Topic “Understanding Disk Storage Options” introduced the types of storage units available on a Windows Server 2003 computer. The types of volumes that reflect RAID configurations are striped volumes, mirrored volumes, and RAID-5 volumes.
Implementing Disk Fault Tolerance
As mentioned in “Understanding Disk Storage Options”, fault tolerance is the ability of a computer or operating system to respond to a catastrophic event, such as a power outage or hardware failure, so that no data is lost and that work in progress is not corrupted. Fully fault-tolerant systems using fault-tolerant disk arrays prevent the loss of data. You can implement RAID fault tolerance as either a hardware or software solution.
Hardware Implementations of RAID
In a hardware solution, the disk controller interface handles the creation and regeneration of redundant information. Some hardware vendors implement RAID data protection directly in their hardware, as with disk array controller cards. Because these methods are vendor specific and bypass the fault tolerance software drivers of the operating system, they offer performance improvements over software implementations of RAID.
Consider the following points when deciding whether to use a software or hardware implementation of RAID:
■Hardware fault tolerance is more expensive than software fault tolerance and might limit equipment options to a single vendor.
■Hardware fault tolerance generally provides faster disk I/O than software fault tolerance.
■Hardware fault tolerance solutions might implement hot swapping of hard disks to allow for replacement of a failed hard disk without shutting down the computer and hot sparing so that a failed disk is automatically replaced by an online spare.
Software Implementations of RAID
Windows Server 2003 supports one RAID implementation (striped, RAID-0) that is not fault-tolerant and two implementations that provide fault tolerance: mirrored volumes (RAID-1) and striped volumes with parity (RAID-5). You can create fault-tolerant RAID volumes only on dynamic disks formatted with NTFS.
With Windows Server 2003 implementations of RAID, there is no fault tolerance following a failure until the fault is repaired. If a second fault occurs before the data lost from the first fault is regenerated, you can recover the data only by restoring it from a backup.
Striped Volumes
A striped volume, which implements RAID Level 0, uses two or more disks and writes data to all disks at the same rate. By doing so, I/O requests are handled by multiple spindles, and read/write performance is the beneficiary. Striped volumes are popular for configurations in which performance and large storage area are critical, such as computer-aided design (CAD) and digital media applications.
Creating a Striped Volume
To create a striped volume, you must have unallocated space on at least two dynamic disks. Right-click one of the spaces and choose Create Volume. The New Volume Wizard will step you through the process of selecting a striped volume and choosing other disk space to include in the volume. Striped volumes can be assigned a drive letter and folder paths. They can be formatted only with NTFS. Up to 32 disks can participate in a striped volume. The amount of space used on each disk in the volume will be equal to the smallest amount of space on any one disk. For example, if Disk 1 has 200 GB of unallocated space, and Disk 2 has 120 GB of space, the striped volume can contain, at most, 240 GB as the size of the stripe on Disk 1 can be no greater than the size of the stripe on Disk 2. All disk space in the volume is used for data; there is no space used for fault tolerance.
Recovering a Striped Volume
Because data is striped over more than one physical disk, performance is enhanced, but fault tolerance is decreased—there is more risk because if any one drive in the volume fails, all data on the volume is lost. It is important to have a backup of striped data. If one or more disks in a striped volume fails, you must delete the volume, replace the failed disk(s) and recreate the volume. Then you must restore data from the backup.
Mirrored Volumes
A mirrored volume provides good performance along with excellent fault tolerance. Two disks participate in a mirrored volume, and all data is written to both volumes. As with all RAID configurations, use separate controllers (by adding a controller, you create a configuration called “duplexing”) for maximum performance. Mirrored volumes relate to RAID-1 hardware configurations.
Create Mirrored Volumes
To create a mirrored volume, you must have unallocated space on two dynamic disks. Right-click one of the spaces and choose Create Volume. The New Volume Wizard will step you through the process of selecting a mirrored volume and choosing space on another disk to include in the volume. Mirrored volumes can be assigned a drive letter and folder paths. Both copies of the mirror share the same assignment.
You can also mirror an existing simple volume by right-clicking the volume and choosing Add Mirror and selecting a drive with sufficient unallocated space.
Once you have established the mirror, the system begins copying data, sector by sector. During that time, the volume status is reported as Resynching.
Recovering from Mirrored Disk Failures
The recovery process for a failed disk within a mirrored volume depends on the type of failure that occurs. If a disk has experienced transient I/O errors, both portions of the mirror will show a status of Failed Redundancy. The disk with the errors will report a status of Offline or Missing, as seen in Below Figure.
After correcting the cause of the I/O error—perhaps a bad cable connection or power supply—right-click the volume on the problematic disk and choose Reactivate Volume or right-click the disk and choose Reactivate Disk. Reactivating brings the disk or volume back online. The mirror will then resynchronize automatically.
If you want to stop mirroring, you have three choices, depending on what you want the outcome to be:
■Delete the volume If you delete the volume, the volume and all the information it contains is removed. The resulting unallocated space is then available for new volumes.
■Remove the mirror If you remove the mirror, the mirror is broken and the space on one of the disks becomes unallocated. The other disk maintains a copy of the data that had been mirrored, but that data is of course no longer fault-tolerant.
