Administering Volume Sets and RAID Arrays

Article
02/20/2014

Archived content. No warranty is made as to technical accuracy. Content may contain URLs that were valid when originally published, but now link to sites or pages that no longer exist.

By William R. Stanek

Archived content - No warranty is made as to technical accuracy. Content may contain URLs that were valid when originally published, but now link to sites or pages that no longer exist.

from Chapter 8, Windows NT Administrator's Pocket Consultant .

When you work with Microsoft Windows NT servers, you'll often need to perform advanced disk setup procedures, such as creating a volume set or setting up a RAID array.

With a volume set, you can create a volume that uses multiple partitions on multiple drives. Users can access this volume as if it were a single drive, regardless of how many drives the actual volume is spread over.
With RAID arrays, you can protect important business data and, sometimes, improve the performance of drives. Microsoft Windows NT supports three different levels of RAID: 0, 1, and 5.

Using Volume Sets

By using volume sets, you can create volumes that span several drives. To do this, you use free space on different drives to create what users see as a single volume. Files are stored on the volume set segment by segment, with the first segment of free space being used first to store files. When this segment fills up, the second segment is used, and so on.

You can create a volume set using free space on up to 32 hard drives. The key advantage to volume sets is that they let you tap into unused free space and create a usable file system. The key disadvantage is that if any hard drive in the volume set fails, the volume set can no longer be used—which means that essentially all the data on the volume set is lost.

Note: Some operating systems, such as MS-DOS, do not support volume sets. If you dual boot your system to one of these noncompliant operating systems, your volume set will be unusable.

Creating a Volume Set

To create a volume set, you use Disk Administrator. The steps follow:

Select an area designated as Free Space. Then hold down the Ctrl key and select additional areas of free space. You must select at least two areas of free space.
Select Create Volume Set from the Partition menu.
You should see the Create Volume Set dialog box. This dialog box specifies the minimum and maximum size for the volume set in megabytes and lets you size the volume set within these constraints. Size the volume set using the Create Volume Set Of Total Size field.
When you're finished, click OK.
Choose Commit Changes Now from the Partition menu to create the volume set.
The volume set areas are designated as Unformatted. To format the volume set, select any area of the volume set, and then choose Format from the Tools menu. You format a volume set as you would a standard partition. For details, see the section titled "Formatting Partitions" in Chapter 7.

Deleting a Volume Set

You delete a volume set as you would a normal partition. Deleting a volume set removes the associated file system and all associated data is lost. Thus, before you delete a volume set you should back up any files and directories that the volume set contains.

To delete a volume set in Disk Administrator, follow these steps:

Select the volume set you want to delete by clicking on one of the areas assigned to the set.
Choose Delete from the Partition menu.
Confirm that you want to delete the partition by clicking Yes.
Commit the change by selecting Commit Changes Now from the Partition menu or by exiting the Disk Administrator and choosing Yes when prompted.

Extending a Volume Set or Logical Drive

Windows NT provides several different ways to extend NTFS volumes that are not part of a mirror set or stripe set. You can extend an individual volume—such as a logical drive—to create a volume set, and you can extend existing volume sets. When you extend volumes, you add free space to them.

Note: When extending volume sets, there are many things you can't do. You can't extend boot or system partitions. You can't extend the active partition on drive 0 or on any volumes that use mirroring or striping. Additionally, you can't extend FAT volumes. As you work with volume sets, please keep these exceptions in mind.

To extend an NTFS volume, complete the following steps:

Select the individual volume or volume set, and then hold down the Ctrl key and select an area of free space to add.

Figure 8-1: Extend volumes using the Extend Volume Set dialog box. The minimum size reflects the current size of the volume or volume set.
Select Extend Volume Set from the Partition menu.
You should see the Extend Volume Set dialog box shown in Figure 8-1. The minimum size reflects the current size of the volume or volume set. Set the new size for the volume set using the Create Volume Set Of Total Size field.
Click OK when you're finished.
You do not need to format the new set, but you do need to commit the changes. Commit the changes by selecting Commit Changes Now from the Partition menu or by exiting the Disk Administrator and choosing Yes when prompted.

Improved Performance and Fault Tolerance with RAIDs

You'll often want to give important data increased protection from drive failures. To do this, you can use RAID (redundant array of independent disks) technology to add fault tolerance to your file systems. With RAID you increase data integrity and availability by creating redundant copies of the data. You can also use RAID to improve the performance of your disks.

Different implementations of RAID technology are available. These implementations are described in terms of levels. Currently, RAID levels 0 to 5 are defined. Each RAID level offers different features. Windows NT supports RAID levels 0, 1, and 5.

RAID 0 can be used to improve the performance of your drives.
RAID 1 and 5 are used to provide fault tolerance for data.

Table 8-1, on the following page, provides a brief overview of the supported RAID levels. This support is completely software-based and is only available on Windows NT servers.

Table 8-1 Windows NT Server Support for RAID

RAID Level	RAID Type	Description	Major Advantages
0	Disk striping	Two or more partitions, each on a separate drive, are configured as a stripe set. Data is broken into blocks, called stripes, and then written sequentially to all drives in the stripe set.	Speed/Performance
1	Disk mirroring	Two partitions on two drives are configured identically. Data is written to both drives. If one drive fails, there is no data loss because the other drive contains the data. (Does not include disk striping.)	Redundancy. Better write performance than disk striping with parity.
5	Disk striping with parity	Uses three or more partitions, each on a separate drive, to create a stripe set with parity error checking. In the case of failure, data can be recovered.	Fault tolerance with less overhead than mirroring. Better read performance than disk mirroring.

The most common RAID levels in use on Windows NT servers are level 1 disk mirroring and level 5 disk striping with parity. Disk mirroring is the least expensive way to increase data protection with redundancy. Here, you use two identically sized partitions on two different drives to create a redundant data set. If one of the drives fails, the data can still be obtained from the other drive.

On the other hand, disk striping with parity requires more disks—a minimum of three—but offers fault tolerance with less overhead than disk mirroring. If any of the drives fail, the data can be recovered by combining blocks of data on the remaining disks with a parity record. Parity is a method of error checking that uses an exclusive OR operation to create a checksum for each block of data written to the disk. This checksum is used to recover data in case of failure.

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