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Article ID: 938475 - Last Review: October 8, 2011 - Revision: 4.0

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Important This article contains information about how to modify the registry. Make sure that you back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:
256986  ( ) Description of the Microsoft Windows registry

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When you use a Windows Vista-based computer to try to access files on a Windows Vista-based file server, you may receive an error message that resembles the following:
DriveLetter:\ is not accessible.
Insufficient system resources exist to complete the API.


This problem occurs if you try to access the file by using a long-term Server Message Block (SMB) request, and if more than 64 concurrent long-term SMB requests are sent to the file server. In this situation, all other long-term SMB requests are not answered.

Note Unlike typical SMB requests, a long-term SMB request remains open until it is answered by the server. For example, a long-term request is sent when a client computer uses the FindFirstChangeNotification function to monitor a shared resource for changes.

In Windows Vista, the Server service intentionally limits the maximum number of concurrent asynchronous network requests to 64. A Windows Vista-based client computer also has this limitation. However, the file server processes typical SMB requests immediately.

Note If both of the file server and the client computer are Windows Vista-based computers, the SMB 2.0 protocol is used for file sharing operations.


To resolve this problem, apply the following hotfix to both the file server and the client computer. After you apply the hotfix, you must modify a registry entry on the server to configure the maximum number of concurrent SMB requests. For more information about how to do this, see the "Registry information" section.

Service pack information

To resolve this problem, obtain the latest service pack for Windows Vista. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
935791  ( ) How to obtain the latest Windows Vista service pack

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. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

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, contact 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 Web site: (
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.


No prerequisites are required.

Restart requirement

You have to restart your computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other previously released hotfixes.

Registry information

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

To resolve this problem, modify the AsynchronousCredits registry entry on the file server to configure the maximum number of concurrent SMB requests. To do this, follow these steps:
  1. Click Start, type regedit in the Start Search box, and then press ENTER.
  2. Locate and then click the following registry subkey:
  3. In the details pane, double-click the AsynchronousCredits entry.

    Note The value type of this entry is REG_DWORD. The value data of this entry is decimal. If the AsynchronousCredits registry entry does not exist, create it.
  4. In the Value data box, enter the maximum number of concurrent SMB requests.

    Note If the value is larger than 64, resource usage on the file server increases. The file server may run out of resources such as paged pool memory. We recommend that you limit number of clients that access the file server before you increase this value.

    This article does not make specific recommendations on the value that you set because of the complexity of the network environment.
  5. Exit Registry Editor.

File information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.
Windows Vista, 32-bit versions
Collapse this tableExpand this table
File nameFile versionFile sizeDateTimePlatform
Package_1_for_kb938475~31bf3856ad364e35~x86~~ Applicable1,78315-Jun-200717:34Not Applicable
Package_2_for_kb938475~31bf3856ad364e35~x86~~ Applicable1,99615-Jun-200717:34Not Applicable
Update.mumNot Applicable1,89715-Jun-200717:34Not Applicable
X86_25d0b76c7c00d970e39bd582c48dc22c_31bf3856ad364e35_6.0.6000.20620_none_cbbc3c6387dbeb8b.manifestNot Applicable69815-Jun-200717:34Not Applicable
X86_9f816cda4d567616d3afaac228e6dba9_31bf3856ad364e35_6.0.6000.20620_none_6a3a12f093b96198.manifestNot Applicable70015-Jun-200717:34Not Applicable
X86_microsoft-windows-smbminirdr_31bf3856ad364e35_6.0.6000.20620_none_7e1f299c6d24b8b7.manifestNot Applicable9,34415-Jun-200717:35Not Applicable
X86_microsoft-windows-smbserver-v2_31bf3856ad364e35_6.0.6000.20620_none_d87a55d61282fd5a.manifestNot Applicable4,71815-Jun-200717:35Not Applicable
Windows Vista, 64-bit versions
Collapse this tableExpand this table
File nameFile versionFile sizeDateTimePlatform
Amd64_872643c0e08b3bfea6e35d908e0eff9e_31bf3856ad364e35_6.0.6000.20620_none_2c94f2fd4b883804.manifestNot Applicable70415-Jun-200717:34Not Applicable
Amd64_c0811949d12dd84ac380a6ab85453b26_31bf3856ad364e35_6.0.6000.20620_none_954a2531d9f6fb41.manifestNot Applicable70215-Jun-200717:34Not Applicable
Amd64_microsoft-windows-smbminirdr_31bf3856ad364e35_6.0.6000.20620_none_da3dc520258229ed.manifestNot Applicable9,61515-Jun-200717:44Not Applicable
Amd64_microsoft-windows-smbserver-v2_31bf3856ad364e35_6.0.6000.20620_none_3498f159cae06e90.manifestNot Applicable4,98315-Jun-200717:44Not Applicable
Package_1_for_kb938475~31bf3856ad364e35~amd64~~ Applicable1,79315-Jun-200717:34Not Applicable
Package_2_for_kb938475~31bf3856ad364e35~amd64~~ Applicable2,00815-Jun-200717:34Not Applicable
Update.mumNot Applicable1,90915-Jun-200717:34Not Applicable


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section. This problem was first corrected in Windows Vista Service Pack 1.


SMB 2.0 is a new version of SMB that is designed for the latest networking environments and for the needs of the next generation of file servers. SMB 2.0 is the default protocol for Windows Vista-based file servers and for file servers that run Windows Server 2008.

For more information about SMB 2.0, visit the following Microsoft Web site: (
For more information about the FindFirstChangeNotification function, visit the following Microsoft Web site: (
For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base:
937082  ( ) You cannot configure the maximum number of concurrent SMB requests that are sent to a Windows Vista-based file server
926646  ( ) You cannot configure the maximum number of concurrent SMB requests that are sent to the Server service in Windows XP
For more information about the terms that are used to describe software updates, 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

  • Windows Vista Ultimate
  • Windows Vista Business
  • Windows Vista Enterprise
  • Windows Vista Ultimate 64-bit Edition
  • Windows Vista Business 64-bit Edition
  • Windows Vista Enterprise 64-bit Edition
kbautohotfix kbvistasp1fix kbfix kbexpertiseadvanced kbqfe kbHotfixServer KB938475
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