Inside Windows Storage: Server Storage Technologies for Windows 2000, Windows Server 2003 and Beyond

10.5 What's Missing?

Microsoft has acknowledged the need for several additional features without yet making any commitment to implement them. This section surveys those features and speculates a bit about their possible future.

10.5.1 SAN Boot

Windows 2000 can be booted from a SAN, but there are many caveats. Microsoft supports SAN boot in a limited scenario. The caveats include the following:

• All hardware and software used must be on the Windows 2000 hardware compatibility list.

• The scenario should not include an FC-AL (Fibre Channel arbitrated loop) environment, primarily because Windows NT kernel has not been modified at all and the delays in an FC-AL environment when the loop is reinitializing can cause kernel timeout errors.

• Windows 2000 needs to have exclusive access to the LUN from which the boot is happening. The required LUN masking must be achieved via switch and HBA management.

• The SAN vendor must be involved in the planning and deployment process and must be the first resource for support issues.

• Having the paging file on a local disk is highly beneficial.

• Most if not all solutions require multipath to be disabled for the duration of the boot process.

Having SAN boot support in the operating system would be highly beneficial. Some better support for latency issues might be one feature. With iSCSI on the horizon, the possibility of a remote SAN boot using a Gigabit Ethernet card is intriguing as well. Of course, having LUN-masking support in Windows NT early in the boot sequence would not be a bad thing to have as well.

10.5.2 Reducing the Layers in the Storage Stack

The Windows NT driver model is highly modular and layered with a view toward providing easy introduction of a new layer as needed. Perhaps the model has been rather too successful in meeting its goal. A typical Windows 2000 server could easily have ten or so drivers between the application and the physical wires. Consider the drivers in the stack:

• The Storport or SCSI miniport driver.

• The Storport or SCSIPort driver.

• The disk class driver.

• The Logical Disk Manager driver, which is really multiple drivers.

• The diskperf driver.

• A virus-checking filter driver.

• The file system driver.

• The file system filter drivers, of which there are more than one. Some examples include the Remote Storage reparse point filter driver, the Single Instance Storage reparse point filter driver, the Symbolic Link reparse point filter driver, and the encrypting file system filter driver.

It might help to see if the functionality of one driver can be subsumed into another driver. Windows XP introduces a limited callback mechanism for filter drivers, and it remains to be seen if this trend is expanded upon. But this is pure speculation for now.

10.5.3 Multipath I/O for iSCSI

The multipath I/O developer's kit does not cater to iSCSI solutions. As the iSCSI market gathers momentum, it remains to be seen what Microsoft does to provide a high-availability and high-performance solution for iSCSI.