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Group Test: NAS Drives
Staff Writers, James Morris, | Oct 19, 2015

Choosing the right RAID level

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Choose the right RAID level 

The RAID capabilities in these NAS appliances combine your physical disks to deliver better performance or greater reliability – or a balance of the two. But it’s important to choose the right configuration for your needs.

RAID0: striping
With RAID0 – the simplest RAID “level” – when you save a file to your NAS, data is written to all disks simultaneously, a process called “striping”. In essence, using two drives in parallel provides twice the read and write performance of a single drive, and on a four-bay device speed can be increased further by using even more drives.

Because of the way striping works, all disks must be the same size. And a striped array can’t normally be expanded after it’s been set up, as this would require previously written data to be redistributed.

The biggest problem with striping, however, is that it’s highly vulnerable to hardware failure. A two-disk system is twice as likely as a single drive to experience a failure in a given period, and if any one disk in a striped array fails, the entire contents are lost.

RAID1: mirroring
RAID1 is the best choice when the safety of data is paramount. A complete copy of your data is written to each disk, figuratively making them all “mirror images”.

Like striping, mirroring assumes disks of identical sizes, but the total storage available represents only the capacity of a single one, and performance is no faster than for a single disk. But when one of your disks fails, mirroring comes into its own: the broken disk can simply be taken offline, and you can continue working as usual. You can even replace the failed drive with a new one and let the controller rebuild the array, restoring your original level of safety.

RAID10: striping and mirroring
RAID0 and 1 suit different uses, but you can combine their advantages on a four-drive NAS device by “nesting” RAID levels. In RAID10, your four disks are set up as two mirrored arrays, and data is striped across them. This strikes a good balance of speed and safety: you get twice the speed and performance of a single drive, but if one drive fails then your data isn’t lost.

RAID5 and RAID6: parity
Another option with more than two drives is RAID5. A four-disk RAID5 array offers the capacity of three disks, and uses the rest of the space to store parity blocks – check digits that are mathematically derived from the contents of the other three disks. These blocks make it possible to recover your data should any single disk fail, while at the same time – since the system can read from and write to all disks simultaneously – you get the speed benefits of striping. RAID6 adds a second parity disk, so two simultaneous disk failures can be accommodated. Since a more sophisticated parity system is used, data corruption can also be detected and corrected. For a four-bay device you might as well use RAID10, but RAID6 gets more efficient in larger arrays: in a 12-bay unit you could use 12 1TB disks to create a 10TB RAID6 array with two levels of redundancy, while the same disks in a RAID10 configuration would yield only 6TB of storage.

What about levels 2 to 4?
The original RAID specification, published in 1987, did specify RAID2, RAID3 and RAID4, but these have proved surplus to requirements. RAID2 used a more complex parity system than RAID5, letting you mathematically determine which disk had failed; since modern disk controllers detect drive failures automatically, there’s now no need for this.
Meanwhile, RAID levels 3 and 4 combine several striped drives with a single parity disk. This means you get the benefit of striped speeds when reading data, but the write speed of the array is bottlenecked by the speed at which the parity disk can be written to. Consequently, RAID3 and RAID4 aren’t widely used.


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