Will Bigger Disk Drives Affect Performance?

By John Haight, Master Consultant for Storage Solutions, Forsythe Solutions Group Inc. , with James Harbaruk, Roland Hudon, Michael Lillie, and Leo Squire, May 2, 2008, 5:00 PM This is a great question and the answer depends on how performance...

May 3, 2008

2 Min Read
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This is a great question and the answer depends on how performance is measured. Generally speaking, large disk drives do not typically hinder the performance of a storage array. Storage arrays are engineered by the manufacturer to sustain determined levels of I/O based on the model of the array.

On one hand, though, it is possible, depending on the situation, that larger drives might impact the ability of the storage array to deliver the expected IOPS (input/output operations per second) to the application. Although different manufacturers have their own recipe for performance, the age-old variables of seek and rotate, or rotational delay (the time it takes for the actual read/write heads to be positioned at the data location on the disk drive) still exist. Theoretically, with the advent of flash drives, this issue will be lessened, if not eliminated.

The rotational delay variables are why most storage array manufacturers will group physical drives and try to spread logical volumes across multiple physical drives with a 15K’ option for the highest performance. The idea is that if you need more performance, then spin it faster. A 300-Gbyte drive spinning at 15,000 RPM definitely performs better than its cousin spinning at 10,000 RPM, as the read/write head passes over the data areas more quickly.

In theory, if you have a requirement for 1 Tbyte of usable capacity for an application that requires higher performance, this could conceivably reside on a single 1-Tbyte disk drive, although performance levels would not be met. It is not uncommon that providing enough disk drives (‘spindles’) to deliver the IOPS required exceeds the capacity that is actually required. This is clearly a negative side effect that affects capacity planning.

On the other hand, most high-end arrays are fronted by large amounts of cache. Therefore, the actual writes to the drive, regardless of the size, are all originally written to cache, which is the point in time when an acknowledgment is given back to the application. It is only later in the process that the actual write is de-staged and written to the physical disk drive. From the perspective of the storage array, provided users are within the ratios of cache to disk specification, there should not be any storage array performance degradation.If you look at the performance metrics of the 300-Gbyte/10K drives available today versus 73-Gbyte/10K drives purchased 3 or more years ago, the 300-Gbyte drives can provide better performance due to larger on-board cache and enhanced/faster disk-to-system interfaces -- for example, 2-Gbit/s Fibre Channel versus 4-Gbit/s Fibre Channel.

So, it is possible to see lower performance of your storage array when using larger disk drives. However, if you understand how you are defining performance, measure it accordingly and choose the appropriate technology and configuration, then the likelihood is you won’t see lower performance.

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