In estimating write traffic, consider the following:
How many updates per second are expected?
What types of updates are expected?
What is the estimated peak update rate?
What is the estimated average update rate?
Are updates concentrated in a particular data center or geographic region?
If one data receives proportionally more update traffic than other data centers, it might be worth placing additional writable servers in this data center to distribute the update load.
Are updates concentrated at a particular time of day?
If write performance is crucial to your enterprise, see Chapter 10, Designing a Scaled Deployment for suggestions on deploying a directory service that is scaled for writes.