Optimizing Disk Drive Performance in Storage Networks
Given that disk drives are relatively slow, people have developed several methods and techniques to increase their performance. The performance enhancements discussed in this section includes:
- Limiting drive contention
- Short-stroking the drive
- Matching rotation speeds
- Aligning zones
Limiting Drive Contention
Multiple applications that are actively performing I/O operations on a single disk drive can generate a significant amount of seek time and rotation latency, causing performance to deteriorate. Therefore, it makes sense to understand which applications are accessing each disk drive in order to avoid contention for a drive's slow mechanical resources.
Disk bottlenecks can occur in storage area networking (SAN) environments where many disk drives provide storage to many systems and applications. Without good planning, it's possible for different partitions on a single disk drive to be assigned to different applications requiring higher 1/O performance levels. In that case, the two applications would wind up competing for the use of a single actuator and a single rotating spindle.
One of the best ways to limit contention for a disk drive is to limit the number of partitions it has. For example, administrators could configure some percentage of high-speed drives to have two partitions one for a high-performance application and the other for a lower-performance application. Unfortunately, the enforcement of this objective would have to be a manual task, as the technology for automating it is not yet available. A facility that allowed automated disk partition policies to be applied to drives might be very useful. The intelligence to do it is in the drive, but the requirement to provide it is not yet obvious to disk drive manufacturers.
NOTE : Most applications have minimal storage 1/O requirements and place a minimal load on disk drives. These "I/O-lite" applications probably don't need to be monitored tot disk drive contention problems. These applications also do not deserve to hog the best parts of high-speed disk drives. Most administrators are not used to creating partitions and letting them sit idle, but that could be done to reserve a percentage of the "choicest cuts'" of some drives in case they are needed by a high-performance application in the future. The question is, if an application does not require optimal I/O performance, why allocate a high-performance disk partition to it?
Short-Stroking the Drive
A technique known as short-stroking a disk limits the drive's capacity by using a subset of the available tracks, typically the drive's outer tracks. This accomplishes two things: it reduces seek time by limiting the actuator's range of motion, and it increases the media transfer rate by using the outer tracks, which have the highest densities of data per track.
Short-stroking is normally done by establishing a single partition on a drive that uses some subset of the total capacity. For instance, you can establish an 80-GB partition on a 200-GB disk drive to increase performance.
Rotation Speeds
There is a tendency in storage networking to treat all storage address spaces equally, which makes them easier to manage. This approach is likely to work very well for the majority of applications, but it falls short for those applications that need the highest performance.
An obvious difference between disk drives is their rotational speed. It would not be a good idea to match slow-speed drives with high-speed drives in support of a high-throughput transaction processing application.
The same concept applies to drive buffer memory. You would not want to use drives with insufficient buffer capacity, slowing down an otherwise fast configuration.
Aligning Zones
The performance differences across different zones on disk drives could be significant for certain applications that expect consistently high i/O rates, such as data warehousing processing. For example, a disk partition on the outside of a disk might have almost twice the performance of a partition on the inside of a disk. In effect, this is the same situation as wanting to match the rotational speeds of disk drives used for high performance applications, except this variable adds the element of matching the zones or the relative position of the partitions on a drive.
