Windows® Internals, Sixth Edition, Part 2

As we’ve stated, dynamic disks are the disk format in Windows necessary for creating multipartition volumes such as mirrors, striped arrays, and RAID-5 arrays (described later in the chapter). Dynamic disks are partitioned using Logical Disk Manager (LDM) partitioning. LDM is part of the Virtual Disk Service (VDS) subsystem in Windows, which consists of user-mode and device driver components and oversees dynamic disks. A major difference between LDM’s partitioning and MBR-style and GPT partitioning is that LDM maintains one unified database that stores partitioning information for all the dynamic disks on a system—including multipartition-volume configuration.

The LDM Database

The LDM database resides in a 1-MB reserved space at the end of each dynamic disk. The need for this space is the reason Windows requires free space at the end of a basic disk before you can convert it to a dynamic disk. The LDM database consists of four regions, which Figure 9-7 shows: a header sector that LDM calls the Private Header, a table of contents area, a database records area, and a transactional log area. (The fifth region shown in Figure 9-7 is simply a copy of the Private Header.) The Private Header sector resides 1 MB before the end of a dynamic disk and anchors the database. As you spend time with Windows, you’ll quickly notice that it uses GUIDs to identify just about everything, and disks are no exception. A GUID (globally unique identifier) is a 128-bit value that various components in Windows use to uniquely identify objects. LDM assigns each dynamic disk a GUID, and the Private Header sector notes the GUID of the dynamic disk on which it resides—hence the Private Header’s designation as information that is private to the disk. The Private Header also stores the name of the disk group, which is the name of the computer concatenated with Dg0 (for example, Daryl-Dg0 if the computer’s name is Daryl), and a pointer to the beginning of the database table of contents. For reliability, LDM keeps a copy of the Private Header in the disk’s last sector.

The database table of contents is 16 sectors in size and contains information regarding the database’s layout. LDM begins the database record area immediately following the table of contents with a sector that serves as the database record header. This sector stores information about the database record area, including the number of records it contains, the name and GUID of the disk group the database relates to, and a sequence number identifier that LDM uses for the next entry it creates in the database. Sectors following the database record header contain 128-byte fixed-size records that store entries that describe the disk group’s partitions and volumes.

A database entry can be one of four types: partition, disk, component, and volume. LDM uses the database entry types to identify three levels that describe volumes. LDM connects entries with internal object identifiers. At the lowest level, partition entries describe soft partitions (hard partitions are described later in this chapter), which are contiguous regions on a disk; identifiers stored in a partition entry link the entry to a component and disk entry. A disk entry represents a dynamic disk that is part of the disk group and includes the disk’s GUID. A component entry serves as a connector between one or more partition entries and the volume entry each partition is associated with. A volume entry stores the GUID of the volume, the volume’s total size and state, and a drive-letter hint. Disk entries that are larger than a database record span multiple records; partition, component, and volume entries rarely span multiple records.

Figure 9-7. LDM database layout

LDM requires three entries to describe a simple volume: a partition, component, and volume entry. The following listing shows the contents of a simple LDM database that defines one 200-MB volume that consists of one partition:Disk Entry Volume Entry Component Entry Partition Entry Name: Disk1 Name: Volume1 Name: Volume1-01 Name: Disk1-01 GUID: XXX-XX... ID: 0x408 ID: 0x409 ID: 0x407 Disk ID: 0x404 State: ACTIVE Parent ID: 0x408 Parent ID: 0x409 Size: 200MB Disk ID: 0x404 GUID: XXX-XX... Start: 300MB Drive Hint: H: Size: 200MB