Global Active Device: A Complete Guide to High Availability, Synchronous Replication, and Data Protection
Global active device, commonly written as Global-Active Device or GAD, is a high-availability storage technology associated with Hitachi Vantara. It is designed to keep data available across two storage systems by maintaining synchronous copies of data volumes and allowing hosts to continue accessing data when one storage system or site has a problem.
The technology is especially useful for organizations that cannot afford long periods of downtime. Banks, healthcare organizations, large retailers, government agencies, manufacturers, and other businesses may depend on applications that need continuous access to storage.
The basic idea is simple: instead of keeping one storage system active and another one waiting as a backup, global active device can create an active-active storage environment in which data is maintained across two systems. Hitachi Vantara describes GAD as a technology that creates and maintains synchronous remote copies of data volumes.
This approach can reduce the impact of storage failures, support planned maintenance, and help organizations move workloads between storage systems without taking applications offline.
However, global active device is not simply a backup feature. It is a specialized high-availability and synchronous replication technology with specific hardware, networking, quorum, configuration, and operational requirements.
This guide explains global active device in simple US English, including how it works, its architecture, benefits, limitations, quorum design, failover behavior, storage virtualization, common use cases, implementation considerations, monitoring, and best practices.
What Is Global Active Device?
Global active device is a storage-based high-availability technology that maintains synchronous copies of data between two storage systems.
In a traditional setup, one storage array may contain the primary copy of data while another array contains a backup or replicated copy. If the primary system fails, the organization may need to perform a recovery or failover process.
GAD takes a different approach.
Two storage systems participate in the environment, and the host can be presented with storage that appears as a single logical resource. The primary and secondary volumes use the same virtual LDEV identification in the virtual storage environment, allowing the host to see the paired volumes as a single volume.
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The two storage systems maintain synchronized data.
This means that when a host writes data, the storage environment is designed to keep the corresponding copies consistent.
The result is an active-active architecture intended to keep applications running even if access to one storage system is interrupted.
What Does GAD Stand For?
GAD stands for Global-Active Device.
It is a Hitachi Vantara storage technology rather than a generic name for every active-active storage solution.
The term is sometimes written in different ways, including:
- Global-Active Device
- Global Active Device
- GAD
- Hitachi Global-Active Device
When people search for “global active device,” they are often looking for information about Hitachi Vantara’s storage replication and high-availability technology.
This distinction matters because active-active replication is a broader industry concept, while GAD is a specific implementation and feature set within Hitachi’s storage ecosystem.
How Global Active Device Works
The easiest way to understand global active device is to imagine two storage systems located at separate sites.
For example:
Site A contains Storage System A.
Site B contains Storage System B.
A server or cluster needs access to a data volume.
Instead of treating the two storage copies as completely separate resources, GAD creates a relationship between the volumes.
The host sees the storage through a virtualized presentation, while the storage systems maintain synchronized copies.
Hitachi documentation explains that GAD uses a virtual storage machine and assigns the paired primary and secondary volumes the same virtual LDEV number. This allows the host to see the pair as a single volume.
This design is important because applications do not need to understand the physical details of where every copy of the data resides.
The Active-Active Concept
The term active-active is central to understanding GAD.
In a traditional primary-secondary arrangement, the secondary system may remain mostly unused until a disaster occurs.
In an active-active arrangement, both sides participate in the availability design.
This can provide several advantages.
For example, an organization can use the environment for:
- High availability
- Planned maintenance
- Workload movement
- Storage migration
- Disaster recovery
- Business continuity
- Reducing service interruptions
The value of this architecture is not limited to major disasters.
It can also help during ordinary maintenance.
An organization may need to perform maintenance on one storage system. Instead of shutting down the application, workloads can potentially continue through the other available path or storage system when the environment is correctly configured.
Hitachi lists continuous server I/O during certain storage failures, server failover and failback without storage impact, and virtual-machine migration without storage impact among GAD’s benefits.
The Main Components of Global Active Device
A GAD environment contains several important components.
Understanding these components makes the overall architecture much easier to understand.
