I just recently setup a 3 drive 4TB MDRAID 5 array for mirroring and an online backup of our server.

I am preparing for a future hardware (drive) failure and wanted to mitigate a recovery failure from a URE.

Typically I think of the process for rebuilding an array to be:

  1. Remove and replace failed drive.
  2. Rebuild array

From my understanding, in a degraded RAID 5 array you can still access data; but when the failed drive has been replaced and a the array is rebuilding, if a URE is detected, the recovery will fail and the data on the array will immediately be rendered unreadable and unrecoverable.

If my understanding is correct then it does not seem prudent to recover the array until all the (readable) data has been duplicated.

This leaves me with a process of:

  1. Duplicate data from array.
  2. Remove and replace failed drive.
  3. Rebuild array

Is there another process that would mitigate rebuild failures (aside from a second drive failure during rebuild)? Is it safe to rebuild array without duplicating the data first? Are my assumptions wrong such as the re build fails on URE but data is still available in degraded state?

  • If the data is precious, don't you already have a backup?! Commented Aug 7, 2017 at 9:31
  • @DavidSchwartz I am unsure of what you mean by "already have a backup". The RAID 5 is providing a location for the most recent full with incremental backups and current mirrors of data from the primary server. The primary data is on multiple non-parity RAID arrays. Our most current backups and mirrors used to be on various drives and smaller RAID arrays and I have just consolidated and migrated the non-sense to a simple RAID 5.
    – Damon
    Commented Aug 7, 2017 at 13:25
  • So then why duplicate the data when the RAID 5 fails? It's already duplicated. Just start the rebuild so you can get the array back to a sane state as quickly as possible. Commented Aug 7, 2017 at 16:56
  • If the RAID array has data that you care about losing and that data isn't replicated anywhere else, you're doing it very wrong. Commented Aug 7, 2017 at 18:26
  • @DavidSchwartz While a large portion of the data will already have been duplicated to other Media or be availible from the primary server there is some data such as the most recent incrementals and few other ancillary items that will only exist in the backups that it would be preferable to not lose. Admittedly if the data was lost due to a failed rebuild it would be data that had been previously deleted by users or versions of current data that is out dated. Sometimes people need to go back though so trying to mitigate rebuild failure with an array that size on raid 5.
    – Damon
    Commented Aug 7, 2017 at 18:26

2 Answers 2


You could prepare your self to the drive failure and to very all other troubles by implementing The 3-2-1 Backup plan, my personal opinion 3-2-1 should be in each business critical environment.

Following 3-2-1 Rule will make life easier, this obviously would cost $, but the outcome should worth it.

You could learn more here: https://knowledgebase.starwindsoftware.com/explanation/the-3-2-1-backup-rule/


  • 1
    Almost there; safe deposit box with tapes is on the agenda along with a possible VPS as a second offsite for daily use files and private high bandwidth cloud sync. Data loss on this RAID 5 would not cause an issue other than downtime and extended recovery time; otherwise a RAID 5 would not be used. But in an effort to maximize its potential, I would like to make sure I am using "best practices" with the rebuild and recovery of a degraded parity RAID.
    – Damon
    Commented Aug 16, 2017 at 0:51

I've realize UREs are a bit more complex and unknown to most as they relate to array failures..

The conclusion is UREs can cause arrays to fail, but not as often as that math in the articles say. But RAID 5 still is a very failure prone RAID array compared to ALL other RAID levels.

So back to basics, what are we mitigating during a RAID 5 rebuild? We are trying to get parity back before a second drive fails. THATs IT! This is a by-any-means-necessary endeavor.

This leads me to solidify my list

  1. Temporarily duplicate data from array, tape is cheapest if array is large and HDD space is not available.
  2. Remove and replace failed drive.
  3. Build new array with new drive from scratch.
  4. Reload files to new array from step 1.

This assumes the array can be taken offline which is not always the case. In the end though, some have found the same that building a new array from scratch and transferring data back in one fell swoop is easier and faster, than attempting a full rebuild on a large multi TB array.

Further, I suspect that reading the data and writing the data off the array sequentially in a degraded state effectively only once would greatly lower the chances of a second drive failure before the data is duplicated compared to a full thrashing rebuild, although the chance is still there.

In the end, its all about risk management which varies on the plethora of specific circumstances. In my particular case, I can usually find time within a 24 hour window to restore my array and thus freshly backing up, rebuilding, and restoring from the fresh backup would be best in my case.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .