Dynamic Data Resiliency provides extended protection (resilience) against failure scenarios by increasing availability and reducing periods without redundancy by adding an additional data copy to mirrored virtual disks. The added copy receives data from the active virtual disk storage source, but the front-end paths from the added copy to the host are disabled and access is not allowed until it is promoted to active status. In the case of a server or storage source failure, high availability to the host can be manually or automatically restored using the additional storage source. The feature also assists administrators by allowing them to move the mirrored data in order to perform maintenance duties in a simple and efficient manner without temporarily compromising redundancy or the need for shared storage sources.
A mirrored virtual disk (comprised of two data copies) loses data redundancy when one of the storage sources becomes unavailable which can leave an application exposed to possible complete loss of access to its data if the second storage source also fails. In this case, restoring high availability requires time to determine the problem, address the cause, take action to correct and then rebuild the failed copy with a log or full recovery as appropriate. Losing both storage server nodes results in complete loss of access to the data with a mirrored virtual disk comprised of two data copies.
Dynamic Data Resiliency provides the following benefits:
- Scale out architecture: The number of data copies for a virtual disk can be increased by adding DataCore Servers with a storage pool to the server group configuration.
- Extended data redundancy or "data resiliency" for mirrored virtual disks is achieved by adding an extra data copy from another DataCore Server storage source to create three synchronous copies of the data, referred to as a 3-copy virtual disk. See Creating 3-Copy Virtual Disks.
- Automatic self-healing: The additional data copy allows any of the storage sources in a mirrored virtual disk to move from one DataCore Server to another and dynamically transition the workload.
See Automatic Self-Healing.
- Mirrored virtual disks provide host access from two DataCore Servers. Virtual disks with more than two copies can maintain, either automatically or manually, two active storage paths to the host if a storage source failure should occur to any one of the DataCore Servers. In this case, no full mirror recovery is required.
- High availability to the host can be maintained with a 3-copy virtual disk when a server must be taken offline for server maintenance or when a server must be permanently removed from the configuration. See Maintenance Mode (Evacuate/Redistribute).
- Load balancing: The additional data copy allows storage sources in a synchronously mirrored virtual disk to be dynamically redistributed among other servers in the server group while maintaining host access and without the need for shared storage access.
Workload can be redistributed among other servers in the group in the following ways:
- Among other servers in the group when servers are added or removed from the group.
- To other servers with more storage resources as pools on overloaded servers reach capacity.
- To other servers prior to and after maintenance.
Dynamic data resiliency requires a minimum of 3 servers each with an available pool of storage in the same server group in order to create three data copies of the virtual disk on different servers.
All servers with storage sources in the same multi-copy virtual disk must have mirror ports with connections to the other servers in the same virtual disk. Additionally, those servers must have front-end ports with connections to the same hosts to which the virtual disk is served or could be served. An adequate number of ports to create redundant paths is recommended.