Wednesday, 9 June 2010

Partitioning for speed

I want to partition the RAID 0 array in order to create a dedicated space for Virtual Memory and Scratch disks for Adobe Photoshop and Premiere. The array is 4x 300GB WD VelociRaptor RAID0.

This is not going to work as intended. To get a better overall performance, he'd be better off splitting the array to two 2x drives each, or maybe even down to standalone drives. Scratch and swap files produce better performance when placed on separate hard drives in such a way that no "spindle" is servicing more than one data stream. In the layout with one 4-disk array, four "spindles" would be serving three or four data streams (source data, swap, scratch, and output data), which is far from ideal because the number of seeks would be too high. Adding partitions to the mix would only ensure there is a certain minimum distance for the disk heads to travel across the partition boundaries. This would actually decrease the performance.

In RAID planning, speed estimations, such as provided by the RAID calculator may be handy but only apply to the simplest case of a single data stream. Also, keep in mind that a RAID setup does not improve access time (command-to-start-read).

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