Windows XP File and Storage Systems

The following sections describe this new dynamic storage model, its benefits, and its drawbacks. You'll also learn about the features and advantages of NTFS v.5.

Windows XP uses a new concept in storage models (introduced by Windows 2000), which Microsoft terms dynamic storage. Although Windows XP still supports the old, familiar models of partitions and drives, upgrading your hard disk to this new storage type brings with it a new way of thinking about partitions and how they are organized.

CAUTION

Converting a hard disk to dynamic storage is a one-way process. To change a dynamic disk back to a basic disk, all volumes must be deleted before converting the drive back to basic storage. Also, note that dynamic disks can be read by Windows XP Pro, Windows Server 2003, and all version of Windows 2000. Windows XP Home, Windows NT, Windows 98/SE/Me, and all earlier versions of Windows cannot access dynamic storage volumes. When you change the boot disk to dynamic, you can no longer multiboot into another operating system because the familiar boot loader screen disappears for good.

One last point of concern when dealing with Dynamic Disks is that they absolutely cannot be used on a multi-booting computer system. Only one installation of Windows can own a set of Dynamic Disks, so if you are planning on using Dynamic Disks as your RAID solution on a multi-boot computer, think about investing in a hardware-based (SCSI) RAID solution.

Dynamic Storage

I can best explain dynamic storage by comparing it to previous methods of structuring a hard disk.

The traditional storage model of disk structure used partition tables. Each hard drive could hold up to four primary partitions or up to three primary partitions and one extended (secondary) partition. Within this extended partition, you created logical drives. The total number of primary partitions and logical drives cannot exceed 32 per hard drive. This disk structure is understood and can be accessed by MS-DOS, Windows, and all versions of Windows NT up to release 4.0. The annoyances and limitations of this partition table methodology are artifacts of Microsoft operating systems, incidentally, not something imposed by hard disks themselves or their manufacturers. Some other operating systems don't suffer the same peculiarities.

NOTE

Storage types are separate from the file systems they contain. Both basic and dynamic disks can contain any combination of FAT16, FAT32, NTFS v4, or NTFS v5 partitions or volumes. All drives are either basic or dynamic.

With dynamic storage, the restraints of primary and extended partitions are gone. Under this storage model, free space on a hard drive is divided into volumes instead of partitions, and these volumes can be noncontiguous and span one or more disks. In addition, volumes on a dynamic disk can be configured as simple, spanned, mirrored, striped, or RAID-5. Basic storage partitions can only be configured as simple partitions unless they are remnants from a previous OS retained during an upgrade.

• A simple volume uses free space available on a single disk. This space can be a single contiguous region or multiple concatenated regions. Under the basic storage model, each partition or logical drive is assigned a separate and distinct drive letter and functions as a distinct region of disk space. Dynamic storage can be configured to see multiple regions of a disk as a single volume, accessed with a single assigned drive letter.

• A spanned volume takes the concept of a simple volume and extends it across multiple disks (up to a maximum of 32). All joined regions on these disks are seen as a single volume to programs accessing them. However, if a single unit in a spanned volume fails, the entire set is lost.

• A mirrored volume is a volume in which data from one disk is mirrored or duplicated on a second disk. This process provides for data redundancy, often called fault tolerance. If one disk fails, the data can be accessed from the second disk. A mirrored volume cannot be spanned; each volume must be contained on a single disk. Programs see only one volume, and Windows ensures that both disks are kept in sync. Mirroring is also known as RAID-1.

• A striped volume is a volume in which data is stored across two or more physical disks. When data is written to a striped volume space, it is allocated alternately and evenly to each of the physical disks. A striped volume cannot be mirrored or spanned via Windows XP (it is possible on hardware-based RAID). Striping, often termed RAID-0, is used to increase storage system throughput. If a single unit in a striped volume fails, the entire set is lost.

• A RAID-5 volume is a fault-tolerant version of a striped volume. When data is written to a RAID-5 volume, it is striped across an array of three or more disks, and a parity value is added. If a hard disk belonging to a RAID-5 volume fails, the data can be re-created by the remaining drives using this parity value. Note the difference here between a mirrored volume and a RAID-5 volume.

NOTE

A mirrored volume contains two disks; if either one fails, the operating system goes to the other for data access.

A RAID-5 volume contains three or more disks, any of which can fail without the system halting. The operating system then reconstructs the missing data from the information contained on the remaining disks.

So, what are the advantages of dynamic storage?

• First and foremost, noncontiguous regions of multiple disks can be linked so that they appear as one large region of disk space to any program. By linking them, you can increase the size of a disk volume on-the-fly, without reformatting or having to cope with multiple drive letters.

• Second, and perhaps more important from an administrator's point of view, is that disk and volume management can be performed without restarting the operating system.

The catch to all this is that on a multiboot system, non-Windows 2000, non-Windows XP Pro, or non-.NET Server systems cannot even see dynamic storage drives. Unlike NTFS which only applies to the formatted partition, dynamic storage affects the entire hard drive. Only Windows XP Pro, Windows Server 2003, and Windows 2000 can see drives configured as dynamic storage. So, if you plan on using dynamic storage, plan ahead and keep other OSes on different hard drives. Plus, you must ensure the boot drive is a basic storage drive as well so the boot menu will function.