RAID is a technology that is used to increase the performance and/or reliability of data storage. The abbreviation stands for Redundant Array of Inexpensive Disks. A RAID system consists of two or more drives working in parallel. These disks can be hard discs, but there is a trend to also use the technology for SSD (solid state drives). There are different RAID levels, each optimized for a specific situation. These are not standardized by an industry group or standardization committee. This explains why companies sometimes come up with their own unique numbers and implementations. This article covers the following RAID levels:

  • RAID 0 – striping
  • RAID 1 – mirroring
  • RAID 5 – striping with parity
  • RAID 6 – striping with double parity
  • RAID 10 – combining mirroring and striping

The software to perform the RAID-functionality and control the drives can either be located on a separate controller card (a hardware RAID controller) or it can simply be a driver. Some versions of Windows, such as Windows Server 2012 as well as Mac OS X, include software RAID functionality. Hardware RAID controllers cost more than pure software, but they also offer better performance, especially with RAID 5 and 6.

RAID-systems can be used with a number of interfaces, including SCSI, IDE, SATA or FC (fibre channel.) There are systems that use SATA disks internally, but that have a FireWire or SCSI-interface for the host system.

Sometimes disks in a storage system are defined as JBOD, which stands for ‘Just a Bunch Of Disks’. This means that those disks do not use a specific RAID level and acts as stand-alone disks. This is often done for drives that contain swap files or spooling data.

Below is an overview of the most popular RAID levels:

RAID level 0 – Striping

In a RAID 0 system data are split up in blocks that get written across all the drives in the array. By using multiple disks (at least 2) at the same time, this offers superior I/O performance. This performance can be enhanced further by using multiple controllers, ideally one controller per disk.

Disk storage using RAID 0 striping


  • RAID 0 offers great performance, both in read and write operations. There is no overhead caused by parity controls.
  • All storage capacity is used, there is no overhead.
  • The technology is easy to implement.


  • RAID 0 is not fault-tolerant. If one drive fails, all data in the RAID 0 array are lost. It should not be used for mission-critical systems.

Ideal use

RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, such as on an image retouching or video editing station.

If you want to use RAID 0 purely to combine the storage capacity of twee drives in a single volume, consider mounting one drive in the folder path of the other drive. This is supported in Linux, OS X as well as Windows and has the advantage that a single drive failure has no impact on the data of the second disk or SSD drive.

RAID level 1 – Mirroring

Data are stored twice by writing them to both the data drive (or set of data drives) and a mirror drive (or set of drives) . If a drive fails, the controller uses either the data drive or the mirror drive for data recovery and continues operation. You need at least 2 drives for a RAID 1 array.

Disk storage using RAID 0 striping


  • RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single drive.
  • In case a drive fails, data do not have to be rebuild, they just have to be copied to the replacement drive.
  • RAID 1 is a very simple technology.


  • The main disadvantage is that the effective storage capacity is only half of the total drive capacity because all data get written twice.
  • Software RAID 1 solutions do not always allow a hot swap of a failed drive. That means the failed drive can only be replaced after powering down the computer it is attached to. For servers that are used simultaneously by many people, this may not be acceptable. Such systems typically use hardware controllers that do support hot swapping.

Ideal use

RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two data drives will be used.

RAID level 5

RAID 5 is the most common secure RAID level. It requires at least 3 drives but can work with up to 16. Data blocks are striped across the drives and on one drive a parity checksum of all the block data is written. The parity data are not written to a fixed drive, they are spread across all drives, as the drawing below shows. Using the parity data, the computer can recalculate the data of one of the other data blocks, should those data no longer be available. That means a RAID 5 array can withstand a single drive failure without losing data or access to data. Although RAID 5 can be achieved in software, a hardware controller is recommended. Often extra cache memory is used on these controllers to improve the write performance.

Disk storage using RAID 5 striping with parity across drives


  • Read data transactions are very fast while write data transactions are somewhat slower (due to the parity that has to be calculated).
  • If a drive fails, you still have access to all data, even while the failed drive is being replaced and the storage controller rebuilds the data on the new drive.


  • Drive failures have an effect on throughput, although this is still acceptable.
  • This is complex technology. If one of the disks in an array using 4TB disks fails and is replaced, restoring the data (the rebuild time) may take a day or longer, depending on the load on the array and the speed of the controller. If another disk goes bad during that time, data are lost forever.

