RAID is single contiguous data storage of large size built up from several hard drives, called “member disks”. The goal of RAID is to combine several slow and unreliable hard drives into a single fast and reliable storage.
There are several variants of RAID which differ on how member disks are grouped into a single storage space. Most well-known array types (also called “layouts”) are RAID0, RAID1, RAID5, and RAID6.
RAID can be created either using hardware (hardware RAID) or with a driver of an operating system (software RAID). There are some limitations on RAID creation applied to both hardware and software implementations of RAID. For example, software RAID5 cannot be bootable while hardware RAID6 is not implemented in low-end RAID controllers.
Many off-the-shelf storage systems have some RAID built-in. This includes multi-disk NAS (Network Attached Storage) devices, like Seagate BlackArmor, Buffalo Terastation, and also some high-end Direct Attached Storage (DAS), like Drobo. If you purchase one of these, there is no need to bother with RAID setup. However, you should be aware of the RAID level used in your device. This way, you know what fault tolerance your device is capable of and what to expect from it when a drive fails.
The main RAID characteristics are:
- The number of member disks,
- Array capacity,
- Array performance in terms of read/write speed,
- Fault tolerance.
All these parameters directly depend on the type of the array. One should choose the array type according to a particular task and hardware/software capabilities.
Before building a RAID, you can estimate what you will get by calculating the parameters above. To do this you can try any of online RAID calculators like www.raid-calculator.com or www.z-a-recovery.com/art-raid-estimator.htm. These free calculators predict the exact array capacity, tolerance to hard drive failures, and approximate read and write speed as well. Based on the predictions, you can decide which option is right for you or at least discard those that don’t suit you. Be warned, though, that sometimes RAID speed predictions do not match the reality. General rules like “the more hard drives, the more speed” and “the more redundancy, the less speed” do hold, but specific performance gains are often overestimated. Also, the performance depends greatly on the specific pattern of disk activity. The most notorious example is “RAID5 and RAID6 do not go well with random writes.”
Besides the standard RAID characteristics there are some less known ones to estimate which you can use another calculator at www.raid-failure.com. This free online calculator gives you the following:
- Probability of RAID0 failure during five years of operating,
- Probability that a RAID 5 completes rebuild successfully,
- Probability to survive a multiple disk failure for RAID10/50/60 arrays.
Thus, long before the creation of your array, you can decide what array layout is more suitable for you, how many disks are needed, what performance you will get and also estimate what to expect from the array as far as failures are concerned.
This article is written by www.ReclaiMe.com
