Diskutime

Diskutime is a performance metric used in data storage to quantify the average time a disk subsystem spends handling an input/output I/O request. It encompasses the mechanical or electronic components required to complete an operation, including motion, rotational delay, and data transfer. The term is used in theoretical models and practical benchmarking to compare disks, arrays, and caching strategies.

Definition and units: Diskutime is typically expressed in milliseconds per operation (ms/op) or microseconds per kilobyte for high-speed systems. A common approach is to sum the average seek time, average rotational latency, and data transfer time for the given operation size: Diskutime = Ts + Tr + Tb. Where Ts is the average seek time, Tr is the average rotational latency, and Tb is the transfer time.

Measurement and calculation: In practice, diskutime can be measured with synthetic benchmarks that simulate random or sequential I/O and with real workloads. Ts is influenced by drive mechanics and control software; Tr depends on rotational speed (for HDDs). Tb is a function of block size, bandwidth, and interface.

Relation to other metrics: Diskutime is closely tied to latency and IOPS. For a given workload, higher diskutime reduces IOPS and increases response times. It can be used together with throughput (MB/s) to characterize overall storage performance.

Applications and limitations: Diskutime helps with capacity planning, hardware comparisons, and tuning cache and RAID configurations. It is an aggregated metric and sensitive to workload mix, caching, queuing, and parallelism; thus it should be used alongside other metrics for a complete assessment.