Ideal use

RAID 5 is a good all-round system that combines efficient storage with excellent security and decent performance. It is ideal for file and application servers that have a limited number of data drives.

RAID level 6 – Striping with double parity

RAID 6 is like RAID 5, but the parity data are written to two drives. That means it requires at least 4 drives and can withstand 2 drives dying simultaneously. The chances that two drives break down at exactly the same moment are of course very small. However, if a drive in a RAID 5 systems dies and is replaced by a new drive, it takes hours to rebuild the swapped drive. If another drive dies during that time, you still lose all of your data. With RAID 6, the RAID array will even survive that second failure.

Disk storage using RAID 6 stripingwith double parity across drives


  • Like with RAID 5, read data transactions are very fast.
  • If two drives fail, you still have access to all data, even while the failed drives are being replaced. So RAID 6 is more secure than RAID 5.


  • Write data transactions are slowed down due to the parity that has to be calculated.
  • Drive failures have an effect on throughput, although this is still acceptable.
  • This is complex technology. Rebuilding an array in which one drive failed can take a long time.

Ideal use

RAID 6 is a good all-round system that combines efficient storage with excellent security and decent performance. It is preferable over RAID 5 in file and application servers that use many large drives for data storage.

RAID level 10 – combining RAID 1 & RAID 0

It is possible to combine the advantages (and disadvantages) of RAID 0 and RAID 1 in one single system. This is a nested or hybrid RAID configuration. It provides security by mirroring all data on secondary drives while using striping across each set of drives to speed up data transfers.

Disk storage using RAID 1 + 0, combining spriping with mirroring


  • If something goes wrong with one of the disks in a RAID 10 configuration, the rebuild time is very fast since all that is needed is copying all the data from the surviving mirror to a new drive. This can take as little as 30 minutes for drives of  1 TB.


  • Half of the storage capacity goes to mirroring, so compared to large RAID 5  or RAID 6 arrays, this is an expensive way to have redundancy.

What about RAID levels 2, 3, 4 and 7?

These levels do exist but are not that common (RAID 3 is essentially like RAID 5 but with the parity data always written to the same drive). This is just a simple introduction to RAID-systems. You can find more in-depth information on the pages of wikipedia or ACNC.

RAID is no substitute for back-up!

All RAID levels except RAID 0 offer protection from a single drive failure. A RAID 6 system even survives 2 disks dying simultaneously. For complete security you do still need to back-up the data from a RAID system.

  • That back-up will come in handy if all drives fail simultaneously because of a power spike.
  • It is a safeguard when the storage system gets stolen.
  • Back-ups can be kept off-site at a different location. This can come in handy if a natural disaster or fire destroys your workplace.
  • The most important reason to back-up multiple generations of data is user error. If someone accidentally deletes some important data and this goes unnoticed for several hours, days or weeks, a good set of back-ups ensure you can still retrieve those files.
1 June 2015

71 responses to “RAID”

  1. Aftab Alam says:

    What an excellent explanation of RAID….. its amazing, easy language and can any body understand.Thank you so much

  2. kader khan says:

    thank you sir.
    for this valuable information.

    the language used is very easy and understandable.

  3. Midhun says:

    Hi, I have read your explanations about RAID configuration and it is very much informative with pictures. I have a doubt that in RAID 5 or in RAID 6, how much space will al1ocate for a parity drive if it is a 1TB drive.
    Another doubt I have that, even though it is not relating to this topic, what is mean by SATA3 6GB/s interface? Is it a 6GB/s transfer speed or any other? My HDD occupied with the same SATA3 interface and I have been getting not more than 50MB/s while copied a file from one logical drive to another since the date of I assembled the PC.

    • Laurens says:

      If I understand your first question correctly, you are wondering if you can use a smaller drive for parity compared to the other drives in the RAID set. The hardware or software RAID controller determines if you can mix different sizes and types of drives. Many require all drives to have the same capacity. Alternatively they use the capacity of the smallest drive across all of them. That means a mix of several 2 TB drives and a single 1 TB leads to all disks only using 1 TB of storage capacity.

      For SATA3, the 6 Gb/s indeed refers to the transfer speed. Please note that it is 6 gigabit per second, not 6 gigabyte per second. It is Gb/s, not GB/s. There is some overhead which means the fastest real transfer speed is around 600 megabyte per second. A hard disk cannot reach that maximum speed, only SSDs are capable of doing that. You should also keep in mind that if you copy files from one logical drive to another on the same HDD, your computer is reading from and writing to the same drive simultaneously. That also slows down the data transfer.