Primary Storage System
The primary storage system contains the primary volume, commonly called the P-VOL.
This is one side of the replicated data relationship.
The primary volume participates in the synchronization process with the secondary volume.
Secondary Storage System
The secondary storage system contains the secondary volume, commonly called the S-VOL.
It maintains the synchronized copy of the data.
Although the terms primary and secondary are used, the overall GAD architecture is designed to support high availability across the two systems.
Host Servers
Hosts are the servers or applications that need access to the storage.
They may run:
- Databases
- Virtual machines
- Enterprise applications
- File services
- Business applications
- Transaction systems
The host does not simply see two unrelated copies of the same storage.
The GAD environment uses virtualization and path-management mechanisms to provide the required access behavior.
Remote Paths
Remote paths connect the participating storage systems.
These paths allow replication information and control information to move between the systems.
Network quality is extremely important because synchronous replication depends on communication between the sites.
Quorum Disk
The quorum disk is one of the most important parts of the architecture.
It helps the storage systems determine which side should continue operating when communication or system failures occur.
Hitachi describes the quorum disk as a heartbeat mechanism that both storage systems access to check the condition of the other system.
This helps prevent a dangerous situation known as split-brain.
Virtual Storage Machine
The virtual storage machine provides a virtual representation of storage resources.
In a GAD environment, the primary and secondary volumes can be presented using the same virtual LDEV number.
This helps the host view the paired storage as one logical resource rather than two unrelated devices.
Why Synchronous Replication Matters
Synchronous replication means data is replicated between storage systems as part of the write process rather than being copied only later.
This is very different from many asynchronous replication designs.
With asynchronous replication, there can be a time difference between the latest data at the primary site and the replicated data at the secondary site.
With synchronous replication, the goal is to maintain matching data between the two systems.
This makes the technology attractive for workloads where data loss is unacceptable or where very low recovery-point objectives are required.
However, synchronous replication also introduces an important trade-off.
The distance and network connection between sites matter greatly.
A synchronous system cannot ignore network latency.
The Importance of Network Distance
Distance is one of the most important design considerations for GAD.
When data must be synchronously replicated between sites, the storage systems need reliable communication.
Greater distance generally means greater network latency.
Greater latency can affect application performance and replication behavior.
This means organizations should not simply choose two facilities because they are geographically far apart.
The sites must also have suitable network connectivity.
A successful design considers:
- Physical distance
- Network latency
- Bandwidth
- Redundancy
- Packet loss
- Path availability
- Network equipment
- Failure scenarios
The exact supported distances and configuration limits depend on the specific storage platform and GAD implementation, so organizations should always check current product documentation for their hardware and software versions.
What Is a Quorum Disk?
A quorum disk is a shared decision-making resource used by the GAD environment.
Its role becomes especially important when the two storage systems cannot communicate normally.
Imagine two sites:
Site A believes Site B has failed.
Site B believes Site A has failed.
If both sides continue accepting writes independently, the data could diverge.
This is the split-brain problem.
The quorum mechanism helps determine which side should continue handling host updates.
Hitachi’s current documentation explains that a quorum disk can be located in a third external storage system and is used to monitor the GAD pair volumes.
Why Split-Brain Is Dangerous
Split-brain is dangerous because both systems may believe they are the active system.
Suppose an application writes:
Customer balance = $500
to Site A.
At the same time, another process writes:
Customer balance = $700
to Site B.
If the systems cannot communicate and both continue independently, the two copies can no longer be treated as identical.
A quorum mechanism helps prevent this type of uncontrolled dual activity.
This is one of the reasons GAD is more than simply copying data between two storage arrays.
GAD Pair Status
Monitoring GAD status is important because administrators need to know whether the pair is healthy.
Current Hitachi documentation describes several GAD statuses, including:
- Simplex
- Mirroring
- Mirrored
- Quorum disk blocked or no quorum disk volume
- Suspended
- Blocked
The exact meaning of each state matters during troubleshooting.
Simplex
Simplex means the volume is not currently operating as a GAD pair volume.
