Spin for spin, 15K drives will always be faster than 10K drives. Dollar for dollar is another story. For the same price, you can buy more 10K drives and create a faster array with more usable space than you can with 15K drives.

Array performance metrics

There are two basic measures of disk array performance, the first being Input/Output Operations per Second (IOPS). This is a measure of how many reads and writes the array can perform over time. The second is bandwidth in MB/s which is a measure of how much data can be read or written over a period of time. The characteristics of the workload have a significant effect on the IOPS and bandwidth that can be supported by the disk array. For the calculations used in this article, we will assume 70%/30% Read/Write, 100% random, 8KB I/O size, and no caching. Since most RAID controllers and drives use cache, you should see better results than the baseline numbers presented here. All performance numbers are based on the wmarow IOPS calculator at http://wmarow.com/strcalc/strcalc.html.

Single drive performance

As a general rule of thumb a 10K drive will be able to produce 140 IOPS and 1.1 MB/s while a 15K drive can produce 172 IOPS and 1.34 MB/s. This is a substantial performance difference, but the 15K drive will cost significantly more than the 10K drive.

Stretching the budget

Using four Dell 600GB 15K drives will result in a RAID 10 array with 1.2TB of usable space. This array will produce about 528 IOPS and 4.1 MB/s and will cost roughly $1,200.

Using four Dell 600GB 10K drives will result in a RAID 10 array with 1.2TB of usable space. This array will produce about 432 IOPS and 3.4 MB/s and will cost roughly $800. Matching the $1,200 expense of the 15K drives, you can buy six 600GB 10K drives and achieve about 649 IOPS, 5.1 MB/s with 1.8TB of usable space.  

Get more space and more performance for the same cost

While the 15K solution produces significantly better performance, the cost is about 1.5 times the cost of the 10K solutions. If you have then room in your budget, that is definitely the best solution. Many companies don’t have that luxury and by using more 10K drives, they can achieve better performance, more space and keep the same cost.

Array design

When choosing a Disk Array there are a variety of factors that can influence the design. This includes the type of workload, required space, performance needed, and of course, the cost. It’s very important to define your performance goals first, and then design the array around that. xByte Technologies can help you with the design of your array.