UnRAID Manual - FAQ

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How do hard drives become disabled?

When a write operation fails on a disk in a protected array, the system will disable the disk. A disabled disk will no longer be used in any way by the system. The disk still appears as a share, and you may still read and write it; however, the array will be running unprotected and another disk failure will cause data loss.

When a read operation fails the system will return the reconstructed data of the failed block. The system then tries to write the reconstructed data back to the failing disk. If this write operation fails then the hard disk will be disabled.

Once a disk is disabled it's contents must be considered invalid (because there have been uncompleted writes). All further read requests to that disk will be serviced by reading Parity and all the other Data disks in order to reconstruct the requested data on-the-fly. All further write requests result in first reading all the other Data disks, and then updating the Parity disk.

Normally, you would replace the hard drive of a disabled disk; however, you can try to re-enable a disabled disk as follows:

  1. Stop the array.
  2. Power down the unit.
  3. Physically remove the failed disk, leaving the slot empty.
  4. Power up the unit.
  5. Start the array.
  6. Stop the array.
  7. Power down the unit.
  8. Re-install your failed disk.
  9. Power up the unit.
  10. Start the array.

When you start the array in step 5, the system will notice the failed disk's slot is empty, and it will clear the identification data for that slot. Thus when you start the array in step 10, the system will treat the disk simply as a new disk.

The system records the presence of a disabled disk in the config/super.dat file.

What happens if two data disks fail at the same time?

Unfortunately you could very well lose all the data on both disks. But unlike other RAID systems, you will not lose the data on disks which didn't fail.

How would I replace a smaller disk and add new disks at the same time?

Sorry, you must do one of those actions first. While such an operation would be possible, our beta testing has shown that there's too great a chance to become confused and mess up your data.

What if there's something really wrong, for example, maybe multiple drives are missing?

If the system detects a configuration where it can not start at all, then you will not be able to start the array. You must either correct the situation or reset the array configuration data.

How do I remove a hard disk that I don't plan on replacing?

In this case you should reset the array configuration data so that the system can generate new parity information.

What is reconstruct?

Reconstruct is a term that refers to re-building the data contents of a disk using the parity information in RAID system. By reading the parity disk plus all the other data disks, the system can generate the data of the target disk.

Suppose my system becomes unusable, can I recover my data?

All data disk hard drives are formatted with the ReiserFS (3.6) file system. You can install your data disk hard drive in another Linux system and recover your data. You can also install your data disk hard drive in a Windows system and recover the data using freely available utilities to mount this file system.