Windows® Internals, Sixth Edition, Part 2

Both the Scsiport.sys and Ataport.sys drivers implement a version of the disk scheduling algorithm known as C-LOOK. The drivers place disk I/O requests in lists sorted by the first sector (also known as the logical block address, or LBA) at which an I/O request is directed. They use the KeInsertByKeyDeviceQueue and KeRemoveByKeyDeviceQueue functions (documented in the Windows Driver Kit) representing I/O requests as items and using a request’s starting sector as the key required by the functions. When servicing requests, the drivers proceed through the list from lowest sector to highest. When they reach the end of the list the drivers start back at the beginning, since new requests might have been inserted in the meantime. If disk requests are spread throughout a disk this approach results in the disk head continuously moving from near the outermost cylinders of the disk toward the innermost cylinders. Storport.sys does not implement disk scheduling because it is commonly used for managing I/Os directed at storage arrays where there is no clearly defined notion of a disk start and end.

Windows ships with several miniport drivers. On systems that have at least one ATAPI-based IDE device, %SystemRoot%\System32\Drivers\Atapi.sys, %SystemRoot%\System32\Drivers\Pciidex.sys, and %SystemRoot%\System32\Drivers\Pciide.sys together provide miniport functionality. Most Windows installations include one or more of the drivers mentioned.

iSCSI Drivers

The development of iSCSI as a disk transport protocol integrates the SCSI protocol with TCP/IP networking so that computers can communicate with block-storage devices, including disks, over IP networks. Storage area networking (SAN) is usually architected on Fibre Channel networking, but administrators can leverage iSCSI to create relatively inexpensive SANs from networking technology such as Gigabit Ethernet to provide scalability, disaster protection, efficient backup, and data protection. Windows support for iSCSI comes in the form of the Microsoft iSCSI Software Initiator, which is available on all editions of Windows.

The Microsoft iSCSI Software Initiator includes several components:

Initiator This optional component, which consists of the Storport port driver and the iSCSI miniport driver (%SystemRoot%\System32\Drivers\Msiscsi.sys), uses the TCP/IP driver to implement software iSCSI over standard Ethernet adapters and TCP/IP offloaded network adapters.

Initiator service This service, implemented in %SystemRoot%\System32\Iscsicli.exe, manages the discovery and security of all iSCSI initiators as well as session initiation and termination. iSCSI device discovery functionality is implemented in %SystemRoot%\System32\Iscsium.dll. An important goal of the iSCSI service is to provide a common discovery/management infrastructure irrespective of the protocol driver being used, which could be the Microsoft software initiator driver or an HBA driver (host bus adapter; iSCSI protocol handling offloaded to hardware, which is generally Storport miniports). In this context, iSCSI also provides Win32 and WMI interfaces for management and configuration. The iSCSI initiator service supports four discovery mechanisms:

iSNS (Internet Storage Name Service) The addresses of the iSNS servers that the iSCSI initiator service will use are statically configured using the iscsicli AddiSNSServer command.

SendTargets The SendTarget portals are statically configured using the iscsicli AddTargetPortal command.

Host Bus Adapter Discovery iSCSI HBAs that conform to the iSCSI initiator service interfaces can participate in target discovery by means of an interface between the HBA and the iSCSI initiator service.

Manually Configured Targets iSCSI targets can be manually configured using the iscsicli AddTarget command or with the iSCSI Control Panel applet.

Management applications These include Iscsicli.exe, a command-line tool for managing iSCSI device connections and security, and the corresponding Control Panel application.

Some vendors produce iSCSI adapters that offload the iSCSI protocol to hardware. The initiator service works with these adapters, which must support the iSNS protocol (RFC 4171), so that all iSCSI devices, including those discovered by the initiator service and those discovered by iSCSI hardware, are recognized and managed through standard Windows interfaces.

Multipath I/O (MPIO) Drivers