Proper Usage and Limitations of Classic Multi-Attach Block Storage in 3AZ Regions
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06.05.2025 
Block storage
Introduction
Classic Multi-Attach 3AZ is a regional multi-zone volume type, available exclusively in Public Cloud regions with three Availability Zones (3AZ). This volume type allows a single volume to be attached simultaneously to multiple instances within the same region, enabling increased resilience and high availability for critical applications.
This feature is designed specifically for active/active or active/passive use cases, where multiple instances need coordinated access to shared data.
To better understand the underlying architecture for multi-zone deployments using regional Block Storage, refer to the following documentation.
It is the user's responsibility to ensure that a cluster-aware file system is used with Classic Multi-Attach volumes.
Failure to do so can result in data corruption.
Use only file systems designed for shared access, such as GFS2 or OCFS2.
Standard file systems (e.g., XFS, EXT4, NTFS) are not supported unless proper fencing mechanisms are in place.
Usage
Creating and attaching a Classic Multi-Attach volume follows the same process as described in the OVHcloud documentation on volume creation. The key difference is that the volume type is "classic-multiattach", not just "classic".
If the volume type is omitted during creation, OpenStack will default to "classic-multiattach" (similar to how "classic" is set by default in regular regions).
In the OVHcloud Control Panel, simply select the classic volume available in the 3AZ region.
Considerations
When working with Classic Multi-Attach in 3AZ regions, the following points should be taken into account:
- Classic 3AZ Multi-Attach volumes can be attached to up to 16 instances simultaneously within the same region.
- The multi-attach feature is only available with this specific volume type and exclusively in 3AZ regions, such as the Paris region.
- Using cluster-aware file systems is required, but may result in performance degradation depending on the workload and configuration.
Limitations
Classic Multi-Attach volumes come with specific limitations that should be considered before deployment:
- Volume retyping is not supported while the volume is in use — switching from a multiattach-capable type to a non-multiattach-capable type (or vice-versa) is not allowed.
- Encryption is not available for multiattach-capable volumes.
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