hesela.devRead-on-arrival
Read-on-arrival is an HDD technique that reads requested sectors in their encountered rotational order and buffers them for logical-order delivery.
The name zero-latency read does not mean zero request time. Reading can begin at a requested sector encountered after positioning instead of waiting for the request's first logical sector. Mechanical transfer, seeking and host delivery still take time. [1] [2]
For a request covering exactly one physical track, the mechanism can collect the track in one revolution after positioning. An arbitrary track-sized logical range may cross a boundary and does not automatically have that property. Atropos explicitly used track geometry and aligned accesses. [3]
Hesela's eight-sector illustration starts at sector 3. Ordered reading takes 13 sector ticks, while read-on-arrival takes 8 and restores logical order. All eight boundary phases are enumerated, with a separate single-sector control that gains nothing. This is constructed arithmetic with no seek, queue or host-transfer cost, not a drive benchmark. [4]
Application benefit remains workload-specific. FAST 2002 added the feature to an emulated disk model; Table 5 reports ten-run PostMark means of 74.523 and 74.469 seconds, with standard deviations of 0.618 and 0.783 seconds. The paper reports a rounded 0.1% runtime reduction. This is not a measured firmware upgrade on physical hardware, evidence of statistical significance, or a modern-drive speedup prediction. [2]
Read-ahead predicts future requests; read-on-arrival changes media order for the current request. They can coexist, but neither term identifies the other's policy. The 1994 paper documents historical caching behavior, not current prevalence. See the HDD explanation for broader context. [1] [5]
Relations
Citations
- Ruemmler and Wilkes: An Introduction to Disk Drive Modeling, IEEE Computer 27(3):17-29 (1994), read caching
- Griffin, Schindler, Schlosser, Bucy and Ganger: Timing-accurate Storage Emulation, FAST 2002, section 6.1 and Table 5
- Schindler, Schlosser, Shao, Ailamaki and Ganger: Atropos, FAST 2004, sections 2.2 and 5
- Hesela: exact eight-sector read-on-arrival illustration, code, assumptions and tests
- Hesela: HDD mechanics, geometry and transfer-rate interpretation
Sources & caveats
- Historical research evidence establishes the mechanism, not an invention date or support in every present-day HDD.
- The companion example is an original deterministic illustration, not a reproduction of the cited empirical experiments.
- The FAST 2002 experiment emulated a Seagate Cheetah X15 with an added read-on-arrival capability; the physical drive lacked that feature.