Configuring Disk Mirroring (RAID 1) on Windows Server 2008
The focus of this chapter of Windows Server 2008 Essentials is the configuration of disk mirroring (RAID 1) on a Windows Server 2008 system. It is assumed in this chapter that the reader has a basic understanding of the difference between basic and dynamic disks within the context of Windows Server 2008. For information on this subject the chapter entitled Creating and Managing Simple and Spanned Volumes on Windows Server 2008 is recommended.
An Overview of Disk Mirroring (RAID 1)
Disk mirroring consists of two identical volumes on two different physical disk drives. Any data is written to both disk drives such that if one disk drive fails the data will still be available on the other disk. Unlike striping, which increases I/O performance, mirroring will actually reduce performance for the simple reason that all data write transactions have to be performed twice - once on each drive in the mirrored set.
Creating a Mirrored (RAID 1) Set using Disk Management
The remainder of this chapter assumes that the disks in question have been converted from basic disks to dynamic disks. For details on performing this task refer to the initial sections of the Creating and Managing Simple and Spanned Volumes on Windows Server 2008chapter of this book.
In this section a mirrored set will be created using two similarly sized disk drives connected to a Windows Server 2008 system (note that if the disks are of different sizes Windows will base the available volume size on the smallest disk drive in the striped set).
Begin the process by invoking the Disk Management snap-in. This can be achieved by typing compmgmt.msc at a command prompt window or Run dialog and selecting Storage -> Disk Management. The system used in this example contains three disks numbered 0, 1 and 2 respectively. Disk 0 is the system disk and disks 1 and 2 will be used to created a striped set.
Within the Disk Management tool right click on the unallocated space in the graphical view of the first disk to be used in the stripe set and select New Mirrored Volume... to invoke the New Striped Volume wizard. Click Next on the welcome screen to proceed to the disk selection screen. As mentioned previously, a mirrored set must be comprised of two disk drives. Initially, the wizard only lists the currently selected disk in the 'Selected column. In order to proceed, one more disk is required. To add a disk to the mirrored set, select a suitable disk from the available disks in the right hand column and click on Add to add it to the selected disks list:
With the necessary disks selected click Next to proceed and assign a drive letter or mount point for the volume. The Next button will then proceed to the Format Volume screen where the file system type and compression options may be selected. To view the summary screen, press the Next button. Assuming the summary is acceptable the Finish button will format the volumes and create the striped volume ready for use.
Creating a Mirrored (RAID 1) Volume from the Command Prompt
Windows Server 2008 Mirrored (RAID 1) volumes may also be created from the command prompt using the diskpart tool. This may be launched either from a command prompt window, or from the Run dialog simply by entering diskpart at the prompt. Once invoked the DISKPART> prompt will displayed, ready to recieve commands.
The first step in creating a mirrored set using diskpart is to identify the disks to be used. This can be achieved using the list disk command:
DISKPART> list disk Disk ### Status Size Free Dyn Gpt -------- ---------- ------- ------- --- --- Disk 0 Online 30 GB 0 B Disk 1 Online 8 GB 8189 MB Disk 2 Online 8 GB 8189 MB
For the purposes of this tutorial we will be creating a mirrored volume based on a set consisting of disks 1 and 2. The mirrored volume is created using the create volume command combined with the mirror and disk= directives. In the case of the disk= directive a comma separated list of disk numbers to be used in the striped set must be provided. The size of the volume may also be specified using the size= directive. Omission of the size= option will cause diskpart to create volume such that it is the size of the smallest disk in the striped set. For example, to create a striped volume comprising of disks 1 and 2:
DISKPART> create volume stripe disk=1,2 DiskPart successfully created the volume.
Having created the striped volume, the list volume command may be used to verify the new configuration:
DISKPART> list volume Volume ### Ltr Label Fs Type Size Status Info ---------- --- ----------- ----- ---------- ------- --------- -------- * Volume 0 RAW Stripe 16 GB Healthy Volume 1 D DVD-ROM 0 B No Media Volume 2 C NTFS Partition 30 GB Healthy System
As shown above the new volume is listed as volume 0 and shown as Stripe. The volume is also listed as RAW because it has not yet been formatted with a file system. This, too, can be achieved within the diskpart tool:
DISKPART> select volume 0 DISKPART> format fs=ntfs label="My Striped Vol" 100 percent completed DiskPart successfully formatted the volume.
Once formatted, the last task is to assign either a drive letter or mount point to the volume by which it will accessed:
To assign a drive letter:
DISKPART> assign letter=E: DiskPart successfully assigned the drive letter or mount point.
To assign a mount point:
DISKPART> assign mount=\bigvol DiskPart successfully assigned the drive letter or mount point.
Once assigned a drive letter or mount point, the new striped volume is ready for use.
Recovering a Striped Volume
When a disk in a striped set fails or goes off-line the volume will be listed as either Missing, Offline, Failed or Unreadable. If the disk is offline or missing the first thing to check is that all drives in the striped set are powered up and that the connectors are firmly in place. If a disk can be brought back on line the next step is to instruct Windows to rescan the disks so that striped volume can be brought back online. Do this by selecting Rescan disks from the Action menu. If the disk is listed as failed, unreadable or shows errors then the disk may have physical errors which suggest the disk needs to replaced and the data on the striped volume restored from the most recent back up.
