How to rebuild RAID sets properly in Snow Leopard

While Snow Leopard will attempt to automatically recover failed RAID 1 arrays, there are times when the process won't work. Erik Eckel explains how to manually rebuild a RAID 1 array when needed.

Little should raise technology administrators' fears faster than failing RAID arrays. Data recovery from a single failed hard disk is tough enough, especially when the impacted users or departments demand the lost data or service be returned to immediate operation.

RAID failures are exponentially more difficult to recover from, compared to a simple single disk catastrophe, which is one of the reasons business-critical systems should be backed up regularly using image-creation software. When RAID issues arise on a Mac workstation or server, the first step is to confirm an issue actually exists. Make sure all drives are properly connected, checking both data and power connections.

But the fact remains; the more drives you have in a RAID set, the higher the odds a disk will fail. Recovery options depend on how the RAID set is configured. If a disk fails in a RAID 0 set, the data's toast. Find the backup, because that's how you're going to recover.

Snow Leopard, like previous versions of Mac OS X, can rebuild failed or degraded RAID 1 sets, though. If configured to automatically attempt healing, Mac OS X will attempt to repair a degraded RAID 1 array in which data mirrored across the two disks encounters inconsistencies. The system attempts to copy data from the good drive to the disk experiencing trouble.

Snow Leopard will also attempt to automatically repair degraded RAID 1 arrays, if so configured, when a disk fails and a hot spare is present. Administrators don't always select the automatic option, or sometimes other errors prevent such dynamic healing of degraded RAID 1 arrays. Mac administrators can manually rebuild RAID 1 arrays when such problems occur.

How to manually rebuild a RAID 1 array

To manually rebuild a RAID 1 array using Mac OS X Snow Leopard, administrators should perform these steps:

  1. Ensure all disks are properly installed and connected.
  2. Open the Mac OS X Disk Utility.
  3. From the left-hand window console, highlight the degraded RAID 1 array. Degraded arrays should display using red text. Then, ensure the RAID tab is selected from within the right-hand window.
  4. One of two resolutions will be available. If the array's degradation is due to inconstant data, "Failed" will display next to the problematic disk. If the trouble is due to a bad or missing drive, "Offline" will appear next to the drive.
  5. Clicking the Rebuild button will prompt Mac OS X to review disk inconsistencies and repair arrays degraded due to inconsistent data. If a drive no longer mounts and is missing, highlight the missing or failed drive and click the - icon, then drag the replacement drive from the left-hand pane to the right-hand window, and then click the Rebuild button to repair the array.

Patience may be required

Due to disk sizes, system components and other active processes and services, the array rebuild process could take just a couple hours or it could stretch on to a couple days. Administrators can and should monitor the array's ongoing status using the Disk Utility.