Files Corrupted By Windows Sharing

Windows doesn't have a long history of interoperability and standards compliance, but the history it does have is somewhat checkered. As a result, most Macintosh administrators develop a knee-jerk aversion to Microsoft products and protocols. So when the first few Photoshop files wind up corrupted on your SMB file shares, complaining of inconsistent permissions issues or just displaying jumbled nonsense, it's easy to blame the problem on the Windows workstations. More often than not, though, it's a configuration error on Mac OS X Server that renders these files unavailable or unreadable.

The SMB (or Server Message Block) protocol is used for file sharing by Windows operating systems. It's also the basis of Windows-compatible file sharing on OS X Server, using the open source "Samba" implementation. When you configure individual share points for the SMB service, you can configure "Protocol Options" including custom naming, guest access, and permission inheritance. Those options also include enabling oplocks (also known as opportunistic locking) by default, which can be a serious and unexpected cause of file damage.

Opportunistic locking can improve performance by allowing multiple users to edit portions of a file concurrently. Unfortunately, they only work when clients edit files via over SMB, meaning that they can cause conflicts and become destructive when enabled on share points that offer AFP or NFS sharing as well. They can also cause performance issues when used with large binary files, the very kinds of files most often associated with the Macintosh platform.

While this issue can become quite serious over time, the solution is trivial. Uncheck the "Enable oplocks" box from the File Sharing configuration in Server Admin, then restart the service from the "Server" menu. You can turn off opportunistic locking this way on each share offered through multiple protocols, and eliminate the threat of file corruption through misconfiguration.

Recommended Reading: For more information on file locking with SMB, check out the Locks And Oplocks chapter of Oreilly's "Using Samba", available free online.