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 disks working in parallel. These disks can be hard discs but there is a trend to also use the technology for 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.

The software to perform the RAID-functionality and control the hard disks 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 2003, 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 RAID 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.

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 disk overhead.
  • The technology is easy to implement.

Disadvantages

RAID 0 is not fault-tolerant. If one disk fails, all data in the RAID 0 array are lost. It should not be used on 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 Photoshop image retouching station.

RAID level 1 – Mirroring

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

RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.

Advantages

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

Disadvantages

  • The main disadvantage is that the effective storage capacity is only half of the total disk capacity because all data get written twice.
  • Software RAID 1 solutions do not always allow a hot swap of a failed disk (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 disks will be used.

RAID level 3

On RAID 3 systems, data blocks are subdivided (striped) and written in parallel on two or more drives. An additional drive stores parity information. You need at least 3 disks for a RAID 3 array.

Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data or access to data.

Advantages

  • RAID-3 provides high throughput (both read and write) for large data transfers.
  • Disk failures do not significantly slow down throughput.

Disadvantages

  • This technology is fairly complex and too resource intensive to be done in software.
  • Performance is slower for random, small I/O operations.

Ideal use

RAID 3 is not that common in prepress.

RAID level 5

RAID 5 is the most common secure RAID level. It is similar to RAID-3 except that data are transferred to disks by independent read and write operations (not in parallel). The data chunks that are written are also larger. Instead of a dedicated parity disk, parity information is spread across all the drives. You need at least 3 disks for a RAID 5 array.
A RAID 5 array can withstand a single disk 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.

Advantages

Read data transactions are very fast while write data transaction are somewhat slower (due to the parity that has to be calculated).

Disadvantages

  • Disk failures have an effect on throughput, although this is still acceptable.
  • Like RAID 3, this is complex technology.

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.

RAID level 10 – Combining RAID 0 & RAID 1

RAID 10 combines the advantages (and disadvantages) of RAID 0 and RAID 1 in one single system. It provides security by mirroring all data on a secondary set of disks (disk 3 and 4 in the drawing below) while using striping across each set of disks to speed up data transfers.

What about RAID levels 2, 4, 6 and 7?

These levels do exist but are not that common, at least not in prepress environments. This is just a simple introduction to RAID-system. 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 if 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.
8 August 2013

44 Responses to “RAID”

  1. 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?

  2. 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.

  3. 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.

  4. Pinar says:

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

  5. 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.

  6. Mark Davis says:

    What you describe as RAID 3 is really RAID 4.

    RAID 2 = bit level striping
    RAID 3 = byte level striping
    RAID 4 = block level striping
    RAID 5 = block level striping with distributed parity

  7. 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.

  8. Ramesh says:

    Raid5 5disc failure how to recovery data pl explain

  9. 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.

  10. 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.

  11. Yousefi says:

    RAID=Redundant Array of Independent Disks

  12. 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.

  13. 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.

  14. Teach says:

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

  15. joshua says:

    RAID is for pussies

  16. durga prasad says:

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

  17. Guest says:

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

  18. Vishu says:

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

  19. RAKHILESH says:

    what is raid7?

  20. 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

  21. Joe C. says:

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

  22. 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….

  23. Geekthinker says:

    It is a safeguard WHEN the storage system gets stolen

    what are you implying…

  24. James says:

    When a product specify this:

    RAID0/1/LARGE/3/CLONE/5/10

    What is LARGE and CLONE?

  25. Meathead says:

    I have a question and can’t seem to find the answer.

    I have two 500GB SATA drives, but one has 16mb cache, while the other has 8mb cache.

    Would these be okay in a RAID 0 ? Or should the drives be identical in all aspects (not just storage space)? Would the performance enhancement not be worth the bother (Compared to 2x 500GB 16mb cache)

  26. Fred says:

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

  27. 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.

  28. siddhesh says:

    nice..dude

  29. Belel says:

    Wowwwww!!!! Av got ma assignt kawai tnx palxxxx…. Thumbx up!!!!!!!!

  30. 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?

  31. boykalbo says:

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

  32. Sharda says:

    which type of RAID use in this time 5 or 5+1…

  33. 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!

  34. miguel says:

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

  35. vaporus says:

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

  36. 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.

  37. 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?

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


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