File locking prevents multiple clients from changing the same information at the same time. When a client opens a file (or part of a file), the file becomes locked so the client has exclusive access.

Before a read or write is performed on a file the lock database to checked to verify the lock status of the file.

Strict locking, enabled by default, helps prevent multiple clients from attempting to write to the same file. When strict locking is enabled, the SMB server checks for and enforces file locks.

Opportunistic locking (oplocks) grants exclusive access to the file similarly to strict locking, but also permits the client to cache its changes locally (on the client computer). This type of locking offers improved performance.

In Mac OS X Server, SMB share points supports oplocks.

To enable oplocks, change the SMB protocol settings for a share point using Workgroup Manager. For more information, see Changing SMB Settings for a Share Point.

IMPORTANT: Do not enable oplocks unless the share point is using only SMB. If the share point uses any other protocol, data can become corrupt.