Managing Microsoft Windows Server 2003 Disk Storage in C#.NET

Creator Code39 in C#.NET Managing Microsoft Windows Server 2003 Disk Storage

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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 Figure 11-8.

Figure 11-8

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.

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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.

Exam Tip Mirrored volumes provide fault tolerance and better write performance than RAID-5 volumes. However, because each disk in the mirror contains a full copy of the data in the volume, it is the least efficient type of volume in terms of disk utilization.

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 volume. The remaining disk 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, so a RAID-5 volume is the least efficient option for write operations. RAID-5 provides improved read performance, however, because data is retrieved from multiple spindles simultaneously. As data in a file is written to the volume, the parity is distributed among all the disks 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.

Exam Tip The formula for calculating the effective storage capacity of a RAID-5 volume is the size of the stripe on one volume times the number of volumes minus one, or size x (volumes 1). For example, if unallocated space of 500 GB on each of four dynamic disks is used to create a RAID-5 volume, parity will occupy 500 GB, so the effective storage capacity of the volume is 1500 GB.

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.

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