Exchange database dismounted due to NTFS file extent limit reached – unexpected outage

Hi everyone,

We experienced a serious outage on our Exchange 2016 server recently, and I wanted to share what we found during the root cause analysis – in case it helps someone else avoid the same scenario.

Summary:
After digging deep, we discovered that the issue was caused by the NTFS file system hitting its internal file extent limit on the .edb file. Once this threshold was reached, the database could no longer grow, and the system dismounted the database unexpectedly. No prior warning, just service interruption.

Details:

The .edb was around 1.2 TB in size.
     This isn’t a limit on database size itself — it’s about how fragmented the file is on disk.
     Once NTFS couldn’t track any more extents, the database stopped working.
     Microsoft doesn’t publish a clear fix for this; only scattered references to similar behavior in past cases.

What we did:

     Created a fresh, clean database.
     Manually moved user mailboxes into the new DB.
     The old database couldn't be mounted anymore, so we brought the system live without historical mail – just to maintain continuity.
     We're now working on extracting data from the unmounted .edb using third-party tools.

Looking for thoughts:

     Has anyone else hit the NTFS extent wall with Exchange?
     How do you monitor extent growth proactively?
     Did switching to ReFS solve this for you long-term?

Open to any input or similar experiences – appreciate it in advance.

Thanks!