    • Cameron says:

      If you are getting poor performance on a Sata 3 controller, it’s likely because your drive is only a Sata 2 drive. To take advantage of Sata 3 speeds, you need both a Sata 3 drive and a Sata 3 controller.

      Also as noted, the 6 gigabit-per-second transfer rate specified for Sata 3 is only what the controller is capable of. A Sata 3 hard disk will never achieve a full 6Gb per second transfer rate, but it will be way faster than a Sata 2 drive. SSDs will get you much closer than any hard drive, but no storage media will actually ever reach the maximum transfer rate of the controller. The type of data being transferred is a significant factor in this as well.

      Also the 6Gb per second Sata 3 transfer rate only applies to sequential reads, which are faster than random reads, particularly on rotating media. Write operations are much slower, as the media itself is the bottleneck.


  4. Kamal says:


    Can you please tell me what is the maximum size for one virtual disk under RAID 1. (virtual disk size limit)

    • Laurens says:

      That depends on the RAID controller that you’ll be using. What is the largest disk size it supports?

  5. vino says:

    Thanks for the great post !!

  6. Madison says:

    I have a 160Gb and a 750gb drives If i RAID 0 with them will I get 910gb of space under one drive or will it be limit to 160gb being to lowest size of the two?

    • Laurens says:

      The storage space added to the array by each disk is limited to the size of the smallest one, which means this would be very unefficient.

  7. Chan says:

    Thank you. Its a well written explanation regarding the RAID function.

  8. Ketki says:

    Excellent Doc

  9. MyRobot says:

    The other disadvantage is that you cannot go back in time and recover a file you accidentally deleted two days ago.

    Previous Versions

  10. Charbel Seif says:

    i have 4*2tb hdd mounted in raid 0 need for performance i need to mirror or secure these data how to do ? RAID 0+1 ? do you recommend ?

    • Laurens says:

      I personally have two external disk enclosures and alternate back-ups of all data on these enclosures. One of them is stored at my parents house and during each visit I swap them out so I always have an off-site backup. There are two disadvantages of just mirroring your data on additional internal disks: your backup is physically in the same location so if the PC gets stolen or there is a fire everything is gone. The other disadvantage is that you cannot go back in time and recover a file you accidentally deleted two days ago.

  11. Br says:

    In addition, I don’t understand using Raid 1 and “a hardware controller.” Please explain.
    And…”cannot be replaced while server is running?

    • Laurens says:

      In the past RAID systems were typically used in servers, not with stand-alone PCs or Macs. That is no longer true so I’ve updated the text. If you attach a separate box containing two or more drives to a computer and those drives are running in a RAID configuration, there is a circuit board in that box that handles the distribution of the data across the drives. That board has its own CPU: it is effectively a mini computer but it typically is called a hardware controller.

  12. Br says:

    Hello, I have 4TB of photo images on a glyph drive that is just short of 20% full. (each image between 300 and 500mg.) Most of it is in an alternate location (3tb) and that is also spent so another external drive without a backup is being used.
    I am wondering if a mirrored 12 or 16 gb raid 1 drive is a good idea (my current 4tb can be moved to the other locale giving me 7tb. Or if that’s just too big and if one drive fails due to corruption they both fail as they’re mirrored.
    Are the removable mirrored drives (CRU and G Tech) a good solution to this issue. I anticipate using at least 1TB in the next year and possibly more. Thanks..

    • Laurens says:

      If I understand it correctly you currently have around 6 TB of data and you expect to add at least 1 TB each year. A mirrored 16 TB RAID 1 system gives you 8 TB of effective capacity, which means you’ll run out of space again pretty soon. Assuming you go for a 4×4 TB disk setup, it would make more sense to choose RAID 5 since that gives you 12 GB of effective space. I am not familiar with the brands you mention. Have a look at Drobo as well – their RAID boxes seem to be pretty popular but there are dozens of alternatives on the market.

  13. Vijitha Ivor says:

    100% Kudos to the Author.. You are a true Technically experienced genius unlike to most Book Worms and High Shouts in IT Blogs… WELL DONE !!

  14. GimmeAnESP says:

    So, if one had two 500gb HDs and a 1tb WD My book along with a 2th My passport ultra, what would be the most essential and productive RAID setup to go with?

