RAID5 Frequently Asked Questions

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Various Questions

RAID 5 Recovery

What happens if one of the RAID 5 member disks fails? How can I find out that one of the RAID5 member disks has failed?

There are two types of RAID failures:

  • one of the member disks has dropped out of the array (for example, it has been taken out or it has completely failed);
  • the disk works partially (e.g. read operations are OK but writes fail).

In either case you will eventually get a warning message telling that one of the member disks has failed. RAID still continues to write and read data since RAID 5 is a redundant array. However, performance of read operations degrades significantly.

When requested to read data from the failed disk, the controller has to read data from all of the available disks and then reconstruct missing data.

To discover in good time that one of the disks in RAID has failed, you need to check member disks state periodically using diagnostic tools provided either by RAID controller vendors (for a hardware RAID) or by operating system (e.g. Disk Management in Windows).

What will happen if two RAID5 member disks fail simultaneously?

If two disks in a RAID5 fail simultaneously, RAID5 array will break and all the array data will be lost. In this case, the only option to see the data once again is to restore it from the backup. Note that RAID doesn't replace backup.

The widespread opinion is that double disk failures occur frequently due to insufficient reliability of the hard disks. In real life, a failure of more than one member disk likely happens due to some catastrophic external event (for example flood or lightning strike). Another common case is an operator error during the disk replacement - one of the healthy disks is pulled out instead of the failed disk.

If a RAID controller has failed, is it possible to transfer RAID5 member disks to the same controller and access the array data?

Often, yes. For example, it is possible to transfer arrays between Intel ICH controllers of the same model or from the earlier models to the later ones. However, sometimes some special actions are required to get the RAID5 accessible on the new controller. Anyway, before you start transferring the array, it is better to consult with the vendor of a particular controller. Another advice is to never do or confirm the action in which the word "initialization" is mentioned.

If a NAS has failed but the member disks are OK, is it possible to transfer disks to exactly the same NAS to access the data?

Usually the same approaches as discussed for RAID controllers above are used.

See also

Useful links

RAID Recovery Freeware

RAID Recovery Guide

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