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Add more disk drives for more performance

Disk drives are mechanical devices in which read and write heads move between the center and periphery of a fast rotating platter to find and access data. Even with the fastest disk drives that operate at 15,000 rpm and the time it takes for mechanical arms to reposition, latency adds up to a few milliseconds, limiting the number of IOPS per disk to a few hundred per second and throughput to less than 100 MBps.

One way to scale performance is by spreading data across multiple disks that work in unison when data is accessed, enhancing the number of IOPS and throughput proportionally to the number of disks involved. Additionally, array vendors have implemented techniques like

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short stroking to minimize arm movements. By placing data on the periphery of a platter, read-write head movements are greatly reduced, resulting in a significant performance boost. While a given performance goal can be achieved with a large number of disks and short stroking, it’s very costly and by only using the outer parts of platters storage utilization is dismal. Prior to the emergence of solid-state drives (SSDs), leveraging a large number of disks and methods like short stroking were used to meet high-performance requirements, and even today it’s used for applications where the high cost of solid-state storage still favors disk drives over SSD. “For sequential access of larger blocks and files, disk is usually more cost-effective,” said Mike Riley, director of strategy and technology of America sales at NetApp.

RAID and wide-striping

Easily overlooked, RAID and RAID levels both impact performance and capacity; changing the RAID level of an existing array to improve either performance or capacity utilization is a feasible option. The number of parity drives, large vs. smaller stripes, the size of RAID groups, and the block size within RAID groups all impact performance and available capacity.

While the characteristics of standard RAID levels are well known (seeRAID levels and their impact on performance and capacity utilization,” above), there are a couple of lesser known trends that deserve special attention in an efficient storage discussion. To start with, the size of a RAID group impacts performance, availability and capacity. Usually, larger RAID groups with a higher number of disks are faster, but require more time to rebuild in case of a disk failure. With high-capacity disks doubling in size every few years, rebuild times are increasing and the risk of more than one drive failing rises. Even though RAID 6 with its dual-parity drives reduces the risk by permitting two concurrent disk failures with some performance penalty, a better approach is eliminating dedicated parity drives. For instance, NetApp’s Dynamic Disk Pools (DDP) distribute data, parity information and spare capacity across a pool of drives, and utilize every drive in the pool for the intensive process of rebuilding a failed drive. Hewlett-Packard (HP) Co.’s 3PAR storage systems deploy a technique called wide striping that stripes data across a larger number of disks and subdivides disks’ raw storage capacity within this pool into small “chunklets.” The 3PAR volume manager uses these “chunklets” to form micro-RAIDs with parity “chunklets.” Because all “chunklets” of a single micro-RAID are located on different drives and are small in size, the risk and performance impact during drive failures and subsequent rebuilds is minimized.

This was first published in August 2012

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