INFO: Restriction on the Number of Concurrent I/O Requests Setting (240314)

SUMMARY

The registry subkey to specify the maximum number of outstanding requests on a bus is:

HKEY_LOCAL_MACHINE\ 
  System\ 
    CurrentControlSet\ 
      Services\ 
        DriverName\ 
          Parameters\ 
            DeviceX\ 
              NumberOfRequests
				

DriverName is the name of the miniport driver, such as AIC78xx, and X (from DeviceX) is the bus number assigned at initialization. This subkey is a REG_DWORD between 16 and 255. Use this adjustment sparingly because the per-request storage allocations consume nonpaged memory.

The Windows NT 4 Device Driver Kit (DDK) documentation stated that the number of outstanding requests (NumberOfRequests) could be between 16 and 512. However, for optimum performance, the setting should be between 16 and 255. Values above 255 will consume additional nonpaged memory and will not produce more concurrent requests than the 255 registry setting.

The Windows NT 4 Scsiport.sys driver uses a byte tag value to identify I/O requests sent to each SCSI miniport. Because the tag value is limited to a byte, the number of concurrent requests is also limited to a byte value.