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Q240314: INFO: Concurrent I/O Requests Number Setting Restrictions

Article: Q240314
Product(s): Microsoft Windows NT
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbhw kbDDK kbOSWinNT400 kbStorageDev kbDSupport kbGrpDSNTDDK kbHardware
Last Modified: 08-AUG-2001

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The information in this article applies to:

- Microsoft Windows NT Workstation version 4.0 
- Microsoft Windows NT Server version 4.0 
- Microsoft Windows NT 4.0 Driver Development Kit (DDK) 
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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.

Additional query words: SCSI miniport

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Keywords          : kbhw kbDDK kbOSWinNT400 kbStorageDev kbDSupport kbGrpDSNTDDK kbHardware 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbAudDeveloper kbWinDDKSearch kbWinNT400DDK
Version           : winnt:4.0
Issue type        : kbinfo

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