  15. Ryan says:

    So, if I’m setting up a server running Windows Server 2011 (for home use), which will be installed on a 500gb seagate IDE HD and there will be a spare 500gb WD HD plus a 2tb WD My passport ultra and 1tb My book, how in the hell would you setup the RAID software for that and what would be the best setup for a home server? I’m familiar with networking, I’ve just never bothered with RAID.

    • Laurens says:

      I would stay away from RAID with such a setup. RAID works best for drives with the same capacity and using the same (type of) controller. If you want higher throughput remove the big drives from their external enclosure and put them internally on SATA 6. That is faster than most USB3 controllers. You could mirror both 500 GB drives if redundancy is what you are after but I’d rather have redundancy on my data than on my software.

  16. Dawn Dubke says:

    Is it possible you can explain to me how to do the following or direct me to a tutorial? I have a 4 drive NAS system that uses EXFAT (FAT64) and was considering RAID 1+0 but really didn’t want to lose all that storage. On the other hand, I have lost many hard drives and all the information from crashes. So I’ve learned it’s not IF your drive crashes but WHEN. I’d love to use the full capacity of the drives while still being able to recover from a crash.
    “If you want to use RAID 0 purely to combine the storage capacity of twee drives in a single volume, consider mounting one drive in the folder path of the other drive. This is supported in Linux, OS X as well as Windows and has the advantage that a single drive failure has no impact on the data of the second disk or SSD drive.”

    • Laurens says:

      You don’t get any redundancy with mounting drives into the file system. If that is your goal, you’ll need to stick to RAID or a real-time backup solution. Mounting drives on Windows 7 is explained on this Microsoft page.

  17. JB says:

    Kindly can have the way forward to configure Mirroring RAID?

    • Laurens says:

      I cannot give you a short and relevant description of how to do this, especially not without any knowledge of your setup. The way RAID needs to be configured in a NAS or SAN system is completely different from doing so on a PC or Mac. If you are a PC user, you typically need to go to the BIOS before the system has the chance to boot and in the BIOS you can then configure which disks should be part of the RAID system. Once that is done, the RAID volume can be partionned and formatted from within the operating system.

  18. sir please explain me, which RAID is most important in all the RAID category?

    • Laurens says:

      That depends on your definition of important. For home usage, RAID 0 is interesting if you are after speed or RAID 1 if you want security. For company servers, RAID 6 is probably the way to go right now.

  19. mudd says:

    Hi, just want to check if i understand.
    So for example..
    A storage box consists of an array of 6 disks, 1 TB each and the effective storage capacity, based on the RAID level used is.

    RAID 1: Not sure.
    RAID 5: 6-1=5TB
    RAID-DP: 6-2=4TB
    RAID 0+1: Not sure.
    RAID 1+0: Not sure.

    Is it correct?

  20. Aryan says:

    The way you have explained using simple terms I really liked it. But what I feel is you should have included RAID 6 as it can withstand failure of more than one disk. Its interesting to learn something that is quite different from that of others.

  21. vaporus says:

    if you were to set up raid 0 on SSD’s would that increase speed any?

    • Laurens says:

      There are lots of heated discussions about that on the web. If you run benchmark software to measure the performance of striped SSD drives, there is a significant speed increase. Many claim however that in real world usage, the advantage is insignificant and doesn’t justify the data security risk.

  22. miguel says:

    hello i have 4 hd 2 are 1t and 2 are 1.5t whats the best raid setup i should use

  23. Loz says:

    Your RAID5 diagram looks wrong to me, but it’s not helped by being unclear which blocks constitute a full stripe-set, (eg is it 1a 1b 1c etc) and by labelling a number of blocks with just the word “parity”. Wikipedia is clearer!

  24. boykalbo says:

    Im thinking of using the Raid 5 for my server for our business, thanks for the infor

  25. Sonia says:

    How can i calculate the effective space if i have 3 hard disk of 600 GB and i want to setup a RAD-5?

  26. jose luis barquin guerola says:

    Please, review the RAID10 definition, it’s worng.

    RAID10=(mirrors in stripe mode), and the advantage is that if a disk fails you only need to recover the mirror wich is on degraded mode. In a RAID01 you need to recover the full mirror (like in RAID01 mode).


  27. Fred says:

    Hi, there is a mistake. RAID 0+1 ain’t RAID10. RAID 1+0 is commonly named RAID10.

  28. Geekthinker says:

    It is a safeguard WHEN the storage system gets stolen

    what are you implying…

  29. Ray Clancy says:

    Running raid0 bootable, partitioned with 2 drives, boot and root, minimum swap on cf cards.

