If you have a very large database, up to 2TB in size, will let you copy, or clone, that database many times, very quickly, making the full database available in multiple SQL Server instances across your development and test environments. And yet in each of these instances, the cloned database takes up only a few tens of megabytes, and creating the clone takes seconds, rather than minutes or hours. How is it possible that the cloned databases could be so small and lightweight, and yet behave exactly like any normal database, complete with all the data?

Is a clone like a snapshot? Are we using some strange compression techniques?

It sounds like magic, but it’s not; SQL Clone does all this by making clever use only of technologies built into Windows. The Heavy Lifting of Traditional Database Provisioning All developers and testers would like to work with a database that is as close as possible to production, in terms of data volume and distribution. There are many advantages to being able to perform integration and acceptance testing with realistic production-like data, from as early as possible in the development cycle. However, for databases of any size, this means copying a lot of bytes of data onto every development or test environment that needs a copy of the database. It also usually means a slow, manual process of restoring the latest backup of the production database onto each of the target SQL Server instances. Figure 1 shows a 200GB production database that we’ve restored to both the QA and Development environments, therefore requiring 600GB of disk space in total. Each of the orange squares represents the same 10GB block of data in the MDF and LDF files (the blocks in this diagram are purely conceptual).

The red square represents data that has been modified in the QA environment. Part of the problem is that we’re copying around the same bytes, unchanged, again and again. We might, in the QA environment, for example, make a few changes to the data, to perform scenario testing. Likewise, in development, we might make a few changes to get the data into a state where you can reproduce a bug. However, the rest of the data will be the same. Database Provisioning With SQL Clone The simple answer to the question why is a clone is so small?

Is that SQL Clone uses disk storage virtualization technologies, built into Windows, to remove the need to copy of all these same bytes onto each instance. Instead, SQL Clone works to virtualize the data on each instance.

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SQL Clone creates one full copy of all the data and metadata that makes up the source database. This is called an image. We copy the bytes only once. From this image, it can then create clones on each of the QA and Development SQL Server instances. Each clone has access to all those same bytes by reading from the image.