There is no active data redundancy through the pair.
Mirroring
Mirroring indicates that the pair is transitioning toward a fully mirrored state.
During this phase, data is being copied or synchronized.
Mirrored
Mirrored indicates normal paired operation.
The primary and secondary volumes contain the replicated data.
Suspended
A suspended state means the pair has stopped normal synchronous operation.
I/O behavior depends on the configuration and current conditions.
The administrator needs to determine why the pair became suspended before taking recovery actions.
Blocked
Blocked is a serious condition in which I/O is not accepted by the affected pair volumes under the documented conditions.
The correct recovery procedure depends on what caused the state.
Administrators should not treat every GAD status as a simple “on” or “off” condition.
Global Active Device and Failover
One of the major reasons organizations use GAD is failover.
Failover means continuing operations through an alternate storage path or system when the preferred storage system becomes unavailable.
The exact behavior depends on the configuration, host multipathing software, storage paths, and failure scenario.
Hitachi documentation explains that alternate path software can operate in an active-active configuration and can redirect host I/O to the appropriate volume when the host cannot access the primary volume.
This is important because storage replication alone does not guarantee application continuity.
The server must also have a way to reach the available storage.
Host Multipathing
Multipathing is an important part of high-availability storage.
A server may have multiple connections to storage.
If one path fails, another path can be used.
In GAD environments, host path-management software helps determine which path should be used.
Depending on the environment, this can involve:
- ALUA
- Hitachi Dynamic Link Manager
- Other supported multipathing mechanisms
Hitachi notes that path-management requirements can differ depending on distances and configuration.
This is why administrators should treat host configuration as part of the overall GAD design rather than as a separate task.
Global Active Device and Virtualization
Virtualization is another important part of GAD.
The physical storage systems may be separate, but the host can be presented with a consistent virtual representation.
The use of the same virtual LDEV number for the paired volumes helps create this consistent view.
From the application’s perspective, the storage does not need to look like two unrelated devices.
This abstraction is valuable because it reduces the amount of storage-location awareness required by applications.
Global Active Device for Virtual Machines
Virtualized environments are a major use case.
Modern organizations often run workloads through virtualization platforms.
If storage can remain available while virtual machines move or storage maintenance occurs, administrators can perform infrastructure work with less disruption.
Hitachi specifically identifies virtual-machine migration without storage impact as one of the benefits of GAD.
The practical value is significant.
Instead of treating storage maintenance as an application outage event, administrators can design the infrastructure so that storage availability continues while underlying resources are changed.
Global Active Device for Disaster Recovery
Disaster recovery focuses on maintaining or restoring business operations after a major failure.
GAD can support disaster-recovery strategies by maintaining synchronized storage between sites.
For example:
Site A → production workload
Site B → synchronized storage
If Site A becomes unavailable, the organization can use the available infrastructure at Site B according to its failover design.
This can reduce the amount of data that would otherwise need to be restored from traditional backup media.
However, GAD should not be considered a replacement for backups.
Replication and backup solve different problems.
GAD Is Not a Backup
This distinction is extremely important.
If data is accidentally deleted from the primary storage system and the deletion is synchronously replicated, the secondary copy may also reflect the deletion.
That means replication does not necessarily protect against:
- Accidental deletion
- Corrupted files
- Malicious changes
- Ransomware
- Application errors
- Bad database transactions
Backups provide historical recovery points.
GAD primarily provides high availability and synchronous replication.
A strong enterprise strategy may use both.
For example:
Production storage → GAD replication → second site
and
Production data → backup system → historical recovery points
These technologies complement each other.
Global Active Device vs. Traditional Disaster Recovery
Traditional disaster recovery often uses a primary site and a recovery site.
The recovery site may contain replicated or backed-up data.
Depending on the technology, failover may require several steps.
GAD is different because its architecture is designed around active-active availability and synchronous data copies.
