Exchange Server 2013 databases become fragmented in Windows Server 2012

Applies to: Windows Server 2012 DatacenterWindows Server 2012 DatacenterWindows Server 2012 Standard


When you deploy Microsoft Exchange Server 2013 on Resilient File System (ReFS) volumes on computers that are running Windows Server 2012, Exchange databases become heavily fragmented. Therefore, end-users experience poor performance on Exchange-related operations. For example, it takes longer than expected to change a view in Outlook Web Access. 


This issue occurs because a large file is broken into small chunks during the allocation request. In an Exchange database, a large file is extended in large chunks, such as 128 megabyte (MB) chunks or 256 MB chunks. This issue occurs when another thread that does transactional logging requests allocations in smaller sizes at the same time.

Therefore, when a 128 MB file allocation request is received, the medium allocator requires space from the large allocator. However, some space is allocated to a small allocation request (such as log file allocation request) at the same time. In this situation, the large file is prevented from contiguous allocation. This behavior results in fragmentation.

Note ReFS uses a hierarchical allocator to manage disk space allocation. There are large, medium and small allocators that differ in the granularity of space they manage. Each allocator requests space from its parent allocator. For example, a medium allocator manages medium-sized chunks that are allocated from the large allocator. At the default setting, the medium allocator requests space from the large allocator in 64 MB chunks.


This hotfix is also available at Microsoft Update Catalog.

Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem.

If the hotfix is available for download, there is a "Hotfix Download Available" section at the top of this Knowledge Base article. If this section does not appear, submit a request to Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft website: Note The "Hotfix Download Available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.


To apply this hotfix, you must be running Windows Server 2012.

Registry information

To use the hotfix in this package, you do not have to make any changes to the registry.

Restart requirement

You must restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace a previously released hotfix.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

More Information

For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates