Harddisk Calculator

by Hans-Jürgen Reggel

This page allows to play around with some calculations.

Sections: Sustained Data Rate  ·  File Size / Fragmentation  ·  Idle Revolutions

Sustained Data Rate

This section shows how the maximum sustained data rate results from the main parameters RPM and sectors per track, together with the timing.

Values for head skew and cylinder skew are hard to find, so the values used in the examples are just guesses.

Harddisk RPM  RPM The most popular spindle speed is 7200RPM. Other popular values are 10,000RPM, 15,000RPM, 5400RPM and 4200RPM.
Sectors per Track The standard sector size is 512 Bytes. Current 3.5" PMR harddisks have about 2100-1000 sectors per track (from outer to inner tracks).
Number of Heads 3.5" harddisks use 3-5 platters, i.e. max. 10 heads.
2.5" 9.5mm harddisks use 1-2 platters, i.e. max. 4 heads; 12.5mm max. 6 heads.
Head Skew  ms Head skew is the time between the last sector of the current track and the first sector of the next track of the same cylinder.
Cylinder Skew  ms Cylinder skew is the time between the last sector of the current cylinder and the first sector of the next cylinder.
Max. Data Rate:   MB/s

File Size and Fragmentation

This is a simplified calculation to show the impact of seek, latency, fragmentation and file size. The faster the harddisk and the smaller the file or fragment size, the more time is lost due to latency and seek.

At 85MB/s, a 100kB file is transferred in 1.15ms. One full revolution of a 10,000RPM harddisk takes 6ms, one full stroke of the WD Raptor takes 10.2ms. Even assuming that the filesystem information is already cached, it can still take up to 16.2ms just to get the head to the first sector of the data!

Harddisk RPM  RPM The calculation adds the average latency (½ revolution) for each file and fragment seek. Enter "0" for flash disks (no latency).
Average Data Rate  MB/s The base for the calculation is the average data rate. For example 65-75MB/s for current 3.5" harddisks, or 34-38MB/s for current 2.5" harddisks.
File Size  MB This is the file size used for the calculation. Small files can be read in a very short time, so the impact of latency and seek gets bigger for smaller files.
Number of Files The calculation adds the average file seek time for each file.
Fragments per file This is the number of fragments per file. The value "1" means no fragmentation.
Avg. file seek  ms This is the time added for each file. This depends on the average seek of the harddisk and the size of the partition holding the data.
Avg. frag. seek  ms This is the time added for each additional file fragment. Because fragments are most likely close to each other, this is a separate value and can be chosen less than the file seek.
Resulting Data Rate:   MB/s

Idle Revolutions

This is the calculation from the Samsung HD501LJ and Seagate Barracuda 7200.8 SATA analysis.
The calculation assumes a maximum transfer rate where all head and cylinder switches succeed, and calculates the values for a certain amount of idle revolutions.

Harddisk RPM  RPM
Sample Size MB
Regular Data Rate MB/s   Revolutions
With 1 idle rev.:   MB/s
With 2 idle rev.:   MB/s
With 3 idle rev.:   MB/s
With 4 idle rev.:   MB/s
With 5 idle rev.:   MB/s
With 6 idle rev.:   MB/s
With 7 idle rev.:   MB/s
With 8 idle rev.:   MB/s
With 9 idle rev.:   MB/s
With 10 idle rev.:   MB/s
With 11 idle rev.:   MB/s
With 12 idle rev.:   MB/s

Hans-Jürgen Reggel   ·   http://www.hjreggel.net/hdtechdat/   ·   2007-02-09 ~ 2007-02-19