RAID Calculator What is RAID? RAID types reference

What is RAID?

RAID, short for the "Redundant Array of Independent Disks", is a technology to combine several hard drives into a single storage unit.

While the hard drives (called "RAID members") are relatively small, slow, and unreliable, the resulting array can be large, fast, and reliable. However note that while standalone drives are miserable but cheap, the drive array is perfect but expensive.

There are different RAID implementations, called RAID levels achieving different goals at different costs. Please read RAID types reference for an overview of these implementations.

Hardware vs. software RAID

The RAID can be implemented either using a special controller (hardware RAID), or by an operating system driver (software RAID).

  • Entry-level hardware RAID (integrated into the motherboard or a cheap RAID card) is similar in performance to the software RAID. Consider entry-level RAID hardware if the operating system does not support software RAID. Most common examples are:
    • Use fault-tolerant arrays (RAID1 or RAID5) in Windows XP or Vista.
    • Boot Windows from a RAID0 or RAID5 (this is not possible using software even in Windows Server editions).
  • Enterprise-level hardware RAID controllers are feature-rich but expensive. They have certain features not possible in software arrays and never implemented in low-cost controllers, like caching, hotswapping, and battery backup. Additionally, certain RAID levels, like RAID50 and RAID60 can only be created with high-end controllers.
  • Software RAID, implemented by the operating system driver, is the cheapest and fairly versatile option. Most modern operating systems have the software RAID capability
    • Windows uses Dynamic Disks (LDM) to implement RAID levels 0, 1, and 5. However, fault-tolerant RAID1 and RAID5 are only available in Windows Server editions.
    • Linux uses either the MD-RAID or LVM for a software RAID.
    However, there are certain limitations of a software RAID.
    • It is not possible to boot an operating system from a software RAID0 or RAID5.
    • No hotswap is possible with software alone, without a hardware support. So, software RAIDs have no hotswap.

Hard drive considerations

  • Try to use identical drives in RAID - preferrably, from the same production batch with consecutvie serial numbers. This ensures the firmware will be identical on all drives, which is good for performance. Some experts say that you should buy drives as diverse as possible instead, because they feel concerned about correlated failures. In fact, your backup should cover correlated failures. If you have no backup, you're a sitting duck regardless of your choice of the hard drives. RAID does not replace a proper backup.
  • If practical, use the drives specifically intended for use in a RAID, like Western Digital RE (RAID Edition) series. These drives have some features which are handy in a RAID but aren't available in desktop hard drives. One example is WD TLER (Time Limited Error Recovery) feature designed to prevent drives from dropping out of the array (WD whitepaper on TLER, Adobe PDF).
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