The key difference is the operating model.
| Feature | Traditional backup/DR | Global Active Device |
| Primary purpose | Recovery | High availability |
| Data replication | Depends on technology | Synchronous |
| Active-active design | Not always | Yes |
| Historical recovery | Strong with backups | Not its primary purpose |
| Storage failover | Depends on system | Core capability |
| Backup replacement | No | No |
| Quorum mechanism | Not always | Important to GAD |
This comparison shows why GAD should be considered part of a broader business-continuity architecture.
Benefits of Global Active Device
GAD offers several important advantages when properly designed and managed.
Higher Availability
The biggest advantage is improved storage availability.
If one storage system becomes unavailable, the other side can potentially continue providing access according to the configuration.
Synchronous Data Protection
Synchronous replication helps keep the paired storage copies aligned.
This can support demanding recovery-point objectives.
Reduced Planned Downtime
Organizations can perform some maintenance operations without taking applications offline.
Storage Migration
GAD can support movement of workloads between storage resources with less disruption.
Active-Active Architecture
Both sides participate in the availability design rather than leaving one system completely idle.
Business Continuity
Critical applications can remain accessible through infrastructure failures that would otherwise cause an outage.
Hitachi’s documentation lists continuous server I/O, server failover and failback, and migration capabilities among the benefits of GAD.
Limitations of Global Active Device
GAD is powerful, but it is not appropriate for every organization.
There are several limitations to consider.
Infrastructure Cost
Two capable storage environments can be expensive.
The organization may also need redundant networking, additional facilities, support contracts, and skilled administrators.
Network Requirements
Synchronous replication requires suitable network connectivity.
Poor latency or unreliable links can affect the design.
Technical Complexity
GAD involves storage, servers, multipathing, networking, quorum, and operational procedures.
Platform Dependencies
Compatibility depends on the specific Hitachi storage products, software versions, host systems, and supported configurations.
It Does Not Replace Backups
GAD should not be treated as the only data-protection mechanism.
Operational Skills
Administrators need a good understanding of pair states, failover, recovery, paths, and quorum behavior.
Global Active Device Requirements
Requirements vary by platform and software version.
Current documentation should always be checked before deployment.
For example, Commvault’s current documentation lists supported Hitachi Vantara VSP platforms and software requirements for environments using GAD-related backup workflows.
Hitachi also provides current compatibility and configuration documentation for GAD environments.
The important lesson is simple:
Do not design a GAD environment based only on an old architecture diagram or a general online article.
Storage platforms evolve.
Supported configurations change.
Software versions change.
Compatibility should be verified before implementation.
Global Active Device Configuration
A typical implementation involves several stages.
Planning
First, define:
- Critical workloads
- Recovery objectives
- Storage capacity
- Site locations
- Network design
- Host requirements
- Quorum placement
- Failure scenarios
Preparing Storage
The storage systems need to be prepared for the GAD configuration.
Configuring Remote Paths
Remote communication between the systems must be established.
Current Hitachi documentation for VSP One Block describes configuring remote paths and quorum disks as part of setting up a GAD environment.
Preparing the Quorum
The quorum resource needs to be configured according to the supported architecture.
Creating GAD Pairs
The appropriate primary and secondary volumes are paired.
Initial Synchronization
The initial copy brings the secondary volume into synchronization with the primary.
Host Configuration
Hosts need the correct paths and multipathing configuration.
Testing
The organization should test normal operation and failure scenarios before depending on the system in production.
Why Testing Matters
A high-availability solution is only valuable if it works when something goes wrong.
Organizations should not wait for a real disaster to discover a configuration problem.
Testing should include controlled scenarios such as:
- Storage path failure
- Storage system failure
- Network path failure
- Site communication failure
- Host path failure
- Quorum-related conditions
- Failover
- Failback
- Recovery synchronization
The goal is to confirm that the expected application behavior actually occurs.
Global Active Device Monitoring
Monitoring should be continuous.
Administrators should watch:
- Pair status
- Replication health
- Remote paths
- Quorum status
- Host paths
- Storage performance
- Capacity
- Alerts
- Error conditions
A healthy monitoring strategy should identify problems before they become outages.
Current Hitachi documentation provides detailed GAD status information and explains what each status means for data redundancy and I/O.
