RAID Recovery™
Recovers all types of corrupted RAID arrays
Recovers all types of corrupted RAID arrays
Last updated: May 07, 2024

Time required for a rebuild RAID array

RAIDs are better than single-drive storage in many aspects, but one thing remains the same on the two – data loss. Without making proper disaster recovery plans, you can lose your important data even on a RAID level that supports high redundancy and fault tolerance. But when you choose RAID, one of the advantages you get is automatic rebuilding after one drive failure.

However, automatic RAID rebuilding is limited to only RAID 5, and when the array only suffered from not more than one drive failure. For other popular RAID levels, you have to manually rebuild the array; how much time does this rebuilding take, and how can you optimize it to ensure data safety? This article explains all you should know about the RAID rebuild procedure.

Basics of RAID Rebuilding

RAID rebuilding refers to the reconstruction of a RAID level after the array has seen a single drive or multiple drives failure. It implies recopying data from the good drives in the array to the newly inserted disk(s). Some RAID levels support automatic rebuilding if only one drive is what failed

Depending on the RAID level, amount of data to be transferred, and disk specifications, the total time it’ll take to successfully rebuild an array can vary greatly, from 1 hour to 24 hours or more. Also, depending on the RAID level, the rebuilding pattern may differ from simply mirroring data from the other drives to the new one(s) or reconstructing lost data from parity information on the other good drives.

Well, some people would rather change to a different RAID level than rebuild a failed array in the same RAID level, but this poses high risks of fatal data loss – unless you have your data backed up externally.

If you configured RAID 5 or RAID 1 and had a single failed drive, simply remove the failed drive and insert a new one, the array will rebuild automatically without any need to shut down the system or interrupt ongoing activities. Aside from RAID 1 and RAID 5, all other RAID levels require you to shut down the system and do the rebuilding manually – using RAID rebuilding tools.

Are RAID Rebuild Times Still Relevant?

Of course, they matter, especially in enterprise environments where multiple drives are used in RAID array setups. Rebuilding an array may take many days, which means productions may be stopped until the rebuild is complete. Also, if HDDs were used for the setup, rebuilds may take longer times this is because hard disk drives are terrible at random data access but offer good speed for sequential I/O.

The procedure to rebuild a RAID involves removing the bad drives first, before any other action. If you’re using a NAS system, there may be some settings to deactivate before uninstalling the drives. Furthermore, some array vendors integrate features such as “predictive sparing” to detect a potential failure and copy data from the suspected failing drive to store as parity, this could save you some rebuilding time.

RAID Levels Rebuilding Time Difference

The common RAID levels used by most people today are RAID 0, RAID 1, RAID 5, and RAID 6. Each of these RAID levels offers unique characteristics and will take a different amount of time to rebuild.

Practically, RAID 0 offers the fastest performance due to its data stripping technique, but it supports zero fault tolerance and your data will be lost if a drive fails in the array – it does not support automatic rebuilding.

On the contrary, RAID 1 is the best type of RAID to use if you’re more concerned about getting a shorter rebuild time; RAID 1 mirrors the same data across all disks in the array, so until the last drive fails, all your data will remain accessible. RAID 1 rebuilds automatically when you remove the failed drive and insert a new one.

RAID 5 and RAID 6 are often described together because they are almost of the same configuration – but RAID 6 is a bit more complex. Both RAID levels use a dedicated parity disk for fault tolerance; particularly RAID 5 can withstand one drive failure because it uses a single parity disk and would rebuild automatically if the single failed drive is removed and replaced with a good one, while RAID 6 can withstand up to two simultaneous drive failures because it uses two dedicated parity disks and would rebuild automatically if the failed drives are removed and replaced.

Rebuild RAID 5 and 6 takes longer than other types of RAID levels because of the overhead due to parity data calculations.

Note: Each RAID level, aside from RAID 0, has a maximum number of drive failures it can withstand and trigger an automatic rebuild if the failed drive(s) gets replaced. If the number of drive failures in a RAID array exceeds its fault tolerance capacity, the entire array will shut down until you rebuild manually.