■Break the mirror If you break the mirror, the mirror is broken but both disks maintain copies of the data. The portion of the mirror that you select when you choose Break Mirror maintains the original mirrored volume’s drive letter, shared folders, paging file, and reparse points. The secondary drive is given the next available drive letter.
Knowing that information, how do you suppose you would replace a failed disk—a member of the mirrored volume that simply died? Well, after physically replacing the disk, you will need to open Disk Management to rescan, initialize the disk and convert it to dynamic. After all that work you will find that you can’t remirror a mirrored volume, even though half of it doesn’t exist. So far as the remaining disk is concerned, the mirrored volume still exists—its partner in redundancy is just out to lunch. You must remove the mirror to break the mirror. Right-click the mirror and choose Remove Mirror. In the Remove Mirror dialog box, it is important to select the half of the volume that is missing; the volume you select will be deleted when you click Remove Mirror. The volume you did not select will become a simple volume. Once the operation is complete, right-click the healthy, simple volume and choose Add Mirror. Select the new disk and the mirror will be created again.
RAID-5 Volumes
A RAID-5 volume uses three or more physical disks to provide fault tolerance and excellent read performance while reducing the cost of fault tolerance in terms of disk capacity. Data is written to all but one disk in a RAID-5. That volume receives a chunk of data, called parity, which acts as a checksum and provides fault tolerance for the stripe. The calculation of parity during a write operation means that RAID-5 is quite intensive on the server’s processor for a volume that is not read-only. RAID-5 provides improved read performance, however, as data is retrieved from multiple spindles simultaneously.
As data in a file is written to the volume, the parity is distributed among each disk in the set. But from a storage capacity perspective, the amount of space used for fault tolerance is the equivalent of the space used by one disk in the volume.
From a storage capacity perspective, that makes RAID-5 more economical than mirroring. In a minimal, three disk RAID-5 volume, one-third of the capacity is used for parity, as opposed to one-half of a mirrored volume being used for fault tolerance. Because as many as 32 disks can participate in a RAID-5 volume, you can theoretically configure a fault-tolerant volume which uses only 1/32 of its capacity to provide fault tolerance for the entire volume.
Configure RAID-5 Volumes
You need to have space on at least three dynamic disks to be able to create a RAID-5 volume. Right-click one disk’s unallocated space and choose New Volume. The New Volume Wizard will step you through selecting a RAID-5 volume type, and then selecting the disks that will participate in the volume.
The capacity of the volume is limited to the smallest section of unallocated space on any one of the volume’s disks. If Disk 2 has 50 GB of unallocated space, but Disks 3 and 4 have 100 GB of unallocated space, the stripe can only use 50 GB of space on Disks 3 and 4—the space used on each disk in the volume is identical. The capacity, or Volume Size reported by the New Volume Wizard will represent the amount of space available for data after accounting for parity. To continue our example, the RAID-5 volume size would be 100 GB—the total capacity minus the equivalent of one disk’s space for parity.
RAID-5 volumes can be assigned a drive letter or folder paths. They can be formatted only with NTFS.
Because RAID-5 volumes are created as native dynamic volumes from unallocated space, you cannot turn any other type of volume into a RAID-5 volume without backing up that volume’s data and restoring into the new RAID-5 volume.
Recovering a Failed RAID-5 Volume
If a single disk fails in a RAID-5 volume, data can continue to be accessed. During read operations, any missing data is regenerated on the fly through a calculation involving remaining data and parity information. Performance will be degraded and, of course, if a second drive fails it’s time to pull out the backup tapes. RAID-5 and mirrored volumes can only sustain a single drive failure.
If the drive is returned to service, you may need to rescan, and then you will need to right-click the volume and choose Reactivate Volume. The system will then rebuild missing data and the volume will be fully functional again.
If the drive does not offer a Reactivate option, or if you have had to replace the disk, you may need to rescan, initialize the disk, convert it to dynamic, then right-click the volume and choose Repair Volume. You will be asked to select the disk where the missing volume member should be recreated. Select the new disk and the system will regenerate the missing data.
Mirrored Volumes versus RAID-5 Volumes
Mirrored volumes (RAID-1) and RAID-5 volumes provide different levels of fault tolerance. Deciding which option to implement depends on the level of protection you require and the cost of hardware. The major differences between mirrored volumes and RAID-5 volumes are performance and cost. Below Table describes some differences between software-level RAID-1 and RAID-5.
TABLE: RAID Performance And Costs
Creating Fault Tolerance for the System Volume
Because RAID-5 is a native dynamic volume, it is not possible to install or start the Windows Server 2003 operating system on a RAID-5 volume created by the Windows Server 2003 fault-tolerant disk technologies.
The only option for creating fault tolerance for the system, without buying hardware RAID, is thus to mirror the system volume. You can mirror the system volume by following the procedures described for creating a mirrored volume: right-click the system volume and choose Add Mirror. Unlike Windows 2000, you do not need to restart, and the BOOT.INI file is updated automatically so that you can start to the secondary drive if the primary drive fails.
If the drives are attached to IDE controllers, and the primary drive fails, you may have to remove that drive, change the secondary drive to the primary controller and set its jumpers or cable position so that it is the master. Otherwise, the system may not boot to the secondary drive.
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