Exploring Storage Snapshot, Cloning & Copy Techniques: Is It a Logical or Physical Copy?

by Dianne McAdam
October 1, 2004

In today’s 24x7 data center, one copy of a critical application volume isn’t enough. To run backup operations, load data warehouses, or test new versions of applications without disrupting the flow of information to users and applications, IT administrators must create multiple copies of primary production volumes.

These copies can be created manually, but doing so costs critical people resources and time, and can lead to data loss due to human error. To circumvent these problems, hardware vendors have developed automated array-based applications that create copies of primary volumes more easily and with less exposure to data loss than manual alternatives.

Many vendors today refer to the result of copy functions as “snapshots.” However, while all snapshot copies may look the same on the surface, the methods used to derive them can vary widely. We realize many software solutions create copies; this article addresses only hardware solutions. For example, snapshots can be physical or logical copies of primary disk volumes; these are distinctly different copying processes.

A physical copy (or “clone”) is a bitfor- bit replica of the primary disk volume. A logical copy is a logical representation of the primary disk volume. While a logical copy seems to create a new disk volume, it does so without actually allocating new physical disk space to that volume.

Who Supports Copies?

Physical copies are created when a new volume, equal in size to the original, is allocated to the disk array for storing a duplicate of the original disk volume. The resulting clone consumes the same exact disk space as the source. In contrast, logical copies are created without initially moving (or actually copying) one byte of data from one location to another. They require little additional disk capacity. Both methods have advantages and disadvantages.

Today, IT administrators have various disk arrays from which to choose, all of which can make physical or logical copies of data. However, because no two vendors’ implementations or terminology are identical, the selection process can be difficult. IT administrators should understand how both types of copy functions work to ensure they select the right method for any given application. Figure 1 lists several storage vendors and their offerings.



Enter the Clones

Many disk arrays create clone volumes equal in capacity to original volumes. In these situations, the array controller first allocates unused disk capacity, then copies data from the original volume to the clone without interrupting normal operations. When the volume is completely copied, the clone is split from the original volume and mounted onto another server, where it can be used for a backup operation, for example.