Registry Hive Fragmentation Leads to Excessive Size

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Processes that use the registry to repeatedly modify the same values in small increments can cause excessive memory fragmentation in the corresponding registry hive. The sum of the fragmented cells can become so large that the hive size winds up many times larger than the sum of actual data in the hive. Eventually, the sum of the hive will exceed the Registry Size Limit (RSL), causing subsequent modifications to fail until the RSL is increased. As an example consider the SAM hive used to store Windows NT accounts. The SAM size frequently exceeds projected sizes due to the fact that it fragments the SAM hive when adding members to large groups.


In the SAM example, when a user is added to a large group it causes the registry value for that group to be replaced and the old value added to a hash of free lists by size of allocation. The freed cell is coalesced with the nearest neighbor cell in memory, if it's a free cell, to create a slightly larger cell to add to the free lists. Over several additions, the free lists can contain several cells that are never used because they're too small. Over time the sum of the free cells become so large that the sum of the hives exceeds RSL.


To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the individual software update. For information on obtaining the latest service pack, please go to:
  • 152734 how to obtain the latest windows nt 4.0 service pack
For information on obtaining the individual software update, contact Microsoft Product Support Services. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:;EN-US;CNTACTMS


Microsoft has confirmed this to be a problem in Windows NT version 4.0. This problem was first corrected in Windows NT version 4.0 Service Pack 5.


Article ID: 197632 - Last Review: November 2, 2013 - Revision: 4.0
Applies to
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows NT Server 4.0 Enterprise Edition
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