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Positioning and Transfer Performance

Much the way different RAID implementations vary in the degree to which they improve read and write performance, they also differ in how much they affect different types of accesses to data, both read and write. Data access to a single hard disk involves two discrete steps: first, getting the heads of the hard disk to exactly where the data is located, which I call positioning; and second, transporting the data to or from the hard disk platters, which I call transfer. Hard disk performance depends on both of these; the importance of one relative to the other depends entirely on the types of files being used and how the data is organized on the disk. For more on positioning and transfer, and related issues, see this section (if you don't understand these two concepts reasonably well, what follows may not mean as much to you as it should...)

The use of multiple hard disks in a RAID environment means that in some cases positioning or transfer performance can be improved, and sometimes both. RAID levels are often differentiated in part by the degree to which they improve either positioning or transfer performance compared to other levels; this becomes another factor to be weighed in matching the choice of RAID level to a particular application. It's important also to realize that the relative performance of positioning or transfer accesses depends also on whether you are doing reads or writes; often writes have overheads that can greatly reduce the performance of random writes by requiring additional reads and/or writes to disks that normally wouldn't be involved in a random write. See here for more details.

In general terms, here's how the basic storage techniques used in RAID vary with respect to positioning and transfer performance, for both writes and reads:

• Mirroring: During a read, only one drive is typically accessed, but the controller can use both drives to perform two independent accesses. Thus, mirroring improves positioning performance. However, once the data is found, it will be read off one drive; therefore, mirroring will not really improve sequential performance. During a write, both hard disks are used, and performance is generally worse than it would be for a single drive.
• Striping: Large files that are split into enough blocks to span every drive in the array require each drive to position to a particular spot, so positioning performance is not improved; once the heads are all in place however, data is read from all the drives at once, greatly improving transfer performance. On reads, small files that don't require reading from all the disks in the array can allow a smart controller to actually run two or more accesses in parallel (and if the files are in the same stripe block, then it will be even faster). This improves both positioning and transfer performance, though the increase in transfer performance is relatively small. Performance improvement in a striping environment also depends on stripe size and stripe width. Random writes are often degraded by the need to read all the data in a stripe to recalculate parity; this can essentially turn a random write into enough operations to make it more resemble a sequential operation.

Which is more important: positioning or transfer? This is a controversial subject in the world of PCs, and one I am not going to be able to answer for you. This discussion of ranking performance specifications is one place to start, but really, you have to look at your particular application. A simple (but often too simple) rule of thumb is that the larger the files you are working with, the more important transfer performance is; the smaller the files, the more important positioning is. I would also say that too many people overvalue the importance of greatly increasing sequential transfer rates through the use of striping. A lot of people seem to think that implementing RAID 0 is an easy way to storage nirvana, but for "average use" it may not help performance nearly as much as you might think.

Next: Stripe Width and Stripe Size

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