RAID

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 standardisation 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.

RAID-systems can be based with an 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

Advantages

  • 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.

Disadvantages

  • 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 a 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

Advantages

  • 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.

Disadvantages

  • 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 (meaning it cannot be replaced while the server keeps running). Ideally a hardware controller is used.

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 check sum 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

Advantages

  • 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.

Disadvantages

  • 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 it writes the parity data 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

Advantages

  • 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.

Disadvantages

  • 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

Advantages

  • 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.

Disadvantages

  • 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.
14 February 2015

46 Responses to “RAID”

  1. 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.

  2. 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.

  3. 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.

  4. 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?

  5. 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.

  6. 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.

  7. miguel says:

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

  8. 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!

  9. boykalbo says:

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

  10. 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?

  11. 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).

    Thanks.

  12. Fred says:

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

  13. Geekthinker says:

    It is a safeguard WHEN the storage system gets stolen

    what are you implying…

  14. 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….

  15. Joe C. says:

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

  16. Joe C. says:

    Hello:

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

    Any input is appreciated.

    Joe
    ATT Corp
    West Demoines IA

  17. RAKHILESH says:

    what is raid7?

  18. Vishu says:

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

  19. Guest says:

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

  20. durga prasad says:

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

  21. joshua says:

    RAID is for pussies

  22. Teach says:

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

  23. 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.

  24. 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.

  25. Yousefi says:

    RAID=Redundant Array of Independent Disks

  26. 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.

  27. 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.

  28. Ramesh says:

    Raid5 5disc failure how to recovery data pl explain

  29. 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.

  30. 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.

  31. Pinar says:

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

  32. 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.

  33. 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.

  34. 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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