If you are designing Exchange 2016 (or have been designing Exchange 2013) environment you are aware of the The Exchange 2016 Preferred Architecture (https://blogs.technet.microsoft.com/exchange/2015/10/12/the-exchange-2016-preferred-architecture/) and articles like Ask the Perf Guy: How big is too BIG? (https://blogs.technet.microsoft.com/exchange/2015/06/19/ask-the-perf-guy-how-big-is-too-big/) which explain pretty much how to design an Exchange solid (and large) Exchange environment.

When it comes to Mailbox databases, the recommended size limit for non-replicated databases is 200GB and for replicated databases 2 TB (when running 3 or 4 copies of a Mailbox database).

One can only guess how Microsoft has designed their Exchange servers in Exchange Online, but we can assume that the Preferred Architecture is written with their Exchange Online experiences in mind.

Sometimes error messages that are generated in Exchange Online can reveal more information. While moving mailboxes from Exchange 2010 to Exchange Online in a hybrid configuration the following error message was returned in a migration batch for a number of Mailbox databases:

Error: MigrationTransientException: Target database ‎’07bdf507-ab94-479b-aeb6-1bfef1458c4c‎’ cannot be used: Current database file size: 1502835900416 Current space available inside database: 100237312 Allowed database growth percentage: 90 Maximum database file size limit: 1622722691784 Is database excluded from provisioning: ‎’False‎’. –> Target database ‎’07bdf507-ab94-479b-aeb6-1bfef1458c4c‎’ cannot be used: Current database file size: 1502835900416 Current space available inside database: 100237312 Allowed database growth percentage: 90 Maximum database file size limit: 1622722691784 Is database excluded from provisioning: ‎’False‎’.

Obviously it’s telling us the migration cannot proceed since the target Mailbox (in Exchange Online!) has reached its size limit. The following sizes are reported:

• Current database file size: 1502835900416 (1,502,835,900,416 bytes, approx. 1.5TB)
• Current space available inside database: 100237312 (100.237.312 bytes, approx. 100MB)
• Maximum database file size limit: 1622722691784 (1.622.722.691.784 bytes, approx. 1.6 TB)

So, the maximum size limit for Exchange 2016 in Exchange is not really used in Exchange Online, but it’s getting close, which is interesting to see.

What I don’t understand is why this issue occurs in the first place. To me it looks like a failing part in the provisioning service but I have to admit I’ve never seen this before in the last couple of years so I expect it’s only one Exchange server that’s failing here.