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I found this when searching the Internet.<br />
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&quot;The days of online maintenance killing itself trying to defragment partially filled 4K pages of data to be on a single ESE page are gone. &nbsp;The same goes for offline defragmentation. &nbsp;<b>We now live in a world where large mailboxes are the norm and any white space will be quickly re-used. </b> Also consider that many organisations are now running 24 * 7, and it is hard to justify the impact caused by taking a database offline.&quot;<br />
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I am hoping that is the answer to my question. Any white space created by migrating mailboxes from DB1 to DB2 will end up being reused by growing mailboxes and therefore keep DB1 at the same size for now.
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I found this when searching the Internet.

"The days of online maintenance killing itself trying to defragment partially filled 4K pages of data to be on a single ESE page are gone.  The same goes for offline defragmentation.  We now live in a world where large mailboxes are the norm and any white space will be quickly re-used. Also consider that many organisations are now running 24 * 7, and it is hard to justify the impact caused by taking a database offline."

I am hoping that is the answer to my question. Any white space created by migrating mailboxes from DB1 to DB2 will end up being reused by growing mailboxes and therefore keep DB1 at the same size for now.

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This is Exchange 2013 with DAG setup, so I know reclaiming whitespace isn't an option. The problem I have is the amount of time required to procure more capacity in the SAN and keeping the drive from running out of space. There are two databases on separate Windows partitions. <br />
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E: 2 TB with 50 GB Free - DB1<br />
F: 950 GB with 125 GB Free - DB2<br />
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Will migrating mailboxes from DB1 to DB2 slow the growth of DB1 by creating whitespace, or is that whitespace completely unusable without a defrag?
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This is Exchange 2013 with DAG setup, so I know reclaiming whitespace isn't an option. The problem I have is the amount of time required to procure more capacity in the SAN and keeping the drive from running out of space. There are two databases on separate Windows partitions. 

E: 2 TB with 50 GB Free - DB1
F: 950 GB with 125 GB Free - DB2

Will migrating mailboxes from DB1 to DB2 slow the growth of DB1 by creating whitespace, or is that whitespace completely unusable without a defrag?

Exchange 2013 Database Whitespace

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.