Hard drive buying guide
- — 22 July, 2009 13:20
- How do hard drives work?
- What is a solid-state drive?
- USB 2.0
- Network-attached storage
- Form factors
- Internal or external?
- Hard drive specifications
- Disk cache
- Native command queuing (NCQ)
- Seek/access time
- Buying the right hard drive
- File systems, fragmentation and slow downs
There are several methods of combining multiple hard drives into a single volume. RAID — standing for Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks — lets users boost drive speed or help to prevent data loss in the event of one or more drive failures. Some basic RAID features are supported by motherboards and multiple-drive external hard drives or NAS devices, but you may need to purchase a hardware RAID card.
RAID is entirely a function of the disk controller; a hard disk does not need to actively "support" RAID in order to work in an array. There are several different levels of RAID, each serving different needs.
RAID 0, called striping, is where part of a file is stored on each disk. When the data is read, it is read in parallel from the disks in the RAID. Because each disk has to do less work, the overall speed of the file transfer is greater. RAID 0 can give you significant performance improvements. The capacity of a RAID 0 array is equal to the equivalent of the number of drives in the array multiplied by the capacity of the smallest drive. In other words, each drive has the same formatted capacity. The drawback to RAID 0 is that if one drive dies then all the data in the array is lost, so it is rarely used for vital information.
RAID 1, or mirroring, replicates all the data across all the drives in the array. This gives you reliability and speed (since you can still read data in parallel off multiple drives), but reduces your capacity. Two 200GB drives in a RAID 1 array, for example, would offer a total capacity of only 200GB, because all of the data is replicated. If one of the drives dies, then no data is lost (because all the data is still on the other). RAID 1 provides speed and reliability but comes at a significant cost in capacity.
To get the best of both RAID 0 and RAID 1, a third RAID level, RAID 1+0, is used. The array, which requires four drives or more, involves mirroring the data written in pairs, so that two of the drives in the array are striped in RAID 0 format, while also having a mirror of the data similar to RAID 1. Since this requires a comparatively large number of drives, it is relatively uncommon in desktop PCs.
The other common RAID formats — RAID levels 5 and 6 — are extensions of RAID levels 0 and 1 across arrays of varying sizes. These are available on 4-8 bay NAS devices.
One other method of combining multiple hard drives into a single volume is through a process dubbed concatenation. This method, referred to as a JBOD (Just a Bunch of Disks) array, simply tells the operating system that the multiple drives are one volume, even though they aren't. This method does not provide any striping or mirroring; data is simply to the drives in sequential order. It will fill the first drive before moving onto the next.