Redirector Uses Core Mode on Large File Transfers

• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Server version 3.5

SYMPTOMS

When you copy a large file to or from a Windows NT Workstation and Windows NT Server version 3.5 computer, the redirector may switch to core mode during the transfer. The use of core mode increases the time required to transfer the file. The switch to core mode occurs after approximately 20 megabytes (MB) have been transferred and is more likely to occur when transferring a file to a Windows NT server, but it can occur in either direction. Core mode over FDDI requires more bandwidth than over Ethernet or token ring, so the symptoms are more pronounced on fiber. Smaller files transfer in raw mode.

CAUSE

Raw I/O requires exclusive access to the virtual circuit, meaning the virtual circuit must be effectively "locked" for the duration of the raw I/O. If other activity is occurring on the circuit at the time the redirector tries to do raw I/O, the circuit cannot be "locked" so the redirector uses core I/O instead.

One reason the virtual circuit cannot be "locked" is that as the file data is being written, the file also needs to be extended which also requires network activity.

The reason the problem surfaces at a large offset into the file has to do with when the lazy writer and modified page writer start their work. When the system starts running short of free pages, the modified page writer wakes up. Unfortunately, the modified page writer likes to write one page at a time. The redirector cannot turn a 4K write into a raw I/O.

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 3.5. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.