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Software vs. Hardware RAID

Software RAID

In general, both Striping and Mirroring, that is Level 0 or Level 1 RAID, respectively, can be achieved in software alone as long as the operating system supports it. This type of RAID is also called zero-channel RAID (because no dedicated hardware channels are involved)

Hardware RAID

Whether any given RAID solution is Hardware or Software often boils down to a matter of semantics. That is, by definition, any dedicated hardware RAID channels will qualify the array as Hardware RAID. However, often enough another distinction is made based on the presence or absence of a dedicated XOR processor. That is, any hardware RAID controller without the additional controller will often be referred to as software RAID, in other words, there is a grayzone. We will cover the significance of the XOR operations a bit later, for now, let’s start with the “lesser” levels of RAID, that is Level 0, 1, 0+1 and JBOD.

JBOD and Spanning

To begin with the easiest, JBOD, short for Just a Bunch Of Drives combines several physical devices to make it look like one single logical device. In other words, the total data capacity of all drives spans across all physical devices. JBOD does not care about the capacity of the individual drives, there does not need to be any match, however, since the HDD controller usually negotiates the ATA protocol with the different devices attached during the initialization sequence, the slowest drive defines the overall interface speed.

RAID Level0 and Striping

The easiest way to increase HDD performance is to use striping, that is distributed storing of data blocks over several drives. In practice, data are split into chunks of predefined block size that are written alternately to the different drives that make up the array. The strategy is to use the cache as buffer and then overcome the slower media transfer rate by doubling the number of drives working on the same transfers in tandem. In addition, the burst transfer rate is increased, with the caveat that this only applies if the drives are on physically separate cables.

Striping distributes all files on a per block basis. Usually the striping block size is defined in the RAID setup utility and as rule of thumb, smaller block sizes improve performance on random accesses whereas larger block sizes such as 32 kB (64 LBAs) to 128 kB (256 LBAs) will result in better performance for streaming media as it is required e.g. in video editing. Serial ATA with its lower command overhead has chnaged this rule of thumb somewhat in that smaller block sizes almost generally give better performance than larger striping blocks. Empirically we have found that for almost all applications a stripe block size of 16 kB gives the best performance. The drawing was done in adaptation of a similar schematic on the PCGuide.

RAID Level0 has a number of drawbacks. First of all, the lowest capacity drive defines the total density of the striped array. Second, Level0 has no fault tolerance, that is, there is no redundancy built into the array and if one drive fails, the entire array will lose the data. Because the statistical probability of a drive failure is independent of any other drives in the system, the probability for a RAID Level0 failure increases as a direct function of the number of devices used. Also keep in mind that RAID Level0 is not a RAID in the true level of the word since there is no redundancy as in “R” offered at all – which makes the term RAID Level0 some kind of a paradox.

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