPO to 30 seconds means that PowerFlex can immediately return to operation at the target system with only 30 seconds of potential data loss.

The following figures depicts the replication scenario under steady state of workload:

Figure 4. 100% RPO compliance for RPO of 30s for an Oracle database during a steady application workload

Figure 5. Replication dashboard view of PowerFlex

Disaster Recovery

In the case of disaster recovery, the entire application can be up and running by failover to secondary, with less than 30 seconds of data loss.

When we do a planned switchover or failover, the volumes on secondary system are automatically changed to read-write access mode and the volumes on source will be changed to read-only. Consequently, we can bring up Oracle database on secondary by setting up the Oracle environment variables and starting the database.

Once we have RCG in the failover or switchover mode, user can decide how to continue with replication:

• Restore replication: Maintains the replication direction from original source to destination.
• Reverse replication: Changes the direction so that original destination becomes the source and replication will begin from original destination to original source.

PowerFlex also provides various other options:

• Pause and Resume RCG: If there are network issues or user need to perform maintenance of any of the hardware. While paused, any application I/O will be stored at source journal and is replicated to the destination only after the replication is resumed.
• Freeze and Unfreeze RCG: If the user requires consistent snapshot of the source or target volumes. While frozen, replication will still occur between source journal and destination journal, nonetheless the target journal holds on to the data and do not apply them to the target volumes.

PowerFlex native volume replication is a unique solution and provides customers with easy to configure and setup without worrying about disaster. 

Irrespective of workload and application, it is designed to support massive scale while providing RPOs as low as 30 seconds.

For more information, please visit: DellTechnologies.com/PowerFlex.