Hard disk drive manufacturers will soon start producing hard disk drives for which the physical sector sizes are larger than the traditional 512 bytes per sector. For example, sectors may be 1 kilobyte (KB), 2 KB, or 4 KB. This change will enable manufacturers to improve the capacity, the performance, and the reliability of their hard disk drives. This article discusses Windows Vista support for large-sector hard disk drives.
Important Dynamic discs are only supported in Windows Vista Ultimate, in Windows Vista Business, and in Windows Vista Enterprise. Generally, dynamic discs are not supported in Windows Vista Home Basic or in Windows Vista Home Premium. However, when you upgrade your computer from Windows XP Media Center Edition to Windows Vista Home Premium, some dynamic discs are supported.
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Background
When the hard disk drive industry makes this shift in the underlying structure of their drives, two kinds of hard disk drives will appear on the market. The simpler kind of hard disk drive will use a large physical sector size internally and expose that same size to the system. This kind of hard disk drive will first appear in enterprise-class drives, such as SCSI drives or Fibre Channel drives. The main drawback of this kind of hard disk drive is backward compatibility. BIOSes, host adapters, operating systems, and business applications may have to be updated to correctly operate together with this kind of hard disk drive.
To reduce backward compatibility issues, some manufacturers will produce hard disk drives that use a large physical sector size internally, but expose only a logical sector size of 512 bytes to the system. These hard disk drives are referred to as emulation devices because of the method that the drives use to write data. This method is frequently called "read-modify-write." For writes that are smaller than a physical sector, the drive must read the physical sector, modify the small, changed part of the sector, and then write the whole physical sector. The main drawback of this kind of hard disk drive is decreased performance. The extra read operation that must occur for writes that are smaller than the physical sector may decrease performance.
Windows Vista support
Windows Vista supports both kinds of hard disk drives if the underlying hardware in the system also supports the drives.
| • | Startup support
Windows Vista will let you start from emulation drives that have a logical sector size of 512 bytes. BIOS vendors and other hardware vendors may have to update their firmware to correctly start together with drives that expose a large logical sector in addition to a large physical sector. Microsoft will test Windows Vista for startup support for these kinds of drives when the necessary hardware support is available.
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| • | API support for querying hard disk drive properties
To act appropriately with large-sector drives, some applications and other components may query for physical sector size, for logical sector size, and for sector alignment. Applications may have strict requirements for the sector size of a hard disk drive. The IOCTL_STORAGE_QUERY_PROPERTY request has been updated to include sector information in the STORAGE_ACCESS_ALIGNMENT_DESCRIPTOR request. For more information, visit the following Microsoft Web site: |
| • | Storage stack, partition, and file system alignment
The file system, the volume manager, and other parts of the storage stack in Windows Vista have been updated to accommodate hard disk drives that have a large sector size. In earlier versions of Windows, the default starting offset for the first partition on a hard disk drive was sector 0x3F. Because this starting offset was an odd number, it could cause performance issues on large-sector drives because of misalignment between the partition and the physical sectors. In Windows Vista, the default starting offset will generally be sector 0x800. However, the starting offset might be different for drives that have special alignments. For emulation drives that have special alignments, dynamic disk operations will not be supported in the initial release version of Windows Vista. This support will be added in a future service pack.
When the file system is formatted, only cluster sizes that are larger than or equal to the underlying physical sector size will be supported. |
Support for these new hard disk drives in Windows Vista does not mean that the drives are automatically supported by applications. Applications that care about the underlying structure of their data storage must be updated to reflect the new hard disk drives. Independent software vendors (ISVs) must update their applications.
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