Raid levels

 

Raid levels 

RAID is a technology that is used to increase the performance and/or reliability of data storage.

A RAID system consists of two or more drives working in parallel. These 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.

RAID 1 is a very simple technology.

The type of 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

RAID level 0 - Striping

RAID O system data is split up into 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/0 performance. This performance can be enhanced further by using multiple controllers, ideally one controller perdisk.

Advantageous of RAID 0 

RAID O 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 0 

RAID O is not fault-tolerant. If one drive fails, all data in the RAID O array is lost.

It shouldn't be used for mission-critical systems.

Disadvantages of RAID 1

RAID level 1 - Mirroring

Data is 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 continuous operation. You need at least 2 drives for a RAID 1 array.

Advantages of RAID 1 

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 rebuilt, they just have to be copied to the replacement drive.

Disadvantages of RAID 1

The main disadvantage is that the effective storage capacity is only half of the total drive capacity because all data gets written twice.

The software of RAID 1 solutions does 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.

Advances of RAID 5 

RAID level 5 - Striping with parity

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 stripped across the drives and on one drive a parity checksum of all the block data is written. The parity data are not written to a fixed drive, they are spread across all drives by using the parity data, the computer can recalculate the data of one of the other data blocks, should that 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.

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

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 5 

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 is lost forever.

Advances of RAID 6

RAID level 6 - Striping with double parity

RAID 6 is like RAID 5, but the parity data is written 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 system dies and is replaced by a new drive, it takes hours or even more than a day to rebuild.

The swapped drive if another drive dies  during that time you lose all of  your data with RAID 6 the RAID array will even survive an second failure.

Advantages of RAID 6

Like with RAID 5, read data transactions are very fast.

If two drives were to fail, you will still have access to all data, even while the failed drives are being replaced.

RAID 6 is more secure than RAID 5.

Disadvantages of RAID 6

Write data transactions are slower than RAID 5 due to the additional parity data that have to be

calculated. In one report I read the write performance was 20% lower.

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.

RAID level 10 - combining RAID 1 & RAID O

It is possible to combine the advantages (and disadvantages) of RAID O 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 10

If something was to 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 process can take as little as 30 minutes for drives of 1 TB.

Disadvantages of RAID 10

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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