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Stripe Size Discussion

RAID Scaling Charts, Part 3: 4-128 kB Stripes Compared

Before talking about stripe size, it is important to mention the term stripe width, which equals the number of drives in a RAID array: for example, five drives equal a stripe width of five. Obviously, this value only changes if you alter the number of drives used in a RAID array; the more you add, the faster your RAID array will be, until you hit a controller or system interface bottleneck. The stripe size is the storage capacity for each individual hard drive in a RAID array, which the user can define with most RAID controllers. Be careful here: stripe size does not represent the storage capacity of an entire stripe set spanning over all used drives. A stripe set consists of all individual stripes as a whole, each of them having the selected stripe size.

A stripe is the smallest chunk of data within a RAID array that can be addressed. People often also refer to this as granularity or block size. It can be compared to the blocks (logical block addressing - LBA) on conventional hard drives. Most RAID controllers allow the user to define her or his favorite stripe size, because it alters the performance characteristics of a RAID array. Typical options are 16, 32, 64 and 128 kB, but many professional RAID controllers also offer smaller stripe sizes, and some even support sizes as large as 256 kB.

The option to alter stripe sizes with on-board RAID controllers such as the Intel Southbridges ICH7R, ICH8R and ICH9R does not make much of a difference in the desktop space. In fact, this feature is often overrated, as the performance changes are marginal on a typical gaming or office PC. However, it can be important for servers, though differences will still only be noticeable between very small and very large stripes with certain applications. A change from 32 to 64 kB can be measured, but isn’t typically noticeable.

In general, your stripe size choice has an impact on several factors:

  • Performance
    Conventional hard drives deliver their best transfer performance when they read or write sequentially, repositioning the heads as little as possible. From this standpoint, it makes the most sense to select the largest stripe size available, especially if your hard drives are good at providing high throughput. However, this only works if the files stored or read are at least as large as an entire stripe. If you will end of storing millions of text files, Word documents, small spreadsheets or similar small files, small stripe sizes will help to distribute all files across multiple drives to keep throughput high.
  • Capacity Used
    The stripe size also defines the amount of storage capacity that will at least be occupied on a RAID partition when you write a file. For example, if you selected a 64 kB stripe size and you store a 2 kB text file, this file will occupy 64 kB. Obviously, the stripe size defines the minimum amount of data that a RAID controller will distribute files across its hard drives, in my example, 64 kB. As long as a file can be written onto a single stripe or a single drive, you won’t have any advantage from running this particular RAID array.

If you want to create a RAID array for quick access to small files, small stripe sizes are favorable, to keep the waste of storage capacity small, and to provide high throughput thanks to a high level of data distribution across many drives. File servers for photos, audio and video should, however, be operated with larger stripe sizes, as this helps to maximize sequential read performance. In the end, the best solution is to experiment with various options: try both a small and a large stripe size and collect performance data for it. We recommend avoiding making little steps: going from 32 kB to 64 kB stripe size won’t have a significant impact.

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