Common GAD Problems
Some common problems can involve:
- Broken remote paths
- Quorum problems
- Suspended pairs
- Host multipathing issues
- Network latency
- Storage failures
- Incorrect configuration
- Capacity limitations
- Unsupported combinations
- Operational mistakes
The correct response depends on the exact condition.
Administrators should avoid making recovery decisions based only on the name of a status.
They should examine the cause, current data state, I/O state, and documented recovery procedure.
Global Active Device and Quorum Failures
Quorum problems deserve special attention.
A quorum disk is not simply another copy of production data.
It is part of the decision-making architecture.
Its job is to help the storage systems determine the correct operating state.
If the quorum becomes unavailable, the GAD environment may enter a different operating condition depending on configuration and timing.
Hitachi’s current documentation specifically identifies a “Quorum disk blocked or no quorum disk volume” state in which the pair can remain mirrored under the documented conditions.
This is why administrators should understand the exact behavior of their platform rather than assuming that every quorum failure causes immediate data loss or immediate shutdown.
Global Active Device and RPO
RPO means Recovery Point Objective.
It answers a basic question:
“How much data can the organization afford to lose?”
Because GAD uses synchronous replication, it is designed for environments that need very low data-loss exposure during certain storage failures.
However, RPO should not be discussed separately from application design.
Database behavior, host configuration, storage consistency, network reliability, and failure type all matter.
Organizations should test actual workloads rather than assuming that a storage technology automatically provides a particular business RPO.
Global Active Device and RTO
RTO means Recovery Time Objective.
It answers:
“How quickly must the business service be restored?”
GAD can help reduce recovery time by keeping another synchronized storage environment available.
But RTO also depends on:
- Server availability
- Application startup
- Network routing
- DNS
- Load balancing
- Cluster configuration
- Staff procedures
Therefore, GAD can be an important part of a low-RTO design, but it is not the entire disaster-recovery solution.
GAD and Business Continuity
Business continuity is broader than storage availability.
A company can have perfectly replicated storage and still experience an outage if:
- The application servers fail
- Network connectivity is unavailable
- Users cannot access the system
- Authentication services fail
- Power is lost at both sites
- A software error damages the application
This means GAD should be viewed as one layer in a broader resilience strategy.
A mature business-continuity design may include:
- GAD
- Backups
- Redundant servers
- Network redundancy
- Power protection
- Monitoring
- Security controls
- Disaster-recovery procedures
- Regular testing
Global Active Device and Cybersecurity
GAD is a high-availability technology, not a complete cybersecurity solution.
Organizations should still protect storage infrastructure through:
- Access controls
- Strong authentication
- Network segmentation
- Secure management interfaces
- Monitoring
- Patch management
- Backup protection
- Incident-response planning
This is especially important because modern cyber incidents can affect replicated environments.
If malicious changes are considered valid writes by the storage infrastructure, replication may reproduce those changes.
That is why independent backups and recovery points remain important.
When Should a Business Consider GAD?
GAD may be appropriate when a business has workloads that require very high availability and can justify the infrastructure required to support synchronous storage replication.
It may be worth considering for:
- Financial applications
- Large databases
- Healthcare systems
- Enterprise ERP systems
- High-volume e-commerce
- Manufacturing systems
- Government applications
- Critical virtualized environments
The decision should be based on business requirements rather than technology popularity.
When GAD May Not Be the Right Choice
GAD may not be necessary for a small business where a short outage is acceptable.
It may also be unsuitable when:
- The sites are too far apart for the required latency
- Budget is limited
- The application does not require high availability
- Skilled storage administrators are unavailable
- A simpler replication solution meets the business requirement
The best infrastructure solution is the one that matches the actual business risk.
Global Active Device vs. Asynchronous Replication
The key difference is timing.
Synchronous replication attempts to keep the copies synchronized as part of the write process.
Asynchronous replication allows the primary system to continue without waiting for the remote copy to be fully updated.
