What Are Dynamic Disks and Volumes?
Updated: March 28, 2003
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
What Are Dynamic Disks and Volumes?
In this section
Like basic disks, which are the most commonly used storage type found on computers running Microsoft Windows, dynamic disks can use the master boot record (MBR) or GUID partition table (GPT) partitioning scheme. All volumes on dynamic disks are known as dynamic volumes. Dynamic disks were first introduced with Windows 2000 and provide features that basic disks do not, such as the ability to create volumes that span multiple disks (spanned and striped volumes), and the ability to create fault tolerant volumes (mirrored and RAID-5 volumes).
Dynamic disks offer greater flexibility for volume management because they use a database to track information about dynamic volumes on the disk and about other dynamic disks in the computer. Because each dynamic disk in a computer stores a replica of the dynamic disk database, Windows Server 2003 can repair a corrupted database on one dynamic disk by using the database on another dynamic disk.
The location of the database is determined by the partition style of the disk.
On MBR disks, the database is contained in the last 1 megabyte (MB) of the disk.
On GPT disks, the database is contained in a 1-MB reserved (hidden) partition known as the Logical Disk Manager (LDM) Metadata partition.
All online dynamic disks in a computer must be members of the same disk group, which is a collection of dynamic disks. A computer can have only one dynamic disk group, also called the primary disk group. Each disk in a disk group stores a replica of the same dynamic disk database. A disk group uses a name consisting of the computer name plus a suffix of Dg0. The disk group name is stored in the registry.
When you move dynamic disks to a computer that has existing dynamic disks, you must import the dynamic disks to merge the databases on the moved disks with the databases on the existing dynamic disks.
The disk group name on a computer never changes, as long as the disk group contains dynamic disks. If you remove the last disk in the disk group or convert all dynamic disks to basic, the registry entry remains. However, if you create a dynamic disk again on that computer, a new disk group name is generated. The computer name in the disk group remains the same, but the suffix is Dg1 instead of Dg0.
When you move a dynamic disk to a computer that has no dynamic disks, the dynamic disk retains its disk group name and ID from the original computer and uses them on the local computer.
For more information about converting basic disks to dynamic disks, including the limitations of dynamic disks, see “Converting a Basic Disk into a Dynamic Disk” in How Basic Disks and Volumes Work.
Types of Dynamic Volumes
A dynamic volume is a volume that is created on a dynamic disk. Dynamic volume types include simple, spanned, and striped volumes. Windows Server 2003 also supports mirrored and RAID-5 volumes, which are fault tolerant. Fault tolerance is the ability of computer hardware or software to make sure that your data is still available, even if there is a hardware failure.
Regardless of the partition style used (MBR or GPT), you can create up to 1000 dynamic volumes per disk group, although boot time increases as the number of volumes increases. The recommended number of dynamic volumes is 32 or fewer per disk group.
The following sections describe the different types of dynamic volumes.
Simple volumes are the dynamic-disk equivalent of the primary partitions and logical drives found on basic disks. When creating simple volumes, keep these points in mind:
If you have only one dynamic disk, you can create only simple volumes.
You can increase the size of a simple volume to include unallocated space on the same disk or on a different disk. The volume must be unformatted or formatted by using NTFS. You can increase the size of a simple volume in two ways:
By extending the simple volume on the same disk. The volume remains a simple volume, and you can still mirror it.
By extending a simple volume to include unallocated space on other disks on the same computer. This creates a spanned volume.
- By extending the simple volume on the same disk. The volume remains a simple volume, and you can still mirror it.
If the simple volume is the system volume or the boot volume, you cannot extend it.
Spanned volumes combine areas of unallocated space from multiple disks into one logical volume. The areas of unallocated space can be different sizes. Spanned volumes require two disks, and you can use up to 32 disks. When creating spanned volumes, keep these points in mind:
You can extend only NTFS volumes or unformatted volumes.
After you create or extend a spanned volume, you cannot delete any portion of it without deleting the entire spanned volume.
You cannot stripe or mirror spanned volumes. For more information about striped or mirrored volumes, see “Striped Volumes” or “Mirrored Volumes” later in this section.