Rebuild Time Formulas

While RAID rebuild time is determined by a wide range of factors, there are formulas that allow you to get the possible duration a rebuild will take to complete. But, to use these formulas, you must know the following specifications of all drives and controllers used in the RAID setup: disk speed, disk size, data size to be rebuilt, RAID controller speed, and the system load.

Also, each RAID level has its formula, but all of them follow the same logic, which is to divide the data size by the rebuild rate. The rebuild rate is the product of the drive’s speed, controller speed, and system load. Now let’s get down to the formula for each RAID level.

Type: RAID 1

  • Number of Drives: 2 x 1TB disks
  • Data Size: 600GB
  • Drive Speed: 150MB/s each drive
  • Controller Speed: 85%
  • System Load: 50%
  • Rebuilt Time Formula: Data size / Rebuild rate = 600GB / (150MB/s x 0.85 x 0.5) = 2.6hrs

Type: RAID 5

  • Number of Drives: 4 x 1TB disks
  • Data Size: 5TB
  • Drive Speed: 150MB/s each drive
  • Controller Speed: 85%
  • System Load: 50%
  • Rebuilt Time Formula: Data size / Rebuild rate = 5TB / (150MB/s x 0.85 x 0.5 x (n-1)) = 7.3 hours

Note: “n” is the number of disks in the array (4) minus the dedicated parity drive (1).

Type: RAID 6

  • Number of Drives: 6 x 1TB disks
  • Data Size: 6TB
  • Drive Speed: 150MB/s each drive
  • Controller Speed: 85%
  • System Load: 50%
  • Rebuilt Time Formula: Data size / Rebuild rate = 6TB / (150MB/s x 0.85 x 0.5 x (n-2)) = 6.54 hours

Note: “n” is the number of disks in the array (6) minus the dedicated parity drives (2).

Rebuild Time Optimization

There are a few tweaks you could do to save some time each time your RAID array undergoes a rebuild. These tweaks must be done aforetime – even before the RAID array setup. The tweaks include:

  • Use drives of the same speed, interface, and capacity. If you can buy faster disks, that would be a lot handy.
  • SSDs are always better
  • Get a high-end RAID controller if you’re setting up hardware RAID or enable hardware acceleration on the system if you’re going for software RAID.
  • Configure any system settings that could result in reduced system workload
  • Monitor your drives' health status and replace them on time before they fail completely.
  • Make backups!
  • Carefully choose a RAID level that suits your needs.

What are RAID Rebuild Tools?

RAID Recovery Software

DiskInternals RAID repair utility is a professional solution that recovers data from all RAID levels. It supports multiple Windows OS and Linux OS file systems, as well as Unicode file names. The software is available for Windows XP, 10, 11 and features a clear-cut interface anyone easily understand.

The software allows you to create disk images of your RAID drives to serve as backup copies you can recover from in disaster times. DiskInternals RAID Recovery features a built-in Recovery Wizard that guides you through recovering your lost data. The software can recover and preview multiple file formats for free.

Tip: RAID 0 or RAID 1: what to pick?


Rebuilding a RAID array can take a few minutes or days, depending on various factors that have been analyzed in this article. To ensure faster rebuilding time, you should use faster disks and high-performance RAID controllers. Finally, the DiskInternals RAID Recovery can be helpful to recover lost files from a failed RAID drive, so you don’t lose any important data.


  • How do I rebuild RAID 5 drive failure?

    Access the RAID controller and record the information of the malfunctioning drive. Then, substitute the defective drive with a new or previously unused drive within the configuration. The RAID controller will begin to reconstruct the RAID array, leveraging data from the remaining drives. Upon completion of the rebuild, your RAID 5 setup will be fully functional once again.

  • What is rebuild time RAID 5 vs RAID 6?

    RAID6 offers the resilience to withstand the failure of two drives without losing any data. In contrast, the rebuild times for RAID5 are significantly quicker, with speeds that can be 50% to 200% faster, influenced by factors such as total storage capacity, the RAID controller used, and the volume of data stored.

  • Why does RAID rebuild take so long?

    The approach to data recovery in RAID systems varies by the RAID level, encompassing straightforward data mirroring or more complex reconstruction using parity data. The duration of the rebuild process is influenced by multiple elements, including the disks' capacity and performance, the volume of data being replicated, the RAID controller's efficiency, and the current system workload.

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