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Tip #1287: What is “Latency?”

Larry Jordan – LarryJordan.com

Hard drives have latency – SSDs do not.

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Latency is directly tied to spinning storage media – the traditional hard drive – and determines how quickly you can access your data. Latency is the average time for the data being accessed to rotate into position under the drive’s magnetic head, after a completed seek.

As PC Tech Guide.com writes:

Disk RPM is a critical component of hard drive performance because it directly impacts the latency and the disk transfer rate. The faster the disk spins, the more data passes under the magnetic heads that read the data; the slower the RPM, the higher the mechanical latencies. Hard drives only spin at one constant speed, and for some time most fast EIDE hard disks spin at 5,400 rpm, while a fast SCSI drive is capable of 7,200 rpm.

Mechanical latencies, measured in milliseconds, include both seek time and rotational latency. Seek Time defines the amount of time it takes a hard drive’s read/write head to find the physical location of a piece of data on the disk. Latency is the average time for the sector being accessed to rotate into position under a head, after a completed seek. It is easily calculated from the spindle speed, being the time for half a rotation.

A drive’s average access time is the interval between the time a request for data is made by the system and the time the data is available from the drive. Access time includes the actual seek time, rotational latency, and command processing overhead time.

EXTRA CREDIT

What makes SSDs so fast is that they don’t spin or have magnetic drive heads. This means that terms like latency and seek time no longer apply. Here’s the full PCTechGuide.com article to learn more.


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