I participated in a webinar two weeks ago, Subversion in the Enterprise, in which we had a limited chance to respond to questions. We’ll answer some of the remaining questions here.

One topic that came up multiple times was enterprises migrating from Rational ClearCase. Since I spent 5 years helping sell, implement and support ClearCase, and now 6 years at CollabNet, I understand both worlds pretty well.

Before focusing on ClearCase though, I encourage you to read some very good migration articles that can be found in the Subversion migration project. I talk to big companies every day about Subversion and many of them are considering to migrate or have although done so. These articles are a good start to prepare for a migration.

When planning a migration, the most important thing you need to do is realistically determine what data you really will benefit from migrating, rather than just assuming you want to migrate everything. For one thing, you don’t want to clutter your new repository. If you move from one house to another, you have garage sales, donate items to charity, and throw things away. The same concept should be applied to version control migrations. What really do you need in your new "house"? Realize that you have to pay the moving costs for whatever you migrate so be sure that the value you realize is worth more than the cost.

I always suggest leaving historic data in the legacy tool and just taking the latest from the live branches or perhaps snapshots of key milestones from current development branches. You should minimize your investment in the legacy tool both in terms of licenses and hardware, but retain it as the historical archive. A potential alternative is to migrate the historical data to its own Subversion repository while migrating limited data into the “production” repository. That way, history is retained (with the clutter stored elsewhere) and the legacy tool is eliminated along with its licensing and support costs.

Any migration will have limits based on the differences between the legacy system and Subversion as well as the extent to which the migration tool’s author(s) decided to map functionality and metadata between the two tools. With ClearCase, it is not a question of "will there be limits?", but rather how many limits. ClearCase doesn’t really support data migration so the development of migration tools is very challenging. That said, I am aware of at least a couple of attempts at providing such a tool. The first was a tool developed by an enterprise specifically for their needs and situation. It had limitations that they were willing to accept but the tool will unlikely be acceptable to most other companies.

Most of the focus these days is on the SVN Importer tool which purports to support migrations from many different version control tools. Unfortunately, a brute force approach - limited to a full migration - is taken to ClearCase and to most of the other systems. Such an approach requires a lot of system resources (e.g., disk and memory) to execute. That has proven itself to limit the size of a source VOB that can be migrated using a relatively “average” machine. One customer has seen that limit at a 150MB VOB, but the limit will vary depending on the hardware used for the migration. This customer had one VOB larger than the tool could support but fortunately they wanted to split the VOB contents anyway so they split it and executed two migrations resulting in two Subversion repositories.

As for case studies of enterprises successfully migrating from ClearCase, I certainly see it happening, particularly with companies that are truly doing distributed development and those who don’t need the unique features of ClearCase. Some companies have migrated history but most are leaving it behind. In my experience, the biggest key to success is not to recreate ClearCase processes in Subversion, but rather revisit your organization’s version control needs and map Subversion’s features and proven best practices to meet those needs.

Keep in mind that companies usually run a pilot before making a commitment to something as critical as a new version control tool. When they commit, they do so based on their own documented experiences. For a migration from ClearCase, I highly recommend you do a pilot first. After that you can join the companies that are enjoying the many benefits Subversion brings over ClearCase, such as atomic commits, real support for globally distributed development and consistent performance with tags and branches.

If you need help with your migration, our Professional Services team has a lot of experience helping customers move to Subversion. Check out our corporate web site for more information.