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Rotational latency (sometimes called rotational delay or just latency) is the delay waiting for the rotation of the disk to bring the required disk sector under the read-write head. [22] It depends on the rotational speed of a disk (or spindle motor), measured in revolutions per minute (RPM).
Major page faults on conventional computers using hard disk drives can have a significant impact on their performance, as an average hard disk drive has an average rotational latency of 3 ms, a seek time of 5 ms, and a transfer time of 0.05 ms/page. Therefore, the total time for paging is near 8 ms (= 8,000 μs).
A hard disk drive failure occurs when a hard disk drive malfunctions and the stored information cannot be accessed with a properly configured computer. A hard disk failure may occur in the course of normal operation, or due to an external factor such as exposure to fire or water or high magnetic fields , or suffering a sharp impact or ...
Rotational latency. This is the time it takes to correctly position the platter underneath the head so that the desired data can be accessed. (The average rotational latency is one-half of a revolution of the disk platter.) In the case of a 15,000 RPM drive, this is approximately 2 milliseconds."
File system fragmentation is more problematic with consumer-grade hard disk drives because of the increasing disparity between sequential access speed and rotational latency (and to a lesser extent seek time) on which file systems are usually placed. [8] Thus, fragmentation is an important problem in file system research and design.
A proposed workaround is for the operating system to artificially starve the NCQ queue sooner in order to satisfy low-latency applications in a timely manner. [10] On some drives' firmware, such as the WD Raptor circa 2007, read-ahead is disabled when NCQ is enabled, resulting in slower sequential performance. [11]
With a CAV-drive the data on the outer tracks are the same angular width of those in the centre, and so less densely packed. Using ZBR instead, the inner zoning is used to set the read/write rate, which is the same for other tracks. This permits the drive to have more bits stored in the outside tracks compared to the inner ones.
Furthermore, sustained transfer rate tests do not give realistic throughput expectations for most workloads: They use I/O loads specifically designed to encounter almost no delays from seek time or rotational latency. Hard drive performance under most workloads is limited first and second by those two factors; the transfer rate on the bus is a ...