Product : DataCore, SSY [SDS]/10.0 PSP12, x86
Feature : Node Failure Protection, Reads/Writes, Data Availability
Content Owner:  Herman Rutten
2-way and 3-way Mirroring (RAID-1)
DataCore SANsymphony software primarily uses mirroring techniques (RAID-1) to protect data within the cluster. This effectively means the SANsymphony storage platform can withstand a failure of any two disks or any two nodes within the storage cluster. Optionally, hardware RAID can be implemented to enhance the robustness of individual nodes.

SANsymphony supports Dynamic Data Resilience. Data redundancy (none, 2-way or 3-way) can be added or removed on-the-fly at the vdisk level.

A 2-way mirror acts as active-active, where both copies are accessible to the host and written to. Updating of the mirror is synchronous and bi-directional.

A 3-way mirror acts as active-active-backup, where the active copies are accessible to the host and written to, and the backup copy is inaccessible to the host (paths not presented) and written to. Updating of the mirrors active copies is synchronous and bi-directional. Updating of the mirrors backup copy is synchronous and unidirectional (receive only).

In a 3-way mirror the backup copy should be independent of existing storage resources that are used for the active copies. Because of the synchronous updating all mirror copies should be equal in storage performance.

When in a 3-way mirror an active copy fails, the backup copy is promoted to active state. When the failed mirror copy is repaired, it automatically assumes a backup state. Roles can be changed manually on-the-fly by the end-user.

DataCore SANsymphony 10.0 PSP9 U1 introduced System Managed Mirroring (SMM). A multi-copy virtual disk is created from a storage source (disk pool or pass-through disk) from two or three DataCore Servers in the same server group. Data is synchronously mirrored between the servers to maintain redundancy and high availability of the data. System Managed Mirroring (SMM) addresses the complexity of managing multiple mirror paths for numerous virtual disks. This feature also addresses the 256 LUN limitation by allowing thousands of LUNs to be handled per network adapter. The software transports data in a round robin mode through available mirror ports to maximize throughput and can dynamically reroute mirror traffic in the event of lost ports or lost connections. Mirror paths are automatically and silently managed by the software.

The System Managed Mirroring (SMM) feature is disabled by default. This feature may be enabled or disabled for the server group.

With SANsymphony 10.0 PSP10 adds seamless transition when converting Mirrored Virtual Disks (MVD) to System Managed Mirroring (SMM). Seamless transition converts and replaces mirror paths on virtual disks in a manner in which there are no momentary breaks in mirror paths.