Written by Uhro

Advice on RAID

RAID is an acronym for Redundant Array of Independent Disks. An NAS with space for more than one hard drive (more than 1 slot) is suitable for a RAID array. A RAID array divides data over multiple disks, stores it on more than one hard drive, or both.

One or more drives

NAS with 1 hard drive

NAS with 1 hard drive

  • Lower costs considering the number of hard drives to buy

  • No need to choose a RAID array

  • Ready for use right away, as the hard drive is has usually already been installed

  • Data is less secure, as it's stored on a single disk

  • You won't have the benefits of a RAID

NAS with 2 hard drives

NAS with multiple hard drives

  • Data is stored securely, as it can be stored on multiple disks

  • Benefit from the advantages of a RAID array

  • Extra speed when transferring data from and to an NAS

  • Higher costs considering the number of hard drives to buy

  • In order to get the advantages of a RAID, you need to configure the NAS (we'll gladly help you with that)

2. What RAID options are there?

Depending on the chosen RAID array, you can distribute data over hard drives (striping), copy the same data on multiple drives (mirroring), or a combination of both options. To help you make a choice between the different RAID arrays, we'll first explain the different options to you.



Striping is a method to distribute data among multiple hard drives. Data is basically "chopped up." The NAS distributes these pieces of data among the available hard drives, turning multiple hard drives into one big one so to speak.

  • Speed boost when reading/writing files
  • If a hard drive breaks, parts of your files will be lost



Mirroring is the opposite of striping and does exactly what the word says. In this method, the NAS will store a copy of the original file on another hard drive. Each drive basically operates independently.

  • Always have one or more identical copies of your data as a backup

  • After replacing a defective hard drive, all data on the NAS can be used again

  • More expensive solution compared to striping

  • If you use two 1TB hard drives in your NAS, you won't have 2TB of storage space available, but 1TB



Basically, parity is about error correction and is always used in combination with striping. The system will create an error correction code (ECC) for the stored bits (stripes) stripes of your data. The NAS will then save this code in so-called parity blocks on one or more of the hard drives. If one of the hard drives fails, you'll temporarily lose the files on that drive. As soon as the defective hard drive has been replaced, the NAS will restore the lost files using the ECC.

3. Which RAID arrays are there?

An NAS with more than one hard drive slot uses one (or a combination) of the options above to store data. The different RAID options such as striping, mirroring, and parity each have a number, for instance RAID 1 or RAID 5. Each RAID array setup has a different effect on how the data is stored and distributed. Choose the option that best suits what you want in terms of (securely) storing your data.


Most NAS support JBOD, even though it's actually not a type of RAID. JBOD stands for Just a Bunch Of Disks and makes the storage space of all hard drives combined available. If one hard drive fails, all the data on that particular drive will be lost. The data on the other hard drives will remain usable.

  • No striping, mirroring, or parity
  • No secure data storage
  • Data will remain partially intact when a drive fails
  • Data on the defective hard drive will be lost


The NAS distributes the files among the available hard drives. Because the drives retrieve or write data simultaneously, you achieve a theoretical doubling in terms of read speed compared to JBOD. As with JBOD, RAID 0 doesn't offer error correction. When a disk fails, all data on all hard drives in the NAS will be lost.

  • RAID option: striping
  • Doubling of read speed compared to JBOD
  • All data will be lost when 1 drive fails
  • You need at least 2 hard drives.


The NAS will save a copy of the files on another hard drive in the RAID 1 setup. If a hard drive fails, the NAS will remain usable without a loss of data. When you replace the defective hard drive, mirroring will add another copy. Because each hard drive in the RAID array holds a copy, you can only use half of the available hard drives to store files.

  • RAID option: mirroring
  • Makes a copy of the data on another hard drive
  • No loss of data when 1 drive fails
  • You need at least 2 hard drives


RAID 5 works the same as RAID 4 in many ways. In the case of RAID 5, the parity is also calculated per "piece" (stripe). The difference with RAID 4 is that the parity blocks are distributed among the hard drives in the NAS. The combination of striping and parity ensures efficient error correction.

  • RAID option: striping and parity
  • The parity is calculated per "piece" (stripe)
  • All data will remain intact if 1 hard drive fails
  • You need at least 3 hard drives
  • Parity blocks are distributed among the hard drives in the NAS


This setup is similar to RAID 5. In the case of RAID 6 however, the NAS creates two parity blocks and stores these on all hard drives in the RAID array. Thanks to this method, two random drives in the setup can fail without a permanent loss of data. The NAS will reconstruct the lost data after new hard drives have been installed.

  • RAID option: striping and parity
  • The NAS creates 2 parity blocks
  • All data remains intact if 1 hard drive fails
  • You need at least 4 hard drives
  • 2 random hard drives in the setup can break down without losing data


RAID arrays can also be combined. The most common combination is RAID 10. This is a combination of RAID 0 and RAID 1 and uses striping and mirroring. In this setup, you combine reliability (RAID 1) with speed (RAID 0).

  • RAID option: striping and mirroring
  • Combination of RAID 1 and RAID 0
  • Data security and speed boost
  • You need at least 4 hard drives

Synology Hybrid RAID

NAS manufacturers also offer their own RAID arrays. For instance, Synology Hybrid RAID (SHR). This array automatically switches from RAID 1 to RAID if you expand the number of hard drives from 2 to at least 3. This array is particularly suitable if you're not yet sure how much data you'll be needing and want to be able to easily change the number of drives.

  • RAID option: striping or mirroring
  • Automatically switches between RAID 1 and RAID 5
  • Data security and speed boost
  • You need at least 2 hard drives

4. Which hard drives are suitable for RAID?

If you're getting started with RAID, you'll have to choose an NAS with room for multiple hard drives. What do you need to consider when buying the hard drives?

  • Always choose hard drives of the same brand and type
  • If you want to use mirroring, consider the storage capacity of the hard drives
  • Check if the hard drives are recommended by the NAS manufacturer
  • An SSD is fast, but hasn't proven itself when it comes to long-term data storage. We recommend only using an SSD as cache.

5. Advice on NAS

In addition to the RAID arrays mentioned here, there are other things you can consider when you're thinking about buying an NAS. For instance, how much data do you want to store, what type of NAS you need, if you need an NAS with or without hard drives, and much more.

Article by:
Uhro NAS Expert.

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