Spanned volumes do not provide fault tolerance. If one of the disks containing a spanned volume fails, the entire volume fails, and all data on the spanned volume becomes inaccessible. The reliability for a spanned volume is less than the least reliable disk in the set.
Striped volumes improve disk input/output (I/O) performance by distributing I/O requests across disks. Striped volumes are composed of stripes of data of equal size written across each disk in the volume. They are created from equally sized, unallocated areas on two or more disks. In Windows Server 2003, the size of each stripe is 64 kilobytes (KB) and cannot be changed.
Striped volumes cannot be extended or mirrored and do not offer fault tolerance. If one of the disks containing a striped volume fails, the entire volume fails, and all data on the striped volume becomes inaccessible. The reliability for the striped volume is less than the least reliable disk in the set.
A mirrored volume is a fault-tolerant volume that provides a copy of a volume on another disk. Mirrored volumes provide data redundancy by duplicating the information contained on the volume. The two disks that make up a mirrored volume are known as mirrors. Each mirror is always located on a different disk. If one of the disks fails, the data on the failed disk becomes unavailable, but the system continues to operate by using the unaffected disk.
Mirrored volumes are available only on computers running the Windows 2000 Server family or Windows Server 2003.
A RAID-5 volume is a fault-tolerant volume that stripes data and parity across three or more disks. Parity is a calculated value that is used to reconstruct data if one disk fails. When a disk fails, Windows Server 2003 continues to operate by recreating the data that was on the failed disk from the remaining data and parity. RAID-5 volumes are available only on computers running the Windows 2000 Server family or Windows Server 2003.
Dynamic Disk and Volume Scenarios
To gain the maximum benefit from dynamic disks and volumes, it is best to use them in computers with more than one disk, which allows you to scale the storage to match your needs. Dynamic disks and volumes are commonly used in the following scenarios.
Create a spanned volume to increase volume size
Spanned volumes are typically created by a user who has at least two disks in their computer. If the user doesn’t require the high read throughput of a striped volume or the fault tolerance offered by mirrored or RAID-5 volumes, the user can create a spanned volume.
If the data volume on a user’s primary disk gets too full, the user can extend that volume onto a second disk in their computer. This enables the user to create a big volume that uses space on two disks. If necessary, they can extend the volume to cover up to 32 disks. If the user has some space left on their primary disk after performing configuration changes, they can use that space and combine it with space on the second disk, creating a spanned volume.
Create a spanned volume to combine two or three small disks into a large volume
If a user has two or more small disks in their computer, they can combine those disks into a single large volume by creating a spanned volume. To the user, the space spread across the disks would look and function like a single volume.
Create a striped volume to accommodate high read/write throughput
Striped volumes are typically created by the user who has at least two disks in their computer. If the user requires high read/write throughput but does not require the fault-tolerance offered by mirrored or RAID-5 volumes, the user can create a striped volume.
Create a RAID-5 volume to protect critical data
RAID-5 volumes are typically created by the user who requires fault-tolerance and who has at least three disks in their computer. If one of the disks in the RAID-5 volume fails, the data on the remaining disks, along with the parity information, can be used to recover the lost data. RAID-5 volumes are well-suited to storing data that will need to be read frequently but written to less frequently. Database applications that read randomly work well with the built-in load balancing of a RAID-5 volume.
Create a mirrored volume to protect critical data
Mirrored volumes are typically created by the user who requires fault-tolerance and who has two disks in their computer. If one disk fails, the user always has a copy of their data on the second disk. Mirrored volumes provide better write performance than RAID-5 volumes.
Create a mirror to migrate data to a larger disk
If a user has run out of room on the simple volume where they store data and there is no room left on the disk to extend the volume, they can move this data to a larger disk instead of creating a spanned volume. The user can create a mirrored volume using the simple volume containing their data and a larger disk. After creating the mirrored volume, the user can break the mirrored volume and extend the new volume to fill the larger disk, leaving a complete copy of the original volume with available space for new data. The space on the original volume can be reclaimed for other uses.