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But if physics is your enemy, CDP is your best friend. A good CDP product places no more load on your VM than a typical virus protection package, and it's able to recover one or all of your VMs instantaneously with no data loss. Server virtualization alone could herald the comeback of continuous data protection.
|CDP product sampler|
AppAssure Software Inc. Replay 4
A look inside CDP
The Storage Networking Industry Association (SNIA) defines CDP "as a methodology that continuously captures or tracks data modifications and stores changes independent of the primary data, enabling recovery points from any point in the past . . . data changes are continuously captured . . . stored in a separate location . . . [and RPOs] are arbitrary and need not be defined in advance of the actual recovery."
Please note that you don't see the word "snapshot" above. While it's true that many of today's CDP systems allow users to create known recovery points in advance, they're not required. To be considered CDP, a system must be able to recover to any point in time, not just to when snapshots are taken.
CDP systems start with a data tap or write splitter. Writes destined for primary storage are "tapped" or "split" into two paths; each write is sent to its original destination and also to the CDP system. The data tap may be an agent in the protected host or it can reside somewhere in the storage network. Running as an agent in a host, the data tap has little to no impact on the host system because all the "heavy lifting" is done elsewhere. CDP products that insert their data taps in the storage network can use storage systems designed for this purpose, such as Brocade Communications Systems Inc.'s Storage Application Services API, Cisco Systems' MDS line and its SANTap Service feature or EMC Clarion's built-in splitter functionality. Some CDP systems offer a choice of where their data tap is placed.
Users then need to define a consistency group of volumes and hosts that have to be recovered to the same point in time. Some CDP systems allow the creation of a "group of groups" that contains multiple consistency groups, creating multiple levels of granularity without sacrifice. Users may also choose to perform application-level snapshots on the protected hosts, such as placing Oracle in backup mode or performing Volume Shadow Copy Service (VSS) snapshots on Windows. (Remember, snapshots aren't required.) Some CDP systems simply record these application-level snapshots when they happen, while others provide assistance to perform them. It's very helpful when the continuous data protection system maintains a centralized record of application-level snapshots, as they can be very useful.
This was first published in August 2010