Slow performance when you access Azure files storage from Windows 8.1 or Server 2012 R2

Applies to: Windows Server 2012 R2 DatacenterWindows Server 2012 R2 StandardWindows Server 2012 R2 Essentials

This article describes an issue that occurs when you access Microsoft Azure files storage from Windows 8.1, Windows RT 8.1, or Windows Server 2012 R2. A hotfix is available to fix this issue. Before you install this hotfix, see the Prerequisites section.


Consider the following scenario:

  • You have a Windows 8.1-based or Windows Server 2012 R2-based client that performs I/O-intensive workload against a Continuously Available Server Message Block (SMB) file share.
  • There is a small but measurable latency on the network connection between the SMB client and the SMB service.
  • The I/O workload frequently opens and closes handles to the same paths under the SMB file share repeatedly, instead of reusing the same file handles.
In this scenario, performance of I/O workload is poor.


This problem occurs because the SMB client sometimes has the option to delay certain network operations to improve overall I/O performance by using cached results or cached handles to remote files. This includes caching handles to remote files after applications have closed their handles to the files.

In some scenarios that involve I/O-intensive workloads and that perform buffered I/O to many files, caching handles may cause Continuously Available SMB shares to take longer than usual to fail over the file server resources from one cluster node to another. This occurs after the application closes the caching handles.

In Windows 8.1 and Windows Server 2012 R2, SMB client skips caching handles that are opened on Continuously Available SMB shares to make sure that failover times are always as quick as possible.


To fix this issue, install this hotfix in order to enable the SMB client to cache closed handles that were opened against Continuously Available SMB shares.

Note In rare workload-specific scenarios, customers may have to increase the “CATimeout” on their Windows Server-based Continuously Available SMB shares in order to provide all SMB clients enough time to reconnect their handles after a failover event.

Hotfix information

Important If you install a language pack after you install this hotfix, you must reinstall this hotfix. Therefore, we recommend that you install any language packs that you need before you install this hotfix. For more information, see Add language packs to Windows.

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

If the hotfix is available for download, there's a "Hotfix Download Available" section at the top of this Knowledge Base article. If this section doesn't appear, submit a request to Microsoft Customer Service and Support to get 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 don't 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 don't see your language, it's because a hotfix isn't available for that language.


To apply this hotfix, install update 2919355 in Windows 8.1 or Windows Server 2012 R2.

Registry information

To use the hotfix in this package, you don't have to make any changes to the registry.

Restart requirement

You may have to restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix doesn't replace a previously released hotfix.


To work around this issue, use the following method:

  • Change the SMB share from a Continuously Available file share to a normal SMB share by changing the properties.

    Note Benefits that are specific to SMB Continuously Available file shares will be lost if a share is converted into a normal file share.


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


For more information about cached handles and Windows Server 2012 Continuously Available (SMB) file shares, see the following Microsoft Knowledge Base article:

Learn about the terminology that Microsoft uses to describe software updates.

File Information

The English (United States) version of this software update installs files that have the attributes that are listed in the following tables. The dates and times for these files are listed in Coordinated Universal Time (UTC). Be aware that dates and times for these files on your local computer are displayed in your local time and with your current daylight saving time bias. The dates and times may also change when you perform certain operations on the files.