Windows® Internals, Sixth Edition, Part 2

The following types of multipartition volumes are available in Windows:

Spanned volumes

Mirrored volumes

Striped volumes

RAID-5 volumes

After describing multipartition-volume partition configuration and logical operation for each of the multipartition-volume types, we’ll cover the way that the VolMgr driver handles IRPs that a file system driver sends to multipartition volumes. The term volume manager is used to represent VolMgr and the VolMgrX extension DLL throughout the explanation of multipartition volumes.

Spanned Volumes

A spanned volume is a single logical volume composed of a maximum of 32 free partitions on one or more disks. The Disk Management MMC snap-in combines the partitions into a spanned volume, which can then be formatted for any of the Windows-supported file systems. Figure 9-10 shows a 100-GB spanned volume identified by drive letter D that has been created from the last third of the first disk and the first third of the second. Spanned volumes were called volume sets in Windows NT 4.

Figure 9-10. Spanned volume

A spanned volume is useful for consolidating small areas of free disk space into one larger volume or for creating a single large volume out of two or more small disks. If the spanned volume has been formatted for NTFS, it can be extended to include additional free areas or additional disks without affecting the data already stored on the volume. This extensibility is one of the biggest benefits of describing all data on an NTFS volume as a file. NTFS can dynamically increase the size of a logical volume because the bitmap that records the allocation status of the volume is just another file—the bitmap file. The bitmap file can be extended to include any space added to the volume. Dynamically extending a FAT volume, on the other hand, would require the FAT itself to be extended, which would dislocate everything else on the disk.

A volume manager hides the physical configuration of disks from the file systems installed on Windows. NTFS, for example, views volume D: in Figure 9-10 as an ordinary 100-GB volume. NTFS consults its bitmap to determine what space in the volume is free for allocation. After translating a byte offset to a cluster offset, it then calls the volume manager to read or write data beginning at a particular cluster offset on the volume. The volume manager views the physical sectors in the spanned volume as numbered sequentially from the first free area on the first disk to the last free area on the last disk. It determines which physical sector on which disk corresponds to the supplied cluster offset.

Striped Volumes

A striped volume is a series of up to 32 partitions, one partition per disk, that gets combined into a single logical volume. Striped volumes are also known as RAID level 0 (RAID-0) volumes. Figure 9-11 shows a striped volume consisting of three partitions, one on each of three disks. (A partition in a striped volume need not span an entire disk; the only restriction is that the partitions on each disk be the same size.)

Figure 9-11. Striped volume

To a file system, this striped volume appears to be a single 450-GB volume, but the volume manager optimizes data storage and retrieval times on the striped volume by distributing the volume’s data among the physical disks. The volume manager accesses the physical sectors of the disks as if they were numbered sequentially in stripes across the disks, as illustrated in Figure 9-12.

Figure 9-12. Logical numbering of physical sectors on a striped volume

Because each stripe unit is a relatively narrow 64 KB (a value chosen to prevent small individual reads and writes from accessing two disks), the data tends to be distributed evenly among the disks. Striping thus increases the probability that multiple pending read and write operations will be bound for different disks. And because data on all three disks can be accessed simultaneously, latency time for disk I/O is often reduced, particularly on heavily loaded systems.

Spanned volumes make managing disk volumes more convenient, and striped volumes spread the I/O load over multiple disks. These two volume-management features don’t provide the ability to recover data if a disk fails, however. For data recovery, the volume manager implements two redundant storage schemes: mirrored volumes and RAID-5 volumes. These features are created with the Windows Disk Management administrative tool.

Mirrored Volumes

In a mirrored volume, the contents of a partition on one disk are duplicated in an equal-sized partition on another disk. Mirrored volumes are sometimes referred to as RAID level 1 (RAID-1). A mirrored volume is shown in Figure 9-13.

Figure 9-13. Mirrored volume