RAID stands for Redundant Array of Inexpensive Disks. A RAID system consists of two or more disks working in parallel. They appear as one drive to the user, and offer enhanced performance or security (or both).
The software to perform the RAID-functionality and control the hard disks can either be located on a separate controller card (a hardware RAID controller) or it can simply be a software driver. Windows NT 4, 2000, 2003 and XP all include a software RAID solution. Hardware RAID controllers cost more than pure software but they also offer better performance.
Most RAID-systems are based on SCSI, although implementations using IDE or SATA disks or FC (fibre channel) disks also exist. There are even systems that use IDE disks internally but that have a SCSI-interface for the host system.
There are different RAID levels, each suiting specific situations. RAID levels are not standardised by an industry group. This explains why companies are sometimes creative and come up with their own unique implementations.
Sometimes disks in a RAID 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 are used as if they were stand-alone disks. This is often done for disks that contain swap files or spooling data.
Most modern SATA based motherboards now support RAID in their BIOS as well so you can get away without a dedicated RAID controller but if your motherboard fails then you'll need another motherboard of the same model to recover your data.
Below is an overview of the most popular levels:
RAID 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, RAID 0 offers superior I/O performance. This performance can be enhanced further by using multiple controllers, ideally one controller per disk.
RAID 0 offers great performance, both in read and write operations. There is no overhead caused by parity controls.
All storage capacity can be used, there is no disk overhead.
The technology is easy to implement.
RAID 0 is not fault-tolerant. If one disk fails, all data in the RAID 0 array are lost. It should not be used on mission-critical systems.
RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, e.g. on a PhotoShop image retouching station.
RAID 1: mirroring
Data are stored twice by writing them to both the data disk (or set of data disks) and a mirror disk (or set of disks). If a disk fails, the controller uses either the data drive or the mirror drive for data recovery and continues operation. You need at least 2 disks for a RAID 1 array.
RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.
RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single disk.
In case a disk fails, data do not have to be rebuild, they just have to be copied to the replacement disk.
RAID 1 is a very simple technology.
The main disadvantage is that the effective storage capacity is only half of the total disk capacity because all data get written twice.
Software RAID 1 solutions do not always allow a hot swap of a failed disk (meaning it cannot be replaced while the server keeps running). Ideally a hardware controller is used.
Beware that different RAID controller cards are not usually compatible with each other so if your RAID card fails then you may not be able to recover ANY data unless you have a replacement card.
RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two disks will be used.
On RAID 3 systems, data blocks are subdivided (striped) and written in parallel on two or more drives. An additional drive stores parity information. You need at least 3 disks for a RAID 3 array.
Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data or access to data.
RAID-3 provides high throughput (both read and write) for large data transfers.
Disk failures do not significantly slow down throughput.
This technology is fairly complex and too resource intensive to be done in software.
Performance is slower for random, small I/O operations.
RAID 5 is the most common secure RAID level. It is similar to RAID-3 except that data are transferred to disks by independent read and write operations (not in parallel). The data chunks that are written are also larger. Instead of a dedicated parity disk, parity information is spread across all the drives. You need at least 3 disks for a RAID 5 array.
A RAID 5 array can withstand a single disk 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 transaction are somewhat slower (due to the parity that has to be calculated).
Disk failures have an effect on throughput, although this is still acceptable.
Like RAID 3, this is complex technology.
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.
RAID 10: a mix of RAID 0 RAID 1
RAID 10 combines the advantages (and disadvantages) of RAID 0 and RAID 1 in a single system. It provides security by mirroring all data on a secondary set of disks (disk 3 and 4 in the drawing below) while using striping across each set of disks to speed up data transfers.
What about RAID 2,4,6 or 7?
These levels do exist but are not that common. This is just a simple introduction to RAID-system. You can find more in-depth information on the pages of ACNC or storage.com.
Integrated RAID in the Intel SE7221BK1-E server board
The RAID integrated into the Intel SE7221BK1-E server board can be accessed by (a) turning on RAID in the main system BIOS (accessed by pressing the F2 key when booting) and (b) configuring the RAID controller by pressing CTRL/E when the prompt to do so is displayed. The settings in this RAID BIOS do not appear to change anything on the actual disks - they are merely BIOS settings and so can be changed, cleared and changed back with no problems with the disks.
When one disk has failed and you replace it with a new disk you just need to clear the configuration and use the EASY CONFIGURATION wizard to recreate a new RAID 1 array. Once you have done this you need to mark the new disk as either READY or FAILED and then select the REBUILD option. When you select REBUILD you need to select the disk to rebuild and press the space bar and then the rebuild will start. On 200Gb seagate disks the rebuild takes around 45 minutes.
We have noticed that when one disk has failed the system runs VERY slow and lots of errors 9which we initially blamed windows for) occur. When you reboot the system it says that the disks are in a degraded state and says that it is rebuilding them. However if you just let the boot=up continue it DOES NOT rebuild them and the problems continue. To correct the problems you need to use the CTRL/E option on reboot and select the REBUILD option in the RAID BIOS and wait the 45mins or so while they rebuild and then reboot again.
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© 2003 L. Leurs