Replication (MVD Sync)

NEC Storage NV Series systems provide replication functions by utilizing "MVD Sync," which is the replication software of Mountainview Data Inc., specially tuned for the NV Series products.

Star Topology Replication

This software regards an entire file system as a replication unit, which is called a "file set." It realizes a star topology, in which multiple replicas can be set up for a file set.

Star Topology Replication

Bidirectional Replication

Bidirectional replication can be performed between NEC Storage NV Series systems, providing for disaster recovery in both directions.

Features of MVD Sync

Replication and Promotion

Because replicas are viewed as read-only file systems from an application side, any operational errors of NFS clients to the replicas are prevented. On the other hand, it is possible to change a replica into an original when original files are destroyed in possible disasters. This function is called the "promotion" function. By using this promotion function upon system failures, replicas can be changed into originals for continuing data distribution to other replicas in a star topology. In this case, no configuration setup is required for other replicas.

Real Time Replication

The MVD Sync has superior performances in real time replications. This is because instant synchronization to a replica can be accomplished by the MVD Sync software, which traces system calls within the kernel to acquire events that may change any data on the disk. Because other typical replication methods such as the ftp mirroring method are short of acquiring these events, not only do synchronization delays tend to increase, but also a large scale scanning of a file system is required to detect changes. This results in volume consumption of disk I/O and CPU resources. Because the MVD Sync software does not require such a process, efficient replication can be realized.

Snapshot Replication

The snapshot function, which is one of the superb features of NEC Storage NV Series products, can also be effectively utilized for replication processes. There are three modes in creating replicas:

  • The "Snap Mode" to replicate all the snapshot generations plus the latest data A replica produced in this mode is a real replica, in which original snapshot structures are also stored in real time. This mode is used for promoting all the files including snapshot generations when original files are in failure.
  • "No Snap" mode to replicate only the latest data excluding snapshot generations When snapshots are used only in original file sets, such as for the purpose of setting static points for backup operations, snapshots can be excluded from replication. In this way, I/O capacities to be necessary for Copy-On-Write control for snapshots as well as storage capacities for snapshot generations are saved in replication.
  • "Snap Only" mode to replicate only snapshot generations This enables you to initiate replication triggered by a snapshot as opposed to the copy-on-close semantics. When a replica side is interested only in static point images of a file system (e.g. Web farm), useless consumption of network traffic or disk I/O usage may be significantly reduced.

As described above, various types of coordination with the snapshot function are provided for individual users to operate in their specific environments and for meeting users' needs.

Disconnected Operation

Replicas can be freely accessed even when a connecting network is disconnected or the system with the original data is shut down based on schedule, because replicas can retain information on the exact progress of replication. When network connections are restored and data transfer becomes available from the original site, the replication process automatically resumes. By applying promotion functions together, an entire original file set can be replaced as a whole system.

Encrypted Data Transfer

The MVD Sync software is designed so as to allow replication operation between two distant systems, which are separated geographically and by networks. The MVD Sync software applies an SSL-based encryption scheme in data transfer in order to protect valuable data from being intercepted or tampered with at any relay servers while data are transmitted over the Internet. The encryption function can be turned off when it is desired to achieve higher processing speeds, if security of an associated LAN system is assured.

* Notes: SSL stands for "Secure Socket Layer," which is the de facto standard for encryption-based communication and authentication methods utilized over the Internet.

Differential Data Transfer

Another function intended for use over the Internet is the Differential Data Transfer function. The MVD Sync software uses the "rsync" protocol for synchronizing data between systems. With this protocol, when there are possibilities of file updates in an original file set, both original and replica systems produce checksums of possible target areas to be exchanged over the network, for locating any updated blocks. After updated blocks are located, only corresponding differential parts can be transferred as a result. This method is particularly useful when replication is made over a limited bandwidth WAN system, especially when encrypted data transfer is also used as described above.

Replication Window Adjustment

When it is required to transmit a large amount of data, it may possibly result in disturbance to NFS or CIFS functions if CPU resources or network resources are overly consumed. In view of this possible problem, the MVD Sync software can allocate upper limits in the monitoring frequency of the replica side to check data updates as well as in the amount of data to be transferred at a time for synchronization. As a result, although real time operation is slightly sacrificed, a totally efficient replication operation is realized because frequent updates to the same data are combined to reduce the amount of data transfer.