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FIX: MS Access ODBC Driver Does Not Recognize DSN MaxBufferSize

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This article was previously published under Q195473

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Although the Microsoft Access ODBC driver (version 3.51.1713) Data Source Name (DSN) setup sets the MaxBufferSize value to 512K, the cache size is not capped at 512K. For example, if an application spins in a loop and inserts records using SQLSetPos, the process private bytes grow to about 13 MBs rather than the expected 512K.

Looking at the registry entries for the DSN, you might see something like the following:
                            MaxBufferSize = 512
AccessTest is the name of the DSN. The MaxBufferSize setting has no effect for the DSN.

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The Microsoft Access ODBC driver is ignoring the DSN MaxBufferSize setting. However, the driver is using the MaxBufferSize setting of Jet under the HKEY_LOCAL_LACHINE\Jet\3.5\Engines\Jet 3.5\MaxBufferSize setting.

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The Microsoft Access ODBC driver uses the MaxBufferSize setting for Jet to determine its maximum buffer size.

The Jet MaxBufferSize is a registry entry in the following path:
HKEY_LOCAL_LACHINE\Jet\3.5\Engines\Jet 3.5
It is normally set to zero (0). The setting of zero indicates that Jet uses a default algorithm to determine the maximum cache size. That algorithm is "(((Total RAM on computer in MB - 12MB) / 4) + 512K)". This returns a value equal to the number of pages that the cache size can increase. Each page is 2048 bytes, so for a computer with at least 64 MB of RAM, this algorithm returns approximately 14 MB. This is the maximum memory that Jet consumes from its startup point.

Following is an example of the calculation for a 64 MB computer:

   (64Mb - 12Mb) / 4 + 512K
   = 52Mb/4 + 512K
   = 13Mb + 512K
   = 13.5Mb

Jet will not allow the "Total RAM on computer" calculated earlier to exceed 64 MB. If the total RAM on the computer is 12 MB or less, then the algorithm creates a 512 KB cache buffer.

Jet will use memory for its cache until it reaches this dynamic setting if the MaxBufferSize registry setting is zero (0). If the MaxBufferSize registry setting is set at 512 KB, for example, the cache size will be limited to 512 KB. Once Jet exceeds the setting, it fires off a background thread to reduce the memory to the point that it goes below the maximum cache size setting.

You can typically minimize the growth of the Jet cache by dropping a HSTMT() statement handle and creating a new one. In this case, the memory at the beginning of the Jet cache will be re-used.

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Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

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For more information about MaxBufferSize, please see the following article in the Microsoft Knowledge Base:
187872� HOWTO: Determine Jet Memory Usage with DAO MaxBufferSize

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Keywords: KB195473, kbjet, kbfix, kbdriver, kbdatabase, kbbug

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Article Info
Article ID : 195473
Revision : 2
Created on : 10/16/2002
Published on : 10/16/2002
Exists online : False
Views : 437