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File system replication

Breaking new ground in data availability for the enterprise


Replicating data between two storage systems  (EDR) such as snap server NAS from Adaptec, connected by a network is a well-known technique for protecting against loss of digital data due to disasters that incapacitate entire storage systems or data centers. In its essence, data replication is simple. Identical copies of a set of data are established on two separate storage systems with a network connection between them. One of the copies is designated as the primary or master copy, and is processed by client applications. The other is the secondary copy, or replica. Each time a client or application modifies the contents of the primary copy, the modification is transmitted to the storage system that hosts the secondary copy, where it is applied, or replicated, keeping the contents of primary and secondary data sets identical, or synchronized. Typically, the storage system that hosts the primary data copy is located in a production data center. The system hosting the replica may be in the same data center, providing protection against failures of the primary storage system, or at a remote disaster recovery site, providing a basis for business continuity if a disaster incapacitates the entire production data center.

Data can be replicated in the form of virtual storage device contents, file system contents, or in the form of data manager-specific objects, such as database transactions or redo logs. This paper contrasts the virtual device and file system forms of replication, and describes the agámiFSR™ file system replication option for agámi Information Server (AIS) systems in detail.

Part I: Data replication basics

If a failure incapacitates the storage system hosting theprimary data copy, or if the data center experiences adisaster that makes it inaccessible, the replica can beactivated to substitute for it, and the enterprise can resume accessing and processing its critical digital information.

The growing importance of data replication

Enterprises are increasingly reliant on continuous access to their digital information for success (and survival). As a consequence, data replication is supplanting restoration of offline tape backups as the preferred technique for recovering from large-scale failures and disasters (offline tapes remain the preferred method for storing long-term data archives, as well as the last line of defense for recovering from a data corruption event such as a virus or software or procedural error).

Intuitively, the reasons for the growing popularity of replication are easy to understand:

􀂉 Recovery time. Backup tapes must be located and restored to disk storage devices before databases and applications can be recovered and clients can reconnect. By contrast, a replica instantly ready to restart when a disaster occurs, so database and application recovery can begin immediately.

􀂉 Recovery point. When backup tapes are restored, the “age” of the recovered data (called the recovery point) is the difference between the time of the disaster and the time at which the backup copy was made. For example, if an enterprise backs up its critical data daily, the recovery point of a restored copy may be as much as 24 hours out of date. For some slow-changingapplications (e.g., library catalogs), this may be tolerable; for most, however, it is not. By contrast, the recovery point for a replica is zero (or nearly so)—the replica is at within a few updates of the state of live data when a disaster occurs.

Instant recovery times and zero recovery points have been a desirable goal since computers were first used to manipulate data. It is only recently, however, that technology has made it possible for most enterprises to approach these ideals affordably. In particular, developments in two underlying technologies have made replication of online data a feasible option for the average enterprise:

􀂉 Networks: Within the data center and its immediate environs, Ethernet provides very low-cost reliable interconnections at gigabit per second transfer rates, with ten-gigabit rates available today on the backbone and on the near horizon for edge connections. Readily available low-cost dark fiber provides high bandwidth across longer distances. High-performanceconnections between storage systems reduce the latency of replicating data remotely to a point where, for many applications, the impact is not discernable.

􀂉 Disk drives: The evolution of disk drive capacity is one of the most remarkable stories in computer technology. Today’s disk drives have a thousand times more capacity than their predecessors of 25 years ago, while occupying less than a half percent of the space. The cost of this capacity has dropped accordingly—from tens of dollars per megabyte to less than a dollar per gigabyte. From a replication standpoint, every new generation of storage devices makes the cost of replicating data close to the cost of storing a single copy using the previous generation’s technology.

Thus, affordable base technology exists to update data at a disaster recovery site as it changes in the production data center. It has remained for storage system developers to create manageable, reliable, affordable systems that exploit these base technologies to deliver the required business benefit of rapid recovery of access to digital assets after large-scale failures and disasters.

Fitting solutions to problems

When costs are weighed against benefits, data replication is usually found to be an appropriate solution to the problems of protecting critical digital data against loss and restoring access to it after a disaster. When making such calculations, however, one should consider the whole cost of the solution, not just that of the storage system and network facilities required to implement it. Recovering access to digital data after a disaster also requires:

􀂉 Getting appropriately skilled administrative personnel to the recovery site

􀂉 At least a small interval of service outage as applications are restarted and client connections are restored

􀂉 In most cases, eventual failback, or return of live data and service to the primary data center These unavoidable conditions and costs make using an online data replica to restore access to data after a failure

expensive and disruptive. It is usually best to treat failover to a distant replica as a solution of last resort, and not as a mechanism for recovering from faults that can be dealt with locally. For example, individual disk failures and uncorrectable read errors can be dealt with locally using RAID data reconstruction. Protection against LAN link and switch failures can be provided by redundant connections of critical systems to independent physical networks.

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