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Difference between ScsiPort and Storport driver

On 12 November 2004 Microsoft released a new Storport driver, but what exactly is the difference between the SCSIPort and Storport drivers ?

In the Microsoft Windows operating system, the ScsiPort driver, in conjunction with vendor-written adapter-specific miniport drivers, was for many years, the only driver delivering SCSI commands to the storage targets. The SCSIport driver, however, was designed to work optimally with the parallel SCSI interconnects used with direct attached storage. It was neither designed to meet the high performance standards of Fibre Channel SAN configurations, nor to work well with hardware RAID.

In addition to the ScsiPort driver, Microsoft Windows Server 2003 and later versions provide Storport (storport.sys),
a storage port driver that is especially suitable for use with high-performance buses, such as fibre channel buses, and RAID adapters.

The difference is the mini-port driver is layered between the NT SCSI port driver and the adapter. Its limitations come from the NT SCSI Port driver (ex. number of LUN's per target).

There are several advantages to using Storport rather than the ScsiPort driver:

• Improved performance, both in terms of throughput and the system resources that are utilized. The Storport driver should also perform on a par with, or better than, a monolithic port driver.
  
• Improved miniport interface that addresses the needs of high-end storage vendors, particularly host-based RAID and fibre channel vendors.

All vendors are encouraged to use Storport where possible, rather than the ScsiPort driver.
Certain restrictions apply, however. Storport cannot be used with adapters or devices that do not support Plug and Play.

Classic SCSI (SCSIPort):
- commands were queued on an adapter/LUN basis
- Maximum number of I/O's per adapter is 256, with 16 LUN's per adapater
- Queue limit on LUN level, with a maximum of 20 outstanding I/O's
- If one LUN reaches it's limits all other LUN's will be blocked as well
- Will remain for direct attached storage

Storport:
- No limitation on adapter queues
- Each LUN has a queue limit of 256 outstanding I/O's
- Designed for Fibre Channel attached storage
- Exchange and SQL will probably not be using Storport directly
- Except for SQL RAW disk functionality which will bypass volume management (for higher performance)

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