Drobo, Intel, Netgear, and Synology all offer what is frequently referred to as abstracted or virtual RAID. You’ll even find a form of abstracted RAID in Windows 7, and Windows 8 Spaces takes the idea even further.
“Abstracted” means that instead of using physical disks as the building blocks of an array, this arrangement employs virtual volumes (or “virtual disks,” in Microsoft’s parlance). Virtual volumes are handy in that they might take up only part of a disk, or they can expand across multiple disks.
For instance, you could have a virtual volume that consumes all of one 500GB disk and half of a 1TB disk. You would then have a second virtual volume that uses the remaining 500GB on the 1TB drive. The RAID software manages them behind the scenes, and they appear as a single storage unit to the user, if so desired.
Abstracted RAID allows you to mix different-capacity drives and varying levels of fault tolerance, as well as to expand capacity automatically without user intervention. Without it, changing RAID levels involves backing up all the data, reconfiguring, and then copying the data back—a time-consuming and sometimes technically challenging activity.
RAID arrays sometimes employ a hot spare, which is simply an extra disk preinstalled in the NAS box or system that serves to replace a failed disk. This setup allows the rebuilding of the array to proceed automatically without user intervention.
You’ll encounter three other RAID options that can be useful. They aren’t often found in consumer-level RAID boxes, though they are present in some business-oriented NAS boxes.
RAID 6 is very much like RAID 5: It has distributed parity info, as RAID 5 has, but it also has twice as much of it, which means that RAID 6 can withstand the loss of two drives. With RAID 6, a second set of parity information is distributed across the drives—to the obvious detriment of total capacity. Nevertheless, in situations where you need the highest level of fault tolerance, RAID 6 is a good choice.
RAID 10, also referred to as RAID 1 + 0, stripes data (RAID 0) across mirrored pairs (RAID 1) of drives. With this arrangement, you get back some of the write speed that RAID 1 can cost you—but you need at least four drives to implement it, and 50 percent of the total drive capacity becomes devoted to redundancy.
Conversely, RAID 0 + 1 mirrors (RAID 1) striped pairs (RAID 0) of drives. As with RAID 10, you regain some of the write speed that RAID 1 can cost you. Again, you need at least four drives, and you spend 50 percent of the total drive capacity on redundancy.
Other RAID Options
The RAID specification includes several other levels aside from the ones addressed above; however, these are not commonly used anymore.
RAID 2 distributes data across multiple drives at the bit level (the smallest unit of computer information with a value of either 0 or 1) instead of at the block level. RAID 2 writes Hamming ECC (error-correcting code) recovery information to dedicated parity disks at the byte level, which requires a lot of processing power.
RAID 3 is another mode that got kicked off the consumer island because it doesn’t use data blocks; it distributes data across multiple drives as bytes (8 bits), and like RAID 2 it stores parity information on a dedicated drive.
RAID 4 fell into disuse because it distributes data across multiple drives as blocks and stores all parity information on dedicated parity drives; if a dedicated parity drive fails, the entire array remains unprotected until it’s replaced and the information is reconstructed. This weakness is also inherent in RAID 2 and 3.
Choosing RAID: A Cheat Sheet
Trying to determine which RAID level is best for you? Here’s our take.
- First off, use hardware RAID over software RAID when you have a choice. Software RAID is fast, but many implementations tend to rebuild at the drop of a hat, reducing performance while in progress.
- Use RAID 0 when all you want is faster performance with large files, and you don’t need fault tolerance. (But be sure to back up your drives regularly.)
- Use RAID 1 when you have only two drives and you want to protect against drive failure.
- Use RAID 5 when you have more than two drives and you want a hedge against drive failure.
- Use abstracted RAID to derive maximum storage from a collection of drives of different capacities.
Editor’s note: This is an update to a story that we originally published April 15, 2010. This version has the most current information on RAID technology.