What is RAID? Know the advantages and disadvantages

RAID is a technology used to increase the performance and / or reliability of data storage. The abbreviation means  Redundant array of cheap disks or Redundant array of independent units . A RAID system consists of two or more drives working in parallel. They can be hard drives, but there is a tendency to use SSD (Solid State Drives) technology as well. There are different levels of RAID, each optimized for a specific situation. These are not standardized by an industry group or standardization committee. This explains why companies sometimes create their own numbers and implementations. This article covers the following RAID levels:

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

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The software for running RAID functionality and controlling drives can 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 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 several interfaces, including SCSI, IDE, SATA or FC (fiber channel.) There are systems that use SATA drives internally, but which have a FireWire or SCSI interface for the host system.

Sometimes the disks in a storage system are defined as JBOD, which means 'Just a bunch of records' . This means that these disks do not use a specific RAID level and act as independent disks. This is usually done for drives that contain swap files or spooled data.

Below is an overview of the most popular RAID levels:

RAID level 0 - Division

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

Benefits

  • RAID 0 offers excellent performance in both 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 a drive fails, all data on the RAID 0 array will be lost. It should not be used for mission critical systems.

Optimal use

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

If you want to use RAID 0 exclusively to combine the storage capacity of two drives into a single volume, consider mounting a drive in the other drive's folder path. This is supported on Linux, OS X and Windows and has the advantage that a single drive failure does not affect the data on the second disk or SSD drive.

RAID level 1 - Mirroring

The data is stored twice by writing it to the data unit (or set of data units) and to a mirrored unit (or set of units). If a drive fails, the controller will use the data drive or the mirror drive for data recovery and continue operation. You need at least 2 drives for a RAID 1 array.

Benefits

  • RAID 1 offers excellent read speed and write speed comparable to that of a single drive.
  • In the event that a drive fails, the data does not need to be rebuilt, it just needs 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 the total capacity of the unit, because all data is written twice.
  • Software RAID 1 solutions do not always allow a failed drive to be hot-swapped. This means that the failed drive can only be replaced after turning off the computer to which it is connected. For servers used by many people simultaneously, this may not be acceptable. These systems generally use hardware controllers that support hot swapping.

Optimal use

RAID-1 is ideal for mission-critical storage, for example, for accounting systems. It is also suitable for small servers on which only two units of data will be used.

RAID level 5

RAID 5 is the most common secure RAID level. Requires at least 3 units, but can work with up to 16. The data blocks are distributed among the units and, in one unit, a parity checksum of all data in the block is recorded. Parity data is not recorded on a fixed unit, it is spread across all units, as shown in the drawing below. Using parity data, the computer can recalculate data from one of the other data blocks, if that data is no longer available. This means that a RAID 5 array can support a single drive failure without losing data or accessing data. Although RAID 5 can be achieved in software, a hardware controller is recommended. Generally, extra cache memory is used on these controllers to improve recording performance.

Benefits

  • Data read transactions are very fast, while data write transactions are slightly slower (due to the parity that must 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 affect throughput, although this is still acceptable.
  • This is a complex technology. If one of the disks in an array using 4 TB disks fails and is replaced, restoring the data (the rebuild time) may take a day or more, depending on the load on the array and the speed of the controller. If another disk is damaged during this time, the data will be lost forever.

Optimal use

RAID 5 is a good overall system that combines efficient storage with excellent security and decent performance. It is ideal for file servers and applications that have a limited number of data units.

RAID level 6 - dual-parity stripe

RAID 6 is like RAID 5, but the parity data is written to two drives. This means that it requires at least 4 units and can support two units dying simultaneously. The chances of two units breaking down at exactly the same moment are obviously very small. However, if a drive on a RAID 5 system dies and is replaced with a new one, it takes hours or even more than a day to rebuild the replaced drive. If another unit dies during this period, you will still lose all of your data. With RAID 6, the RAID array will survive until the second failure.

Benefits

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

Disadvantages

  • Data write transactions are slower than RAID 5 due to the additional parity data that needs to be calculated. In a report, I read that the recording performance was 20% lower.
  • Drive failures affect throughput, although this is still acceptable.
  • This is a complex technology. Rebuilding an array in which a drive has failed can take a long time.

Optimal use

RAID 6 is a good overall system that combines efficient storage with excellent security and decent performance. It is preferable to RAID 5 on file servers and applications that use many large drives for data storage.

RAID level 10 - combining RAID 1 and RAID 0

It is possible to combine the advantages (and disadvantages) of RAID 0 and RAID 1 in a single system. This is a nested or hybrid RAID configuration. It provides security by mirroring all data on secondary units while using the distribution on each cluster to accelerate data transfers.

Benefits

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

Disadvantages

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

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

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

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RAID does not replace backup!

All RAID levels, except RAID 0, provide protection against a single drive failure. A RAID 6 system still survives 2 disks dying simultaneously. For complete security, you still need to back up data from a RAID system.

  • This backup is useful if all drives fail simultaneously due to a power surge.
  • It is a safeguard when the storage system is stolen.
  • Backups can be kept offsite in a different location. This can be useful if a natural disaster or fire destroys your workplace.
  • The most important reason for backing up multiple generations of data is user error. If someone accidentally deletes some important data and it goes undetected for several hours, days or weeks, a good set of backups will ensure that you can still recover those files.

To learn more, read the page on best backup policy .

reference: Prepressure

Felipe Santos
Felipe Santos is a Cloud and Security Architect, with experience in Windows Server, Cluster, Storages, Backups Veeam and Office 365 environments.
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