Sunday, January 2, 2011

Windows Server 2008 R2 Partition or Volume

When referring to Windows disks, administrators might consider partitions and volumes interchangeable. In fact, even though the graphical user interface makes no clear distinction and might refer to everything as a volume, volumes only exist on dynamic disks and partitions only exist on basic disks. This is especially important when managing disks using the diskpart.exe command-line utility, which defines a clear delineation between partitions and volumes.

Mount Point
When a new volume is created in Windows, it can be assigned a drive letter or mounted into an existing empty folder on an existing volume. When a volume is mounted into folder, this is known as a mount point or junction point. Mount points can be very useful in situations where administrators want to simplify disk access for end users, but must also make use of a number of small disks versus a single large disk. For example, on a database server with three disks, an administrator might assign disk1 the D drive, disk2 would be mounted in d:\data, and disk3 would be mounted in d:\logfiles. Any administrator would only need to connect to the D drive to access the databases or log files. One thing that administrators must test before using mount points is to see that all clients, applications, and backup agents support the use of mount or junction points and can successfully access and back up data stored within them. With many backup applications, enabling a backup job to back up data stored on a mounted volume is not the default and can cause major problems if the correct backup configuration is not selected before a failure occurs.

Simple Volumes
A simple volume is a single partition created on a single basic or dynamic disk. On a basic disk, simple volumes can be extended to include free, unallocated space that exists in a sequential section of the disk. To extend a simple volume to a noncontiguous, unallocated space on the same disk or a different disk, the disk will need to be upgraded to a dynamic disk.

Spanned Volumes
A spanned volume is treated as a single drive, but the volume spans two or more disks or different noncontiguous areas of the same disk. Spanned volumes provide no disk fault tolerance but can be used to meet disk storage needs that exceed the capacity of a single disk or volume. Spanned volumes are slowest when it comes to reading and writing data and are recommended only when the space of more than a single disk is necessary or an existing simple volume needs to be extended to add disk space and there is no available, unallocated space located next to the volume. For instance, if an application, file share, or service is dependent on the drive letter, does not support the moving of data or system files to another drive, and the current drive is nearly full, a simple volume can be upgraded to a spanned volume and extended with unallocated space on the same or another disk to add additional disk space. A simple volume that has been extended with unallocated space on the same disk is still considered a simple volume. If the simple volume is extended to a different disk, it is automatically converted to a spanned volume. The allocated space on each of the disks can be different sizes, and there is no space lost when creating a spanned volume. One thing to keep in mind, though, is that a spanned volume can never be reverted to a simple volume.

Striped Volumes
A striped volume or RAID-0 compatible volume requires two or more Windows dynamic disks and provides the fastest of all disk configurations. Striped volumes read and write data from each of the disks simultaneously, which improves disk access time. Striped volumes utilize all the space allocated for data storage but provide no disk fault tolerance. If one of the disks should fail, the entire data set would become inaccessible. Stripe sets require the exact amount of disk space on each of the allocated disks. For example, to create a 15GB stripe set array with three disks, 5GB of unallocated space would be required on each disk.

Fault-Tolerant Volumes
When fault-tolerant disk arrays managed by hardware controllers are not available, faulttolerant volumes can be created using multiple Windows dynamic disks. Fault-tolerant volumes in Windows are able to maintain data availability in the event of a single disk failure. Windows Server 2008 R2 supports two types of fault-tolerant volumes, including mirrored volumes and RAID-5 volumes.

Mirrored Volumes
Mirrored or RAID-1 compatible volumes require two separate disks to create. Furthermore, the size of the volume must be equal and available in one contiguous, unallocated section of each of the disks. Mirrored volumes duplicate data across each disk and can withstand the failure of a single disk. Because the mirrored volume is an exact replica of the first disk, the total space capacity is the capacity of one disk.

RAID-5 Volumes
Software-based RAID-5 volumes require three or more Windows dynamic disks and can provide faster disk read access than a single disk because all disks in the set can be read at the same time. Write performance can be slower than a single disk because of the parity stripe that must be generated and written. The space allocated to the RAID-5 volume on each disk in the volume must be equal and contiguous unallocated space. For example, to create a RAID-5 volume that requires 100GB on each disk, a disk with two separate areas of 50GB of unallocated space cannot be used to participate in the volume.

RAID-5 sets can withstand the failure of a single disk in the volume. During a disk failure, the remaining disks in the volume will continue to provide access to data but at a slower or degraded rate. This capability is achieved by reserving a small portion of each disk’s allocated space to store data parity information that can be used to rebuild a failed disk and to continue to provide data access. This is called a parity stripe. RAID-5 parity information requires the total space of a single disk in the array. For example, if five 10GB dynamic disks are used to create a single RAID-5 volume, 40GB would be available for data storage. The reserved 10GB would be spread evenly across all five disks. The formula for usable capacity of a RAID-5 array is (N - 1) * S, where N is the total number of drives in the array and S is the capacity of the smallest drive in the array.

Source of Information :  Sams - Windows Server 2008 R2 Unleashed

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