What is RAID ?

RAID = Redundant Array of Inexpensive Disks

Mechanical hard drives are slow. To improve performance we can put multiple hard-drives to work as one. This technique is called RAID and is used for servers. There are multiple types of RAID configurations optimized for speed, safety or a balance between the two.

Backup on RAID

A backup disk must be a single disk on Windows. Sometimes you can use 2 independent disks for alternative backups. For windows NT you can specify several disks for backup that are automatically used by the system and do not have assigned a letter like the other disks.

We do not recommend RAID array for desktop computers but only for servers or workstations. One large HDD can be superior to a bad RAID array.

RAID Inconvenience

  • For RAID you use several disks that can make computer more heavy;
  • You need a computer case with 4 or more HDD locations to create a RAID;
  • RAID consume more power then a single disk in a computer;
  • You should not use SSD for any RAID other then RAID0.

There are 2 ways to create a RAID array:

Software RAID and hardware RAID. For hardware RAID you need a specific card that is to control several disks. New PC motherboards have support for RAID using SATA controller. Software RAID is available as a feature of the operating system.

RAID Parameters

There are 2 important parameters you have to consider when you decide what kind of RAID to create:

  • RAID LEVEL
  • CHUNK SIZE

CHUNK SIZE

To get good performance you must have a reasonable chunk size.

For video editing: You want a lot of bandwidth, so you want a small chunk size. Anywhere from 512 bytes (one block) to 8 KB is a small chunk size. This will allow multiple disks to contribute to data stream at once increasing the data bandwidth.

For a database: You want to maximize your IOPS, which ideally means sending each I/O to only one disk. So you want a large chunk size – at least 64 KB or more. That large chunk will mean that most I/Os get serviced by a single disk and more I/Os are available on  the remaining disks.

RAID LEVELS

RAID capacity, performance and reliability are in balance. Depending on your goal one kind of RAID is better then another. Here are the most important RAID flavours:

JBOD

Just a bunch of disks (JBOD) is known as “spanned volume” on Windows. It is the best choice for a desktop PC. It increase the size of the volume and keep the performance at maximum. This is the most suitable RAID for a beginner who has doo many disks in his computer.

RAID 0

Known as “striped volume” on Windows. RAID 0 is good for speed, you can use two or more hard drives.The storage capacity grow linear with the number of drives. On Linux RAID 0 can be created using MD-RAID, LVM or BTRFS.

The major limitation of RAID 0 compared to other RAID levels is that does not offer any redundancy. RAID 0 can be used to increase storage capacity, sequential read and sequential write.

RAID 1

Known as “mirrored volume” for Windows, RAID 1 is good for data safety and it increase the reading speed but you can use only 2 hard drives. RAID 1 offer 100% redundancy but will not increase your writing speed or disk space.

This is available only on Windows professional edition and Windows server edition. It is not available on Windows home edition.

Note: RAID chunk size do not have effect on performance for RAID 1.

RAID 5

RAID 5 is using 3 or more hard drives and has a special algorithm to distribute data across these disks. If one of the disk fail it can be replaced and array is rebuild by the system. The tricky part is to know which hard disk has failed. If you replace the wrong hard drive you will probably loose all the data.

RAID 6

RAID 6 is using 4 or more hard drives and is using double redundancy. So if any disk fail you can replace the disk but if you replace the wrong disk data is not lost. This RAID is using some more processing power to write and read from the HDD due to redundancy algorithm. This is the most used RAID in enterprise class NAS or server storage.

RAID 10

A server can use up to 4 disks to create RAID 10. The speed for write will be up to 2x faster then a single disk while the speed for reading is 4x faster. RAID 10 can use 4, 6 or 8 HDD for storing videos and pictures.

RAID 10 is not good for servers but is very good for workstations or home servers.