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StartIo Routines in Higher-Level Drivers

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Any higher-level driver can have a StartIo routine. However, such a driver is unlikely to be interoperable with existing lower-level drivers and is likely to exhibit poor performance characteristics.

A StartIo routine in a higher-level driver has the following effects:

  • Incoming IRPs can be queued by calling IoStartPacket from the driver's DispatchXxx routine(s) and IoStartNextPacket from its IoCompletion routine(s), thereby causing IRPs to be processed one at a time through the StartIo routine.

  • The driver's I/O throughput could become noticeably slower during periods of heavy I/O demand, because its StartIo routine can become a bottleneck.

  • The driver's StartIo routine calls IoCallDriver with each IRP at IRQL = DISPATCH_LEVEL, thereby causing all lower-level drivers' dispatch routines also to run at IRQL = DISPATCH_LEVEL. This restricts the set of support routines that lower drivers can call in their dispatch routines. Because most driver writers assume their drivers' dispatch routines run at IRQL < DISPATCH_LEVEL, the higher-level driver is unlikely to be interoperable with many existing lower-level drivers.

  • The StartIo routine reduces overall system throughput because it and the dispatch routines of all lower-level drivers in its chain are run at IRQL = DISPATCH_LEVEL.

    For more information about the IRQLs at which standard driver routines are run, see Managing Hardware Priorities.

None of the system-supplied higher-level drivers has a StartIo routine, because it can slow IRP processing for the driver itself, for all drivers above and below it, and for the system overall.

Most higher-level drivers simply send IRPs to lower-level drivers from their dispatch routines and do any necessary clean-up processing in their IoCompletion routines.

However, higher-level drivers can set up internal queues for IRPs that request particular kinds of operations, or set up internal queues to hold IRPs bound for a set of heterogeneous underlying devices like the SCSI port driver. For more information, see Queuing and Dequeuing IRPs.