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To stay competitive in today’s business climate, organizations must find ways to meet their business needs while controlling IT costs. IT departments face flat budgets and, at the same time, find that their organizations have become increasingly dependent on uninterrupted access to business-critical data. New governmental regulations are requiring some types of businesses to implement resilient disaster-tolerant systems and processes. Adding to this burden is the accelerating pace of change, which must be effectively managed for a business to succeed.
In today’s world, prudent IT administrators prepare to recover from two types of disasters as part of a complete Business Continuity and Availability (BC and A) plan. The first is a localized disaster, affecting a building or a small set of buildings. The second is a wide-area disaster, such as a hurricane or a regional power outage. Enterprises must replicate data to alternate data centers, located at a variety of distances from the primary data center, while maintaining acceptable data currency standards.
Until now, available remote replication technology has involved drawbacks. Synchronous remote replication solutions are designed to maintain total data currency, but can induce application performance degradation when replicating over long distances. By contrast, asynchronous and journal remote replication solutions are designed to provide acceptable performance and data consistency over long replication distances, but cannot guarantee complete data currency.
Today however, the limitations of the previous solutions can be overcome by the HP StorageWorks 3 Data Center Replication (3DC) architecture. This technology provides data currency and consistency. This technology protects against both local and wide-area disasters by simultaneously supporting short-distance synchronous replication and long-distance asynchronous replication emanating from the same source volume.
This paper provides details on the planning, configuration, and maintenance of a 3DC solution.
HP StorageWorks XP Continuous Access Synchronous (previously known as HP StorageWorks Continuous Access XP) is an HP StorageWorks XP “array-to-array” mirroring product that provides synchronous remote replication. It is ideal for replicating data over local or metropolitan networks with small latencies (typically less than 2 ms) and provides replication with full data currency on both the local and remote data center. Because every I/O written to the local array must be sent over the link to the remote array, completed on the remote array, and confirmed to the local array before the I/O can be completed, the latency of the link affects each I/O. Therefore, host performance is directly impacted by the amount of network latency between the data centers.
Most companies prefer synchronous replication because of the high currency of data on the remote site. However, the data centers must be located close to each other to reduce latency performance impact. This situation often leads to data centers being placed in the same infrastructure, which places companies at risk of no operational data in the event of a large disaster.
Seamless integration into a campus/metropolitan cluster environment allows for fast, automated failover between synchronously connected data centers with high levels of data currency. This arrangement is ideal for protection against limited disasters that affect only a single data center.
Figure 2 shows the XP Continuous Access Synchronous (Sync) process steps.
1. The primary data center server writes to the primary data volume.
2. The primary array writes data to the remote array.
3. The remote array acknowledges to the primary array that the write was received.
4. I/O is acknowledged by the primary array to the server.
HP StorageWorks XP Continuous Access Asynchronous (Async, previously known as HP StorageWorks Continuous Access XP Extension) is an HP StorageWorks XP “array-to-array” mirroring product that provides asynchronous remote replication by holding any unsent data in an area of the cache called the sidefile. It provides an ideal solution for longer distance over 50–200 km (31–124 miles) replication, with far less latency impact on the performance of the primary server. The remotely replicated data is consistent (but most of the time not current) and can lead to some transactions being lost during a disaster. However, the added advantage of the distance between data centers makes it possible to have most of the data to restart an application or recover in the event of a large-scale disaster. Meanwhile, the host I/O is not affected by the latency between data centers.
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