PC Ext: Explanation of Opportunistic Locking on Windows NT

An illustration of how level II oplocks work:

1. Station 1 opens the file, requesting oplock.
2. Since no other station has the file open, the server grants station 1 exclusive oplock.
3. Station 2 opens the file, requesting oplock.
4. Since station 1 has not yet written to the file, the server asks station 1 to Break to Level II Oplock.
5. Station 1 complies by flushing locally buffered lock information to the server.
6. Station 1 informs the server that it has Broken to Level II Oplock (alternatively, station 1 could have closed the file).
7. The server responds to station 2's open request, granting it level II oplock. Other stations can likewise open the file and obtain level II oplock.
8. Station 2 (or any station that has the file open) sends a write request SMB. The server returns the write response.
9. The server asks all stations that have the file open to Break to None, meaning no station holds any oplock on the file. Because the workstations can have no cached writes or locks at this point, they need not respond to the break-to-none advisory; all they need do is invalidate locally cashed read-ahead data.

The following registry entries are used to enable or disable oplocks for Windows NT Workstation or Server. These registry keys may not exist by default. To access the registry run REGEDT32.EXE from the File menu, choose Run in Program Manager or File Manager.

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

Workstation Service Entries

\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet
   \Services\LanmanWorkstation\Parameters

UseOpportunisticLocking   REG_DWORD   0 or 1
Default: 1 (true)
 

Indicates whether the redirector should use opportunistic-locking (oplock) performance enhancement. This parameter should be disabled only to isolate problems.

Server Service Entries

\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet
   \Services\LanmanServer\Parameters

EnableOplocks   REG_DWORD   0 or 1
Default: 1 (true)
 

Specifies whether the server allows clients to use oplocks on files. Oplocks are a significant performance enhancement, but have the potential to cause lost cached data on some networks, particularly wide-area networks.

MinLinkThroughput   REG_DWORD   0 to infinite bytes per second
Default: 0
 

Specifies the minimum link throughput allowed by the server before it disables raw and opportunistic locks for this connection.

MaxLinkDelay   REG_DWORD   0 to 100,000 seconds
Default: 60
 

Specifies the maximum time allowed for a link delay. If delays exceed this number, the server disables raw I/O and opportunistic locking for this connection.

OplockBreakWait   REG_DWORD   10 to 180 seconds
Default: 35
 

Specifies the time that the server waits for a client to respond to an oplock break request. Smaller values can allow detection of crashed clients more quickly but can potentially cause loss of cached data.