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As CDP becomes a viable option for enterprise servers, organizations need to decide how extensive their deployment will be as it may require the use of one, two or even three CDP products to protect all desktops and servers. The makeup of the CDP host agent is a tip-off to the scope of data protection the CDP software can provide. File-system-based CDP host agents take advantage of existing TCP/IP networks and can recover data at the file-system or volume level, but may cause excessive server and network overhead in high-transaction environments.

CDP products with block-based agents minimize overhead by capturing changes at the volume level and transmitting all changes over IP or FC SANs. Block-based CDP agents can scale to meet the performance requirements of most mission-critical applications, but can introduce significant cost and complexity.

CDP for different applications
The selection of a desktop and laptop CDP product will hinge on a number of variables, including the backup software in use, if management can be centralized and if it's desirable to have users manage their own restores (see "Laptop considerations," below).

Laptop considerations

Implementing continuous data protection (CDP) software

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on corporate desktops and laptops eliminates the need to be connected to the network at the specific times of day required by scheduled backups. However, there are certain features of these programs that you should carefully assess before selecting a specific desktop or laptop product.

Administrator vs. user-controlled backup policies. Atempo Inc.'s Live Backup allows administrators to remain in control when selecting files that should be backed up on users' desktops and laptops. IBM Corp.'s Tivoli Continuous Data Protection for Files gives that control to users and lets them select which files they wish to back up.

Backup software integration. CDP products differ in how--or even if--corporate backup software manages them. Symantec Corp.'s Desktop and Laptop Option acts as a backup agent directly managed by Symantec's Backup Exec or Veritas NetBackup products. Atempo's Live Backup is managed separately from its Time Navigator software; however, Atempo says it plans to merge the Live Backup and Time Navigator repositories this year so they can be managed with one interface.

Target media. Most CDP products back up primarily to disk, but disk can come in a variety of formats. Some, like IBM Tivoli Continuous Data Protection for Files, are now available in scaled-back versions on memory cards that plug into laptop USB ports and initiate laptop backups, storing the data on the memory card.

Bandwidth throttling. Because CDP products copy changes to data continually throughout the day, server and network performance could be affected. Atempo Live Backup lets users adjust how much network bandwidth CDP can use when connections are available (cache writes to disk when it's offline). Tivoli's Continuous Data Protection for Files only transmits changes for files it identifies as "original art," such as Microsoft Word and Excel documents, anytime a network connection is available; it transmits less-critical files stored on a disk cache, such as temporary and media files, once a day.

Selecting the right CDP product architecture for enterprise servers often comes down to whether the server is connected to a network or an FC SAN. If connected to an FC SAN, you need to determine if there are sufficient changes to app data to justify the deployment of an FC SAN-attached CDP appliance.

CDP products for network-attached servers come in many different architectures. CA XOsoft, for example, installs at the file-system level and is configurable as a standalone product and as a replication target on a secondary server.

VMware users may want to consider FalconStor's CDP Virtual Appliance for VMware. Although both CA XOsoft and FalconStor Software support VMware and the installation of host CDP agents on guest OSes, FalconStor installs its host CDP agent at the block level. The agent then stores the changed data to a volume presented over the corporate IP network by the FalconStor CDP Appliance. Once the CDP software creates a copy of the data on that virtual volume, the administrator may break off that virtual CDP LUN from the source and present it to any other operating system on that ESX server for recoveries or testing and development (see "CDP and VMware," below).

CDP and VMware
Here are three key issues you should consider before implementing continuous data protection (CDP) on VMware servers.

CDP host agent. CDP host agents are only compatible with VMware guest OSes and will require administrators to install host agents on each guest OS. This may require the allocation of additional storage to each guest OS for locally cached writes by the CDP host agent.

Hypervisor agent. This feature, under consideration by CDP vendors, will protect all guest OSes hosted by a VMware ESX server. However, restores can only occur for the entire VMware ESX server, not for individual guest OSes on the VMware server.

CDP server and network overhead. CDP doubles the number of writes on each guest OS, so administrators should ensure that the VMware server has sufficient server and network resources to accommodate the extra load.

This was first published in September 2008

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