Windows® Internals, Sixth Edition, Part 2

Three more upcoming chapters will cover additional topics related to the I/O system: storage management, file systems (including details on the NTFS file system), and the cache manager.

Chapter 9. Storage Management

Storage management defines the way that an operating system interfaces with nonvolatile storage devices and media. The term storage encompasses many different devices, including optical media, USB flash drives, floppy disks, hard disks, solid state disks (SSDs), network storage such as iSCSI, storage area networks (SANs), and virtual storage such as VHDs (virtual hard disks). Windows provides specialized support for each of these classes of storage media. Because our focus in this book is on the kernel components of Windows, in this chapter we’ll concentrate on just the fundamentals of the hard disk storage subsystem in Windows, which includes support for external disks and flash drives. Significant portions of the support Windows provides for removable media and remote storage (offline archiving) are implemented in user mode.

In this chapter, we’ll examine how kernel-mode device drivers interface file system drivers to disk media, discuss how disks are partitioned, describe the way volume managers abstract and manage volumes, and present the implementation of multipartition disk-management features in Windows, including replicating and dividing file system data across physical disks for reliability and for performance enhancement. We’ll also describe how file system drivers mount volumes they are responsible for managing, and we’ll conclude by discussing drive encryption technology in Windows and support for automatic backups and recovery.

Storage Terminology

To fully understand the rest of this chapter, you need to be familiar with some basic terminology:

Disks are physical storage devices such as a hard disk, CD-ROM, DVD, Blu-ray, solid state disk (SSD), or flash.

A disk is divided into sectors, which are addressable blocks of fixed size. Sector sizes are determined by hardware. Most hard disk sectors are 512 bytes (but are moving to 4,096 bytes), and CD-ROM sectors are typically 2,048 bytes. For more information on moving to 4,096-byte sectors, see http://support.microsoft.com/kb/2510009.

Partitions are collections of contiguous sectors on a disk. A partition table or other disk-management database stores a partition’s starting sector, size, and other characteristics and is located on the same disk as the partition.

Simple volumes are objects that represent sectors from a single partition that file system drivers manage as a single unit.

Multipartition volumes are objects that represent sectors from multiple partitions and that file system drivers manage as a single unit. Multipartition volumes offer performance, reliability, and sizing features that simple volumes do not.

Disk Devices

From the perspective of Windows, a disk is a device that provides addressable long-term storage for blocks of data, which are accessed using file system drivers. In other words, each byte on the disk does not have its own address, but each block does have an address. These blocks are known as sectors and are the basic unit of storage and transfer to and from the device (in other words, all transfers must be a multiple of the sector size). Whether the device is implemented using rotating magnetic media (hard disk or floppy disk) or solid state memory (flash disk or thumb drive) is irrelevant.

Windows supports a wide variety of interconnect mechanisms for attaching a disk to a system, including SCSI, SAS (Serial Attached SCSI), SATA (Serial Advanced Technology Attachment), USB, SD/MMC, and iSCSI.

Rotating Magnetic Disks

The typical disk drive (often referred to as a hard disk) is built using one or more rigid rotating platters covered in a magnetic material. An arm containing a head moves back and forth across the surface of the platter reading and writing bits that are stored magnetically.

Disk Sector Format

While the disk interconnect mechanisms have been evolving since IBM introduced hard disks in 1956 and have become faster and more intelligent, the underlying disk format has changed very little, except for annual increases in areal density (the number of bits per square inch). Since the inception of disk drives, the data portion of a disk sector has typically been 512 bytes.