AWS DataSync is a data transfer service that simplifies, automates, and accelerates moving data between on-premises storage systems and AWS storage services over the internet or AWS Direct Connect1. AWS DataSync can transfer data to Amazon FSx for Windows File Server, which is a fully managed file system that is accessible over the industry-standard Server Message Block (SMB) protocol. Amazon FSx for Windows File Server is builton Windows Server, delivering a wide range of administrative features suchas user quotas, end-user file restore, and Microsoft Active Directory (AD) integration2. This solution meets the requirements of the question because:

It can migrate more than 100 TB of data to AWS within a reasonable time frame, as AWS DataSync is optimized for high-speed and efficient data transfer1.

It can preserve the intricate directory structure and the millions of small files stored in deep hierarchies of subfolders, as AWS DataSync can handle complex file structures and metadata, such as file names, permissions, and timestamps1.

It can avoid changing the applications to access the data after migration, as Amazon FSx for Windows FileServer supports the same SMB protocol and Windows Server features that the company’s on-premises file storage uses2.

It can reduce the operational overhead, as AWS DataSync and Amazon FSx for Windows File Server are fully managed services that handle the tasks of setting up, configuring, and maintaining the data transfer and the file system12.