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 0 1 5 10 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.
- 1 RAID is no substitute for back-up!
- 1.1 What is RAID 0?
- 1.2 Which RAID configuration will provide fault tolerance and performance?
- 1.3 What is the RAID 6?
- 1.4 What is disk striping?
- 1.5 RAID level 0 – Striping
- 1.6 Advantages of RAID level 0 – Striping
- 1.7 Disadvantages of RAID level 0 – Striping
- 1.8 Ideal use of RAID level 0 – Striping
- 1.9 RAID level 1 – Mirroring
- 1.10 Advantages of RAID 1 – mirroring
- 1.11 Disadvantages of RAID 1 – mirroring
- 1.12 Ideal use of RAID 1 – mirroring
- 2 RAID level 5 – Striping with parity
- 3 RAID level 6 – Striping with double parity
- 4 RAID level 10 – combining RAID 1 & RAID 0
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.
raid 0 vs raid 1
What is RAID 0?
Which RAID configuration will provide fault tolerance and performance?
What is the RAID 6?
What is disk striping?
This article explains the main difference between these raid levels along with an easy to understand diagram.
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 of RAID level 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.
Disadvantages of RAID level 0 – Striping
- 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 of RAID level 0 – Striping
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.
Advantages of RAID 1 – mirroring
- 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 of RAID 1 – mirroring
- 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 of RAID 1 – mirroring
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 – Striping with parity
RAID 5 is the most widely recognized secure RAID level. It requires no less than 3 drives yet can work with up to 16. Information pieces are striped over the drives and on one drive an equality checksum of all the square information is composed. The equality information are not composed to an altered drive, they are spread over all drives, as the drawing beneath appears. Utilizing the equality information, the PC can recalculate the information of one of the other information squares, ought to those information never again be accessible. That implies a RAID 5 exhibit can withstand a solitary drive disappointment without losing information or access to information. In spite of the fact that RAID 5 can be accomplished in programming, an equipment controller is suggested. Regularly additional store memory is utilized on these controllers to enhance the compose execution.
Advantages of RAID level 5 – Striping with parity
- 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 of RAID level 5 – Striping with parity
- 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 of RAID level 5 – Striping with parity
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 resemble RAID 5, yet the equality information are composed to two drives. That implies it requires no less than 4 drives and can withstand 2 drives kicking the bucket all the while. The odds that two drives separate at the very same minute are obviously little. Be that as it may, if a drive in a RAID 5 frameworks bites the dust and is supplanted by another drive, it takes hours to reconstruct the swapped drive. On the off chance that another drive kicks the bucket amid that time, regardless you lose the greater part of your information. With RAID 6, the RAID exhibit will even survive that second disappointment.
Advantages of RAID level 6 – Striping with double parity
- 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 of RAID level 6 – Striping with double parity
- 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 of RAID level 6 – Striping with double parity
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.
Advantages of RAID level 10 – combining RAID 1 & RAID 0
- 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 of RAID level 10 – combining RAID 1 & RAID 0
- 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.
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