Windows® Internals, Sixth Edition, Part 2

When you install Windows on a computer, one of the first things it requires you to do is to create a partition on the system’s primary physical disk (specified in the BIOS or UEFI as the disk from which to boot the system). To make enabling BitLocker easier, Windows Setup will create a small (100 MB) unencrypted partition known as the system volume, containing the Boot Manager (Bootmgr), Boot Configuration Database (BCD), and other early boot files. (By default, this volume does not have a drive letter assigned to it, but you can assign one using the Disk Management MMC snap-in, at %SystemRoot%\System32\Diskmgmt.msc, if you want to examine the contents of the volume with Windows Explorer). In addition, Windows Setup requires you to create a partition that serves as the home for the boot volume, onto which the setup program installs the Windows system files and creates the system directory (\Windows). The nomenclature that Microsoft defines for system and boot volumes is somewhat confusing. The system volume is where Windows places boot files, such as the Boot Manager, and the boot volume is where Windows stores the rest of the operating system files, such as Ntoskrnl.exe, the core kernel file.

Note

If the system has BitLocker enabled, the boot volume will be encrypted, but the system volume is never encrypted.

Although the partitioning data of a dynamic disk resides in the LDM database, LDM implements MBR-style partitioning or GPT partitioning so that the Windows boot code can find the system and boot volumes when the volumes are on dynamic disks. (Winload and the Itanium firmware, for example, know nothing about LDM partitioning.) If a disk contains the system or boot volumes, partitions in the MBR or GPT describe the location of those volumes. Otherwise, one partition encompasses the entire usable area of the disk. LDM marks this partition as type “LDM”. The region encompassed by this place-holding MBR-style or GPT partition is where LDM creates partitions that the LDM database organizes. On MBR-partitioned disks the LDM database resides in hidden sectors at the end of the disk, and on GPT-partitioned disks there exists an LDM metadata partition that contains the LDM database near the beginning of the disk.

Another reason LDM creates an MBR or a GPT is so that legacy disk-management utilities, including those that run under Windows and under other operating systems in dual-boot environments, don’t mistakenly believe a dynamic disk is unpartitioned.

Because LDM partitions aren’t described in the MBR or GPT of a disk, they are called soft partitions; MBR-style and GPT partitions are called hard partitions. Figure 9-8 illustrates this dynamic disk layout on an MBR-style partitioned disk.

Figure 9-8. Internal dynamic disk organization

Dynamic Disk Volume Manager

The Disk Management MMC snap-in DLL (DMDiskManager, located in %SystemRoot%\System32\Dmdskmgr.dll), shown in Figure 9-9, is used to create and change the contents of the LDM database. When you launch the Disk Management MMC snap-in, DMDiskManager loads into memory and reads the LDM database from each disk and returns the information it obtains to the user. If it detects a database from another computer’s disk group, it notes that the volumes on the disk are foreign and lets you import them into the current computer’s database if you want to use them. As you change the configuration of dynamic disks, DMDiskManager updates its in-memory copy of the database. When DMDiskManager commits changes, it passes the updated database to the VolMgrX driver (%SystemRoot%\System32\Drivers\Volmgrx.sys). VolMgrX is a kernel-mode DLL that provides dynamic disk functionality for VolMgr, so it controls access to the on-disk database and creates device objects that represent the volumes on dynamic disks. When you exit Disk Management, DMDiskManager stops.

Figure 9-9. Disk Management MMC snap-in

Multipartition Volume Management

VolMgr is responsible for presenting volumes that file system drivers manage and for mapping I/O directed at volumes to the underlying partitions that they’re part of. For simple volumes, this process is straightforward: the volume manager ensures that volume-relative offsets are translated to disk-relative offsets by adding the volume-relative offset to the volume’s starting disk offset.

Multipartition volumes are more complex because the partitions that make up a volume can be located on discontiguous partitions or even on different disks. Some types of multipartition volumes use data redundancy, so they require more involved volume-to-disk–offset translation. Thus, VolMgr uses VolMgrX to process all I/O requests aimed at the multipartition volumes they manage by determining which partitions the I/O ultimately affects.