    Desire to –grow to a third device.

    Such was possible with the following:

    mdadm –grow /dev/md0 –raid-devices=3 –add /dev/sdxx

    Error occurs: mdadm /dev/md0; could not set level to raid4…..
    Normal procedure is to use raid4 to resync and then revert back to raid0.

    Why does this procedure fail? It used to work….

  30. Joe C. says:

    One additional ques: I assume RAID 5 is the same as RAID 4+1? Thanks

  31. Joe C. says:


    Can someone tell me if RAID 4+2 is the same as RAID 6?

    Any input is appreciated.

    ATT Corp
    West Demoines IA

  32. RAKHILESH says:

    what is raid7?

  33. Vishu says:

    How to define RAID 5 performance for 8*600 GB? For rotating Disc.

  34. Guest says:

    RAID 1+0 is *NOT* the same as 0+1

  35. durga prasad says:

    it excllent way of showing raid 10 in diagrams
    also in clarity

  36. joshua says:

    RAID is for pussies

  37. Teach says:

    Thanks for this information, I think Raid 5 is on my machine

  38. ITHA says:

    RAID 1 + 0 through a HW controller for the first layer and mobo for the second layer would technically show you as having 2 disks since the RAID card would only show 2 to the motherboard which would then combine those two, therefore you would only see 2 drives unless you opened up the computer itself.

  39. Abid Chaudhary says:

    Dear All,
    I am having a problem with HP prolaint Ml150 G3 server its o/s windows server 2003 is not booting properly it has 2 hard disk 250 GB each hot swap able is it possible that I access both or any one hard disks data by attatching it to some other system or if possible then through which operating system.

  40. Yousefi says:

    RAID=Redundant Array of Independent Disks

  41. Raoule says:

    you can achieve raid10 with 2 disks but it makes no sense to do that, this would slow your drive down which negates the purpose of striping in the first place, you will have 2 read/write operations on same disk making your drive work harder than it has to which would cause failure sooner.

    that type of setup would be great for testing purposes, (not for speed obviously), if you are lacking resources.

  42. Kingsley says:

    I need to deal with very large data set with typical file size of 1-7gb, hundreds of them, in a workstation. Both read and write. I can only fit 4 disks for RAID purpose, what’s the best option? Would Raid 3 be better than Raid10? It seems like Raid 3 can write in parallel in more than 1/2 of the total number of disks and lose only 1/4 of total storage.

    • Mark Davis says:

      Raid 5 only requires a minimum of 3 disks. With 4 disks, you will only loose 1/4 of your HD space. As for file size, that is upto whatever file system you put on the volume created by you Raid array.

  43. Ramesh says:

    Raid5 5disc failure how to recovery data pl explain

  44. Russell Mujee says:

    I have used RAID 6 in one of my server. This has allowed me to create two hot swap disks. I deceided to use it on case scenarios such as: if two active disks fail at the same time. or if two disks will fail at diferent intervals but the chance of getting a new replacement is in the process and has not arrived yet. Featured in a NEC server rack mount.

  45. Bob Twain says:

    I have head that the government is now doing work on RAID -17 (yes, negative 17). This technology is based on tensors and promises to put all other RAID to shame.

  46. Pinar says:

    Actually you can have RAID 1+0 with only two disks.

  47. max says:

    you can get 1+0 on two disk using two partitions. this can be done with software raid. dunno if HW controllers can support this.

  48. Laurens says:

    Don’t the HP tools state “1(+0)”? On some controllers such as HP ones, all available options can be selected even if there aren’t enough disks available. With 2 disks, selecting RAID 1+0 effectively gives you a RAID 1 set. The disks won’t be striped.

    • generalacc says:

      Many thanks, glad your answer was 1st hit google in my search 🙂

    • Ashe' Gupta says:

      I am a newbie when it comes to NAS. I am trying to configure 4 drives of 3 TB each. If I use RAID 1, what is my effective capacity?

    • Laurens says:

      As stated in the RAID 1 section: ‘The main disadvantage is that the effective storage capacity is only half of the total drive capacity because all data get written twice.’. That means your effective capacity will be 6 TB.

  49. puzzled says:

    In the HP ACU page, I see on our array two disks, labeled as RAID 1+0. However, if I understand it correctly, RAID 1+0 is a four disk minimum. How can you have RAID 1+0 with two disks? Isn’t this essentially RAID 1?

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