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Replication alternatives

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"We did a recent study that shows that in 2007, 83.7% of worldwide revenue for storage-based replication was done using array-to-array replication, followed by host-based replication with 11.5% and network-based replication with 4.8%," said James Baker, research manager, storage software at Framingham, Mass.-based IDC. But according to the same study, both host- and network-based replication are catching up. Host-based replication is expected to grow at a compound annual growth rate (CAGR) of 18.2% until 2012, while a CAGR of 15.4% is anticipated for network-based replication. Both are expected to expand significantly faster than the 10% forecasted annual growth for array-based replication.

Mode: Replication can occur synchronously, where data is written to the primary and secondary storage systems simultaneously; or it can be performed asynchronously, where data is replicated to replication targets with a delay. In synchronous replication, the primary storage system only commits I/O writes after the replication target acknowledges that data has been written successfully. Synchronous replication depends on sufficient bandwidth and low latency, and supported replication distances range from 50 km to 300 km. It's typically used in applications where zero RPOs and RTOs are required, such as high-availability clusters and mission-critical applications that demand 100% synchronicity between the primary and target systems. Conversely, asynchronous replication writes

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data to the primary array first and, depending on the implementation approach, commits data to be replicated to memory or a disk-based journal. It then copies the data in real-time or at scheduled intervals to replication targets. Unlike synchronous replication, it's designed to work over long distances and greatly reduces bandwidth requirements. While the majority of array- and network-based replication products support both synchronous and asynchronous replication, host-based replication offerings usually only come with asynchronous replication.

Type: Replication products can replicate blocks of data on volumes or logical unit numbers (LUNs), or replication can be performed at the file level. With the exception of network-attached storage (NAS), which can support both block- and file-based replication, array-based replication products usually operate at the block level. The same is true for network-based replication products. In contrast, most host-based replication offerings operate at the file-system level. Block-based replication is platform-agnostic and will work seamlessly across various OSes. File-based replication products are very much operating system-specific and the majority of available host-based replication products are written for Windows. Unlike file-based replication, block-based replication products have no knowledge of the attached platform, file system or apps, and depend on auxiliary services like snapshots for any type of application integration. As a result, most storage arrays with replication support also provide snapshot capabilities that are more or less integrated with the file system and key apps like Exchange and SQL Server databases.

Data replication trends

These data replication-related trends are gradually changing data protection and disaster recovery.

  • The use of replication-based data protection is increasing.
  • Replication-based data protection is merging with traditional data protection, and traditional backup products are increasingly able to manage replicas and snapshots along with backups.
  • The integration of replication and applications to provide application-consistent recovery is on the rise.
  • Replication-based data protection is becoming an important option for protecting virtualized server environments.
  • As storage-as-a-service (SaaS) and cloud-based computing become more prevalent, hosted replication offerings will become more common.
  • Because replication relies on available bandwidth, wide-area network (WAN) optimization offerings from the likes of Blue Coat Systems Inc., Cisco Systems Inc., Citrix Systems Inc., F5 Networks Inc., Juniper Networks Inc., Packeteer (now a Blue Coat company), Riverbed Technology Inc. and Silver Peak Systems Inc. are used to complement replication products to preserve valuable WAN bandwidth.

This was first published in April 2009

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