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Database consistency
FC SAN-attached CDP appliances maintain three levels of recoverable database images: crash consistent, transaction consistent and CDP consistency groups.

A crash-consistent image is the default-recoverable database image that all block-based CDP products provide. Should a database corruption occur, storage administrators may recover the database to any past point-in-time. However, full database recoveries still depend on the database admin to replay the database transaction logs to complete the recovery to a state where the restored image is usable by the database.

Transaction-consistent database recoveries minimize the database admin's involvement in recoveries. To create points in the CDP journal that are identifiable as recoverable transaction-consistent database images, CDP products offer application-specific host agents for databases. The CDP host agent monitors the database for periods when it enters a transaction-consistent state and then inserts a bookmark into the CDP journal. When performing recoveries, the CDP software identifies and displays these bookmarks so admins can restore images that are immediately accessible.

To create a consistency group, an admin selects and aggregates the virtual CDP LUNs that mirror the LUNs on which the production database resides. Each consistency group has its own journal that tracks data changes

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to any of the LUNs belonging to that consistency group. A critical factor is placing the CDP consistency group journal on back-end disk that matches or exceeds the performance of production LUNs.

Critical to Siemens' implementation was the creation and configuration of three CDP consistency groups to keep the database consistent across more than 200 RecoverPoint CDP LUNs. Siemens' Knoerer matched his 200 production database LUNs with virtual RecoverPoint LUNs residing on EMC Clariion CX700 disk.

However, Knoerer placed his consistency group journal on virtual CDP LUNs that were mapped back to the DMX-3 because he needed the higher performance of the DMX-3 storage to keep pace with the large number of changes in the Oracle database.

Future challenges
How to integrate with VMware is under discussion for most CDP vendors. While CDP host agents are supported on guest OSes, CDP host agents that operate at the hypervisor level on VMware and capture all traffic from all guest OSes are a work in progress.

The next frontier that CDP software needs to address, perhaps beginning in FC SAN-attached environments, is how to keep apps that run across multiple servers consistent. Eric Burgener, senior analyst and consultant at Hopkinton, MA-based Taneja Group, says there's a debate going on in the CDP community about the best way to create a consistent image across multiple servers.

FC SAN-attached CDP products will involve lengthier testing and configuration periods on a per-server basis. Admins will need to allocate new FC ports on FC SAN directors for the CDP appliance and new storage capacity to mirror the source server's production volumes; they'll also need to deploy new storage that matches the performance of the production database to keep the CDP journal.

In the end, reduced recovery time and backup software integration will be the deciding factors in product selection. FC SAN-attached CDP products offer almost immediate recoveries and allow applications to fail over and operate on virtual volumes presented by the CDP appliance with minimal or no application performance degradation. If simplified backup and recovery is your primary objective, backup products with integrated CDP promise to dramatically lower recovery time objectives and recovery point objectives.

This was first published in September 2008

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