Windows® Internals, Sixth Edition, Part 2

Because volumes are logical conveniences that Windows uses to represent contiguous areas on one or more physical disks, the volume manager must translate offsets that are relative to a volume to offsets that are relative to the beginning of a disk. If volume 2 consists of one partition that begins 4,096 sectors into the disk, the partition manager would adjust the IRP’s parameters to designate an offset with that value before passing the request to the disk class driver. The disk class driver uses a miniport driver to carry out physical disk I/O, and reads the requested data into an application buffer designated in the IRP.

Some examples of a volume manager’s operations will help clarify its role when it handles requests aimed at multipartition volumes. If a striped volume consists of two partitions, partition 1 and partition 2, the VolMgr device object intercepts file system disk I/O aimed at the device object for the volume, and the VolMgr driver adjusts the request before passing it to the disk class driver. The adjustment that VolMgr makes configures the request to refer to the correct offset of the request’s target stripe on either partition 1 or partition 2. If the I/O spans both partitions of the volume, VolMgr must issue two associated I/O requests, one aimed at each disk. This is shown in Figure 9-17.

In the case of writes to a mirrored volume, VolMgr splits each request so that each half of the mirror receives the write operation. For mirrored reads, VolMgr performs a read from half of a mirror, relying on the other half when a read operation fails.

Figure 9-17. VolMgr I/O operations

Virtual Disk Service

A company that makes storage products such as RAID adapters, hard disks, or storage arrays has to implement custom applications for installing and managing their devices. The use of different management applications for different storage devices has obvious drawbacks from the perspective of system administration. These drawbacks include learning multiple interfaces and the inability to use standard Windows storage management tools to manage third-party storage devices.

Windows includes the Virtual Disk Service (or VDS, located at %SystemRoot%\System32\Vds.exe), which provides a unified high-level storage interface so that administrators can manage storage devices from different vendors using the same user interfaces. VDS is shown in Figure 9-18. VDS exports a COM-based API that allows applications to create and format disks and to view and manage hardware RAID adapters. For example, a utility can use the VDS API to query the list of physical disks that map to a RAID logical unit number (LUN). Windows disk-management utilities, including the Disk Management MMC snap-in and the DiskPart and DiskRAID command-line tools, use VDS APIs.

Figure 9-18. VDS service architecture

VDS supplies two interfaces, one for software providers and one for hardware providers:

Software providers implement interfaces to high-level storage abstractions such as disks, disk partitions, and volumes. Examples of operations supported by these interfaces include creating, extending, and deleting volumes; adding or breaking mirrors; and formatting and assigning drive letters. VDS looks for registered software providers in HKLM\SYSTEM\CurrentControlSet\Services\Vds\SoftwareProviders, which contains subkeys whose names are GUIDs. Within each subkey is a value named ClsId, which specifies the COM class ID, and these are listed in HKEY_CLASSES_ROOT\CLSID\. Windows includes the VDS Dynamic Provider (%SystemRoot%\System32\Vdsdyn.dll) for interfacing to dynamic disks and the VDS Basic Provider (%SystemRoot%\System32\Vdsbas.dll) for interfacing to basic disks.

Hardware vendors implement VDS hardware providers as DLLs that register under HKLM\SYSTEM\CurrentControlSet\Services\Vds\HardwareProviders and that translate device-independent VDS commands into commands for their hardware. The hardware provider allows for management of a storage subsystem such as a hardware RAID array or an adapter card, and supported operations include creating, extending, deleting, masking, and unmasking LUNs.

When an application initiates a connection to the VDS API and the VDS service isn’t started, the Svchost process hosting the RPC service starts the VDS loader process (%SystemRoot%\System32\Vdsldr.exe), which starts the VDS service process and then exits. When the last connection to the VDS API closes, the VDS service process exits.

Virtual Hard Disk Support