Using a pilot group also allows you to test the whole process of migrating data and then working with mail, calendar, and contacts in the new Gmail environment. Any technical or communication issues you encounter with a small group are more easily managed and corrected, and with that experience you can provide a smoother transition for the rest of your users.
Once you have data from your pilot test, you can plan the migration of the rest of your users, including the hardware resources you’ll need, and the amount of time you’ll need to devote to the process.
Examples of the topological approaches you can take to setting up client machines between your mail server infrastructure and Google Apps, and information about the parallel processing of users on those client machines
A single instance of Google Apps Migration for Microsoft® Exchange runs on an intermediary client machine between your source server and Google Apps. You can migrate the data from one or more servers, and you can deploy one or more clients for each server. You must use at least one client per server, and each client migrates a unique list of users.
You can run only one instance of Google Apps Migration for Microsoft® Exchange on each client machine. If you try to run multiple instances on a single client, those instances overwrite one another’s configuration files (there is a single location for configuration files).
In addition, each instance has to reference a unique list of users in order to avoid corrupting the status information for each user’s data.
The following four diagrams illustrate approaches you can take.
Single server, single client
This is the most basic configuration. Use this configuration when all your data is on a single server, and a single migration client meets your needs. Depending on your network latency and client capacity, a single client can migrate up to 500-1000 users.
Google Apps Migration for Microsoft® Exchange
Single server, multiple clients
In this configuration, all data is pulled from a single source server, but you use multiple GAMME clients to migrate data. Be sure that every client has a completely separate user list. If multiple clients handle the same user, this will cause data corruption.
Google Apps Migration for Microsoft
Exchange Administration Guide