Oplocks (opportunistic locks) enable the redirector on a CIFS client in certain file-sharing scenarios to perform client-side caching of read-ahead, write-behind, and lock information. A client can then work with a file (read or write it) without regularly reminding the server that it needs access to that file, which improves performance by reducing network traffic.
By default, oplocks are enabled for each qtree. If you disable oplocks for the entire filer, oplocks are not sent even if you enable oplocks on a per-qtree basis.
If a process has an exclusive oplock on a file and a second process attempts to open the file, the first process must relinquish the oplock and its access to the file. The redirector must then invalidate cached data and flush writes and locks, resulting in possible loss of data that was to be written.
CIFS oplocks on the filer are on by default. You might turn CIFS oplocks off under either of the following circumstances (otherwise, you can leave CIFS oplocks on):
You are using a database application whose documentation recommends that oplocks be turned off.
You are handling critical data and cannot afford even the slightest data loss.