An opportunistic lock (also called an oplock) is a lock placed by a client on a file residing on a server. In most cases, a client requests an opportunistic lock so it can cache data locally, thus reducing network traffic and improving apparent response time. Opportunistic locks are used by network redirectors on clients with remote servers, as well as by client applications on local servers.

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By default, opportunistic locking is enabled for server message block (SMB) clients that run one of the Windows operating systems that is listed in the "Applies to" section. Opportunistic locking lets clients lock files and locally cache information without the risk of another user changing the file. This increases performance for many file operations but may decrease performance in other operations because the server that grants the opportunistic lock must manage the breaking of that lock when another user requests access to the file.

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