What is RAID?
RAID is a stands for Redundant Array of Independence disks. In fact, RAID is a way that combines many independent and relatively small disks into a single large-volume volume. Disks included in the array are called array members. Disks can be merged into the array in different ways known as RAID levels. Each RAID level has its own characteristics
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What is RAID? |
Fault tolerance which is the ability to stay from one or several disk failures.
Performance shows the change in read and write speed for the entire array compared to a single disk.
The ability of an array to be determined by the amount of user data that can be written to the array. The array capacity depends on the RAID level and does not always match the sum of the RAID member disk sizes. To calculate the capacity of a particular RAID type and set of member disks, you can use a free online RAID calculator. This article covers the following RAID levels:
- RAID 0 – Strip
- RAID 1 – Mirroring
- RAID 5 – Stripe with parity
- RAID 6 – Tape with double parity
- RAID 10 – Merge mirroring and strips
A software that performs RAID function and drive control can either be on a separate controller card (hardware RAID controller) or it can be just a driver. Some versions of Windows, such as Windows Server 2012 and 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 (Fiber Channel). There are systems that use SATA drives internally, but have a FireWire or SCSI interface for the host system.
Sometimes, disks in the storage system are defined as JBOD, which means “set of disks”. This means that these disks do not use a certain RAID level and function as stand-alone disks. This is often done for drives that contain swap files or spool data.
Here’s an overview of the most common RAID levels
RAID Level 0 – Striping
In RAID 0, the system data is divided into blocks that are written across all drives in the array. Using multiple disks (at least 2) at the same time, this provides superior I / O performance. This performance can be further improved by using multiple controllers, ideally one controller per disk.
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Raid 0 – Strip |
Advantages
RAID 0 provides great performance, both in read and write operations. There is no load due to equivalence 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 array 0 is lost. It should not be used for mission systems.Ideal use
RAID 0 is ideal for indecisive storage of data to be read / written at high speed, such as image retouching or video editing station.If you want to use purely RAID 0 to consolidate the storage capacity of twee drives into one volume, consider installing one drive in the path of the other drive folder. This is supported in Linux and OS X as well as Windows and has the advantage that failure of one drive has no effect on the data of the second disk or SSD drive.
RAID Level 1 – Mirroring
Data is stored twice by typing it on both the data drive (or set of data drives) and the mirror drive (or set of drives). If the drive fails, the console uses either the data drive or the mirror drive to restore the data, and the playback continues. You need at least two drives for a RAID 1 array.
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Raid 1 – Mirroring |
Advantages
RAID 1 provides excellent read speed and write speed similar to that of a single drive.
If the drive fails, you do not have to re-create the data; you should only copy it to the alternate drive.
RAID 1 is a very simple technique.
Disadvantages
The main disadvantage is that effective storage is only half the total drive capacity because all data is written twice.
RAID 1 software solutions do not always allow a hot swap of a failed drive. This means that the failed drive can only be replaced after the computer that is connected to it is turned off. For servers that many people use simultaneously, this may not be acceptable. These systems typically use hardware controllers that support fast exchange.
Ideal Use
RAID-1 is ideal for mission critical storage, for example for accounting systems. It is also suitable for small servers where only two data engines will be used.
RAID Level 5 – Stripe with parity
RAID 5 is the most common secure RAID level. Requires at least 3 drives but can work with up to 16. Data blocks are grouped across drives and on a single drive, parity checksum is written for all cluster data. Parity data is not written to a hard drive, it is distributed on all drives, as the diagram below shows. With parity data, your computer can recalculate data from one of the other data blocks, if that data is not available. This means that a RAID 5 array can withstand a single drive failure without data loss or data access. Although RAID 5 can be achieved in the software, it is recommended to use a hardware controller. Additional cache on these controllers is often used to improve write performance.
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Raid 5 – Stripe with parity |
Advantages
Data read parameters are very fast while data write parameters are somewhat slower (because of the parity to be calculated).
If the drive fails, you can still access all the data, even while the failed drive is replaced and the storage controller recreates the data on the new drive.
Disadvantages
Drive failure has an impact on productivity, although this is still acceptable.
This is complex technology. If a disk in an array with 4 TB disks fails and is replaced, data recovery (rebuild time) may take a day or more, depending on the load on the array and the speed of the console. If another disk worsens during that time, the data will be lost forever.
Ideal use
RAID 5 is a good comprehensive system that combines efficient storage, excellent security and decent performance. It is ideal for file servers and applications with a limited number of data drives.
RAID Level 6 – Stips with dual parity
RAID 6 is similar to RAID 5, but parity data is written on two drives. This means that it requires at least 4 drives and can withstand the death of two drives simultaneously. The chances of two drives crashing at the exact same moment are, of course, very small. However, if a drive dies in RAID 5 systems and is replaced by a new drive, it takes hours or even more than a day to re-create the swapped drive. If another drive dies during that time, you can still lose all your data. With RAID 6, the RAID array will survive the second failure.
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Raid 6 – Strips with double parity |
Advantages
As with RAID 5, reading data parameters is very fast.
If two drives fail, you can still access all data, even while replacing failed drives. So RAID 6 is more secure than RAID
Disadvantages
Data write transactions are slower than RAID 5 because additional parity data must be calculated. In one report I read that the writing performance was 20% lower.
Drive failure has an impact on productivity, although this is still acceptable.
This is complex technology. Rebuilding a group in which a single drive fails may take a long time.
Ideal use
RAID 6 is a good comprehensive system that combines efficient storage, excellent security and decent performance. RAID 5 is preferred on file servers and applications that use many large drives to store data.
It is possible to combine the advantages and disadvantages of RAID 0 and RAID 1 in one system. This is a nested or mixed RAID configuration. Provides security by reversing all data on secondary drives while using tape across each set of drives to speed up data transfers.
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Raid 10 – Merge mirroring and strips |
Advantages
If something goes wrong with one of the disks in the RAID 10 configuration, the rebuild time is very fast because all that is needed is to copy all the data from the remaining mirror to a new drive. This can take less than 30 minutes for drives of 1 TB.
Disadvantages
Half of the storage goes to mirroring, so compared to large RAID 5 or RAID 6 arrays, this is an expensive way to get redundancy.
What about RAID levels 2, 3, 4 and 7?
These levels exist but are not common (RAID 3 is essentially similar to RAID 5 but with parity data that is always written on the same drive). This is just a simple introduction to RAID systems. You can find more in-depth information on Wikipedia or ACNC pages.
RAID is not a replacement for backup!
All RAID levels except RAID 0 provide crash protection in a single drive. RAID 6 system even survived two discs simultaneously die. For complete security, you still need to back up your data from the RAID system.
This backup will be useful if all drives simultaneously fail due to increased power.
It is protection when the storage system is stolen.
Off-site backups can be kept in a different location. This can be helpful if it is a natural disaster or a fire that destroys your workplace.
The most important reason to back up multiple generations of data is user error. If someone accidentally deletes some important data unnoticed for several hours, days, or weeks, a good set of backups ensures that you can still recover these files.