A simplified comparison looks like this:
| Area | Synchronous replication | Asynchronous replication |
| Data synchronization | Very close to real time | Delayed |
| Network sensitivity | Higher | Generally lower |
| Distance flexibility | More constrained | Often more flexible |
| Data-loss exposure | Very low under supported conditions | Some recent changes may not be replicated |
| Typical goal | High availability | Disaster recovery |
Neither method is universally better.
The correct choice depends on application requirements, distance, network performance, and acceptable data-loss risk.
Global Active Device and Storage Migration
Another useful capability is migration.
Organizations may need to replace older storage hardware with newer systems.
Instead of shutting down applications, a properly designed GAD environment can support movement between storage resources.
This can reduce operational disruption.
However, migration still requires planning.
Administrators need to understand:
- Capacity
- Compatibility
- Host paths
- Virtual IDs
- Pair relationships
- Performance
- Failback
A migration should be tested before production workloads are moved.
Best Practices for Global Active Device
Organizations planning a GAD deployment should follow several practical principles.
Understand the Business Requirement
Do not start with the technology.
Start with the business problem.
Ask:
What applications must remain available?
How much downtime is acceptable?
How much data loss is acceptable?
What happens financially if the system is unavailable?
Design for Failure
Think about what happens when:
- One storage array fails
- A network path fails
- A site loses connectivity
- A host loses a path
- The quorum becomes unavailable
- Both sites experience problems
A good architecture is designed around failure scenarios.
Keep Backups Independent
Do not rely on GAD as the only form of data protection.
Maintain separate backup and recovery capabilities.
Test Failover
A failover process that has never been tested is not a reliable recovery process.
Monitor Continuously
Use alerts and dashboards to identify replication and path problems early.
Document Everything
Document:
- Storage topology
- Network paths
- Host configuration
- Quorum placement
- Pair relationships
- Failover procedures
- Recovery procedures
Keep Documentation Current
Technology changes.
Firmware changes.
Software changes.
Hardware changes.
The documentation should change with them.
The Future of High-Availability Storage
The need for highly available storage is likely to continue growing.
Businesses increasingly depend on digital services that operate around the clock.
Cloud applications, online payments, automated manufacturing, streaming services, healthcare platforms, and large e-commerce systems all depend on reliable data access.
This creates demand for architectures that can reduce downtime without requiring long manual recovery processes.
Technologies such as active-active storage, synchronous replication, virtualization, automated failover, and intelligent monitoring will continue to play important roles.
At the same time, organizations are becoming more aware that availability and backup are different problems.
A modern strategy needs both.
Final Thoughts
Global active device is a specialized high-availability storage technology designed to keep synchronized copies of data across storage systems and help critical applications continue operating when storage failures or maintenance events occur. Its active-active design, synchronous replication, virtual storage presentation, quorum mechanism, and multipathing capabilities make it useful for demanding enterprise environments, but it also requires careful planning, suitable infrastructure, compatible hardware and software, reliable networking, skilled administration, and regular testing. The most important lesson is that GAD should not be treated as a standalone backup solution; instead, it works best as one part of a broader strategy that combines high availability, independent backups, disaster recovery, cybersecurity, monitoring, and tested business-continuity procedures.
FAQs About Global Active Device
What is global active device?
Global active device, or GAD, is a Hitachi storage technology that keeps synchronized copies of data between two storage systems to support high availability and continuous access.
Is global active device a backup solution?
No. GAD is mainly designed for high availability and synchronous replication. Businesses should still maintain separate backups for protection against accidental deletion, corruption, ransomware, and other data-loss events.
What is the purpose of the quorum disk in GAD?
The quorum disk helps the storage systems determine the correct operating state during communication or storage failures. It helps prevent split-brain situations where both systems might incorrectly operate as independent active systems.
Does GAD support virtual machines?
Yes. GAD can support virtualized environments and can help with certain virtual-machine migration and storage maintenance scenarios without requiring application downtime.
What is the main benefit of global active device?
The main benefit is improved storage availability. By maintaining synchronized data across two storage systems, GAD can help critical applications continue operating during certain storage failures and planned maintenance events.
