Sync Mirroring (High-Availablity)
Real-time I/O replication for High-Availability
- Eliminates SAN or storage as a single point of failure when combined with multipath I/O drivers
- Enhances survivability using physically separate nodes
- Updates two distributed copies simultaneously
- Mirrored virtual disks behave like one, multi-ported shared drive
- May be combined with clustered file shares to achieve high-availability NAS
When it comes to non-stop access, synchronous mirroring deserves most of the credit. It handles the real-time replication of I/Os for the ultimate in high-availability. Having two nodes storing the data simultaneously in conjunction with multipath I/O drivers on the host computers eliminates single point of failure or disruption.
SANsymphony-V allows you to configure redundant storage pools by synchronously mirroring between two DataCore nodes. For any mirrored virtual disk, one DataCore node owns the primary copy and another holds the secondary copy. Those are maintained in lock step.
In the diagram below, Node A owns the primary mirror labeled “P” and Node “B” holds the secondary copy labeled “S” for a given virtual disk. The preferred path from the host computer to the virtual disk is assigned to the node that holds the primary copy of the mirrored set.
Under normal operation, all read and write requests issued to that virtual disk will be serviced by the primary copy. The secondary copy need only keep up with new updates arriving from the mirroring function. Generally, nodes are configured to control primary copies for some virtual disks and secondary for others, thereby evenly balancing their read workloads.
Should any errors be encountered on the preferred path, the host’s multipath I/O drivers automatically fail over to the alternate path without disrupting applications. The same is true if a node or its disks need to be taken out-of-service for maintenance or upgrades.
Clustered file systems may be layered on top of redundant DataCore nodes to achieve highly available Network Attached Storage (NAS). These configurations offer fast, uninterrupted access to your NFS and/or CIFS file shares.
From a physical standpoint, best practices call for the DataCore nodes to be maintained in separate chasses at different locations with their respective portion of the disk pool so that each can benefit from separate power, cooling and uninterruptible power supplies (UPS). The physical separation reduces the possibility that a single mishap or facility problem will affect both members of the mirrored set. Round-trip network latencies govern the maximum distance between mirrored nodes. Current technologies support inter-node distances up to 100 kilometers.