[IMGCAP(1)]When I first started writing this post, it was going to be on Network Attached Storage (NAS). That’s a topic of which every network user should have at least a basic knowledge. But when I started the post, I realized that BAS really doesn’t make a lot of sense beyond having a huge amount of disk space on the network unless you understand R.A.I.D.
Because R.A.I.D, not huge amounts of available storage, is the underlying reason that NAS is something all business (and many home) network administrators should understand, and implement.
[IMGCAP(2)]R.A.I.D. is one of those technologies that gets brushed off with “it increases storage space” or “it backs up your data”. It can do either or both, but what R.A.I.D. really consists of are ways that multiple disk drives can be configured.
First off, R.A.I.D. is an acronym. Ask three people what it means, though, and you’ll probably get three different answers. For the purposes of this and future posts, I’m going to go with “Redundant Array of Individual Disks” and leave out the periods—RAID. The two operative words in the acronym as far as we’re concerned are array and redundant. “Array” is important because you can’t RAID just a single drive, you need at least two.
And “Redundant” is important because it’s the entire point of RAID.
Leveling the Field
When you see the term RAID, it’s usually accompanied by a level, such as RAID 0, RAID 1, RAID 5, or RAID 10. That level number describes the way that the array is configured. Setting up a RAID array is usually done at the drive controller (hardware) level with a RAID controller on the computer’s motherboard, or an external RAID controller plugged into an expansion slot.
Sometimes you’ll see the letter jumble “JBOD”. That stands for “Just a Bunch of Disks” and is not really a RAID array, just two or more drives that are spanned by the controller to look like a single contiguous drive. With JBOD, when the first drive fills up, the controller continues the volume into the next one, and so on. JBOD is not a really good way to set up multiple drives, as if one drive fails, it may be difficult, or impossible to recover the data from the bad drive. If you have a file written across two drives and one drive goes bad, that file is likely toast.
RAID Level 0, usually just expressed as RAID 0, is an actual array, and requires two or more physical drives. There can be an even or an odd number of drives in a RAID 0 array. Unfortunately, a RAID 0 configuration does not provide redundancy by itself. With RAID 0, the controller writes a block of data on one drive, the next block of data on a second drive, and if there are drives beyond the two, blocks of data will be written sequentially on those until the controller shifts back to the first drive. This results essentially in converting the drives into a single drive, since a large enough file can span several drives. The process or writing blocks of data onto sequential drives is called striping, and the big advantage of this type of RAID array is that one drive can be read while another is being written, which speeds up drive access times, sometimes considerably. RAID 0 suffers from the same problem as JBOD—if a drive fails, chances are you’ll lose significant data.
RAID 1 and RAID 5 are the two most frequently used arrays in business computing. RAID 1 requires an even number of drives, and writes the exact same data, at the exact same time, onto two drives. This is called mirroring, and if one of the paired drives fails, the data on the other drive is safe. Sometimes drives are combined in an array where striped drives are also mirrored (which requires a minimum of four disks). The array is called RAID 0 + 1, or more commonly, RAID 10.
RAID 5 is where we really start to get into redundancy, and requires a minimum of three drives in the array. Information about the data files is stored on all three drives. With a RAID 5 array, if one of the drives fails, the stored parity information can be used to rebuild the entire array. RAID 6 extends this by storing parity information across four (or more) drives, and provides the ability to rebuild the array if two of the drives fail simultaneously. RAID 5 and RAID 6 provide redundancy of data, and an effective fall-back as long it’s the drive that fails and not the controller. We’ll look at the controller issue as well as choosing what level of RAID to implement next time when we discuss Network Attached Storage.
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