Many Windows applications (including the Windows 98 explorer shell) seem to confuse flushing buffer contents to disk with doing a sync to disk. Under UNIX, a sync call forces the thread to be suspended until the kernel has ensured that all outstanding data in kernel disk buffers has been safely stored onto stable storage. This is very slow and should only be done rarely. Setting this parameter to no (the default) means that smbd 8 ignores the Windows applications requests for a sync call. There is only a possibility of losing data if the operating system itself that Samba is running on crashes, so there is little danger in this default setting. In addition, this fixes many performance problems that people have reported with the new Windows98 explorer shell file copies. The flush request from SMB2/3 clients is handled asynchronously, so for these clients setting the parameter to yes does not block the processing of other requests in the smbd process. sync always no