If some Domains you serve have a large number of Accounts (5,000 or more), you should consider storing accounts in Account Subdirectories rather than in a flat domain directory. This recommendation is based on the method that file systems use to maintain the list of entries within a directory index, and the maximum recommended number of entries is largely dependent on the type of file system in use.
For example, a file system with a hashed directory index is capable of efficiently accessing more directory entries than those file systems that use only a flat file for directory indexing. Some file systems can easily access an index of over 50,000 entries, while others become sluggish at only 1,000.
This same principle also applies to sites with over 2,000 or more domains on the server or cluster. In this scenario, it is recommended to use Domain Subdirectories
You can store subdirectories on multiple logical volumes, if necessary for storage volume or performance - just replace the moved subdirectories with their symbolic links. You can also move domain directories from the Domains directory and replace them with symbolic links.