With Exclusive Oplock, if a file is opened in a non-exclusive (deny none)
mode, the redirector requests an opportunistic lock of the entire file. As
long as no other process has the file open, the server will grant this
oplock, giving the redirector exclusive access to the specified file. This
will allow the redirector to perform read-ahead, write-behind, and lock
caching, as long as no other process tries to open the file.
When a second process attempts to open the file, the original owner will
be asked to Break Oplock or Break to Level II Oplock. At that point, the
redirector must invalidate cached data, flush writes and locks, and
release the oplock, or close the file.
Opportunistic Locking level II, provides a method for granting read access
to a file by more than one workstation, and these workstations can cache
read data locally (read-ahead). As long as no station writes to the file,
multiple stations can have the file open with level II oplock.
An illustration of how level II oplocks work:
- Station 1 opens the file, requesting oplock.
- Since no other station has the file open, the server grants
station 1 exclusive oplock.
- Station 2 opens the file, requesting oplock.
- Since station 1 has not yet written to the file, the server asks
station 1 to Break to Level II Oplock.
- Station 1 complies by flushing locally buffered lock information to
- Station 1 informs the server that it has Broken to Level II Oplock
(alternatively, station 1 could have closed the file).
- 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
- Station 2 (or any station that has the file open) sends a write request
SMB. The server returns the write response.
- 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
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
Server Service Entries
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
MinLinkThroughput REG_DWORD 0 to infinite bytes per second
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
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
OplockBreakWait REG_DWORD 10 to 180 seconds
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.