Insider Insights: z/Journal Interviews Storage Specialist Fred Moore

by Bill Carico
December 1, 2005

z/Journal: Tape cartridges and disk drives are quickly heading past the 1TB capacity level. What challenges and benefits go along with terabyte storage?

 

FM: Disk performance hasn’t kept pace with the growth in disk capacities. More densely packed data means fewer disk actuators for a given amount of storage. Disk storage capacity witnessed exponential improvement in the ’90s. Since 1992, the areal density of magnetic disk recording has increased more than 60 percent annually. Storage device performance is improving at less than 10 percent annually. Device performance is normally measured in total random I/Os per second and can surpass 100 I/Os per second while the average access time (average seek, latency and data transfer) are just below 10ms per I/O on newer drives. Continual increases in capacity, without corresponding performance improvements at the drive level, create a performance imbalance that’s defined by Access Density. Access Density is the ratio of performance, measured in I/Os per second, to the capacity of the drive, usually measured in gigabytes (Access Density = I/Os per second per GB). The few remaining disk manufacturers are primarily focused on driving capacity higher to lower the price per gigabyte purchased. Less regard is given for performance at the drive level. This approach is good for storing data but what about retrieving data? If capacity and performance doubled, the access density would remain unchanged. Scaling disks involves more than increasing capacity. The access density challenge in turn forces users to allocate less storage on a disk drive to maintain performance and therefore add more drives to keep the actuator count high enough to ensure concurrency of I/O activity.

Like disk drive capacities, tape cartridge capacities are growing at unprecedented rates, and the capacity of a tape cartridge has exceeded that of a disk drive; this trend is expected to continue for the foreseeable future. Increasing tape capacities, without providing any offsetting performance improvements, lowers the effective retrieval capability of a given tape technology. Native tape cartridge roadmap capacities are close to surpassing 1TB. That means 2TB, as nearly all tape data is compressed 2:1. The key tape metric, Tape Throughput Density, is the ratio of tape cartridge capacity in gigabytes divided by the tape data rate in megabytes per second (cartridge capacity divided by data rate). Throughput density can be applied to any tape format to determine if a tape is better suited for high-performance applications or more archival applications that demonstrate write-once, read-seldom access profiles. As a guideline, a throughput density of 10:1 or less indicates higher performance tape with good retrieval capability, while greater than 10:1 indicates high-capacity and archival tape. Keeping the capacity/data rate ratio below 10 is a challenge for most tape providers. The issue for tape capacity is similar to that for disk drives— increasing capacity without providing a corresponding performance offset impacts performance and tends to reduce the allocation levels of the tape. This is less critical for tape, since it’s used for less performance-sensitive applications.