An Overview of RAID 5
RAID 5 consists of three or more volumes each located on a separate physical disk. As with RAID 0, RAID 5 also uses disk striping, whereby blocks of data are divided up into stripes with each stripe written to a different disk. RAID 5, however, differs quite considerably from RAID 0. Under RAID 5, not only are the data stripes written, but also parity information relating to the data. The key to RAID 5
fault tolerance is the fact that the parity information for a particular data stripe is always written to a different drive from the drive containing the corresponding data stripe. This means that if a disk fails, the corresponding parity information stored on another disk can be used for error detection and data correction (also referred to as
regeneration).
Whilst RAID 5 has considerable advantages over RAID 0 there are one or two drawbacks that should be taken into consideration when considering this storage option. Firstly, there is the inevitable performance overhead inherent in calculating and storing parity information for each data stripe written to disk. Secondly, the loss of more than one disk in a RAID 5 array will leave insufficient parity data on the remaining disks to regenerate the original data. That said, RAID 5 does provide considerable advantages that generally outweigh the disadvantages.
Configuring RAID 5 Using Windows Server 2008 Disk Management
A Windows Server 2008 RAID 5 configuration may be set up using the
Disk Management snap-in. This is accessed either from the Server Manager or
Computer Management tools. To launch the Server Manager, open the Start menu and click on the Server Manager option, or click on the Server Manager icon in the task bar. Alternatively launch Computer Management from Start -> All Programs -> Administration Tools -> Computer Management or run
compmgmt.csc at the command prompt or in a Run dialog. In all cases the Disk Management tool can be found under the
Storage category.
As previously noted, RAID 5 implementation requires a minimum of 3
disk drives. For the purposes of this tutorial a system containing four disk drives is assumed. In this scenario, disk 0 is the system disk and disks 1 through 3 are available for use in the RAID 5 configuration. Before proceeding the disks will need to be initialized using either the MBR or GPT partition style. Assuming these prerequisites are met the first step is to right click on one of the 3 disks in the Disk Management graphical view. In the resulting popup menu select the
New RAID-5 Volume... option to invoke the
New RAID-5 Volume wizard. On the wizards welcome page click on the
Next button to proceed to the
Disk Selection screen. This screen contains a list of disk drives available for inclusion in the
disk array together with a list of selected disks. Currently only the current disk is included in the
Selected list. Two more disks must be added to the selected disks before the RAID 5 array can be built. Select disks from the
Available list and click on the
Add> button to add the disk to the selected list. Once sufficient disks (in this case disks 1, 2 and 3) are selected the
Next button will activate to allow the remainder of the configuration to be completed. Note that disk 0 (the system disk) is not included in the RAID 5 array:
With the disk selections completed, the
Next button proceeds to the
drive letter and mount point assignment screen. Once these settings are configured click
Next to proceed to the
Format Volume screen. Select the appropriate file system and compression options and click on
Next to proceed to the
Summary screen. Review the information displayed and click on
Finish to initiate the RAID 5 creation process. During this process the Disk Management graphical view will list the disks as
Formatting and then
Resynching. The amount of time these phases will take depends on the size of the volumes in question. Once the process is complete the status will change to
Healthy and the RAID 5 volume is ready for use.
Configuring RAID 5 from the Command Prompt using DiskPart
In addition to configuring RAID 5 from within Disk Management, the configuration may also be implemented from the command prompt using
DiskPart. DiskPart may be launched either from a command prompt or a Run dialog simply by typing
diskpart. Once invoked, DiskPart will display the
DiskPart> command prompt and is ready to receive commands.
The first step in the configuration process is to identify the disks attached to the system using the
list disk command:
DISKPART> list disk
Disk ### Status Size Free Dyn Gpt
-------- ---------- ------- ------- --- ---
Disk 0 Online 30 GB 15 GB *
Disk 1 Online 8 GB 8189 MB
Disk 2 Online 8 GB 8189 MB
Disk 3 Online 8 GB 8189 MB
For the purposes of this chapter disks 1, 2 and 3 will be used to create a RAID 5 configuration. Each of these
disks needs to be converted to
dynamic disks before the configuration can proceed. This is achieved by selecting each disk in turn and executing the
convert dynamic command:
DISKPART> select disk 1
Disk 1 is now the selected disk.
DISKPART> convert dynamic
DiskPart successfully converted the selected disk to dynamic format.
DISKPART> select disk 2
Disk 2 is now the selected disk.
DISKPART> convert dynamic
DiskPart successfully converted the selected disk to dynamic format.
DISKPART> select disk 3
Disk 3 is now the selected disk.
DISKPART> convert dynamic
DiskPart successfully converted the selected disk to dynamic format.
Once the disks have been converted to dynamic disks the next step is to create the RAID 5 volume using the
create volume raid command. This command also takes as a parameter the
disk= directive followed by a list of disks to be used in the array. The
size= directive may also be specified to declare the size of the volume. If this value is omitted the volume will be sized to match the smallest contiguous block of unallocated space on the designated disk drives:
DISKPART> create volume raid disk=1,2,3
DiskPart successfully created the volume.
Once the command has completed and displayed the
DISKPART> prompt the system will have begun the resynching process. This can take a considerable amount of time depending on the size of the volume. During this process, the status of the volume will be listed as
Rebuild when the
list volume command is executed:
DISKPART> list volume
Volume ### Ltr Label Fs Type Size Status Info
---------- --- ----------- ----- ---------- ------- --------- --------
* Volume 0 RAW RAID-5 16 GB Rebuild
Volume 1 C NTFS Simple 15 GB Healthy System
Volume 2 D DVD-ROM 0 B No Media
Once the resynch is complete the volume status will change to
Healthy at which point the volume may be formatted (the RAW type listed above indicates the volume has yet to be formatted). The volume may be formatted using the
format command as follows:
DISKPART> format fs=ntfs label="RAID 5 Vol"
100 percent completed
DiskPart successfully formatted the volume.
Once the volume has been formatted the RAID 5 configuration is ready for use.
Fixing RAID 5 Problems
In order to function, all the disks in a RAID 5 configuration must be online and healthy. If the RAID 5 set displays
Failed Redundancy and the volume is listed as
Offline,
Missing or
Online (Errors) there is a problem which needs to be resolved.
If the status is
Offline or
Missing, check that the disk is connected and powered up. If the problem is resolved select
Rescan Disks from the
Actions menu. Once the rescan is complete right click on the problematic drive and select
Reactivate. All being well the drive status will change to
Regenerating as the data is rebuilt using the parity information on the other drives in the array. If, once regeneration is complete, the status does not return to
Healthy, right click once again and select
Regenerate Parity.
If a disk in a RAID 5 configuration is listed as
Failed or
Online (Errors) repeat the above steps. If this fails the disk may be unrecoverable and will need to be replaced. To achieve this, right click on the volume on the failed disk and select
Remove Volume from the popup menu. Once the volume has been removed, right click on a suitable block of unallocated space on another dynamic disk which is not already part of the RAID 5 array, has sufficient space and has a matching partition style and select
Repair Volume. This will rebuild the RAID 5 configuration using the space on new disk drive together with the remaining healthy disks from the original RAID 5 configuration.
