Disk Access Time

What is Disk Access Time?

Disk access time simply refers to the total time taken by the computer system as a whole to process a request for data sent by the processor and then retrieve that particular data from the storage device.

Technically, disk access time comprises the time taken to move the read and write heads to the chosen track, which is called the seek time, and the time taken to spin the disk so that the sector is found by the head, which is called the latency.

Understanding Disk Access Time

What is Disk Access Time

Disk access time refers to the time taken by the computer to find the first data character once a data request is initiated.

In simple words, this access time of the hard disk involves the following:

Typically, the access time taken by the disks is actually given as an average, always.

This is because it mainly depends on the seek time and rotational latency of the disk. Both of these parameters typically vary on the basis of the current location of the read and write heads on the platter.

Usually, the hard disk drives are known as direct access devices and they need different times to position the read and write heads of the disk, also called the head assembly or comb, on top of a particular data or track of the designated cylinder.

Apart from that, it also needs the disk to rotate so that the selected track of the particular record can be accessed by the comb.

The average disk access time is a very useful metric that helps in the proper analysis and planning of the computer application in real time, in an online inquiry system, for instance.

On the other hand, a minimum access time is very important for the proper and sequential use of disk drives.

Over a period of three decades or so, the rotational speed of the hard disk drives may not have improved significantly, with 3600 RPM being the standard, but the bit densities on each individual track have increased notably in the same period, by as much as fivefold.

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This means that the average speeds for data transfer have also increased significantly. However, this has not reduced the access times for each track by a considerable margin.

Still, within this period, the average disk access times have been reduced by half. This is mainly due to the notable and widespread change in the design of the actuators in the hard disk drives.

Typically, in order to move the head assembly mechanism, the hard disk drives now come with voice coil actuators in place of the traditional hydraulic actuators.

Variance

The disk access times typically differ depending on the type of the drive. For example, the different types of Hard Disk Drives, the respective access times for each will be as follows:

However, for the Dynamic Random Access Memory or DRAM-based storage drives, the average disk access time will not be enough to measure.

And as for the Solid State Drives, just as their name implies, they do not have the usual components of an HDD.

Therefore, in the SSDs, the delay in getting the data will be very short and may be measured in nanoseconds.

The actual rate of reading data from the storage array will also be pretty short. So, the disk access time for an SSD will be much less in comparison to the disk access time of an HDD.

However, the probable limitation will be in the capabilities and speed of the other components within the SSD as well as the interface connecting it to the rest of the computer system in terms of its speed and bandwidth.

As a result, for the SSDs, the average disk access time will be in the tens of microseconds.

Disk Access Time Formula

The disk access time can be calculated easily by using a simple formula: Disk Access Time = Access Time + Data Transfer Time.

However, there are other factors to consider to calculate the actual disk access time, and when these are considered, the formula would be:

Disk Access Time = Seek time + Rotational delay + Transfer time + Controller overhead + Queuing delay.

Here, the different elements of the equation have to be calculated differently, as explained hereunder.

Average seek time is calculated as follows:

Average seek time = 1/3 x time taken for one full stroke.

For example, if the time taken by the read and write head to move from one track to the next track is t units and the number of tracks in total is k, then the average seek time will be:

{Time taken to move from track 1 to track 1 + Time taken to move from track 1 to last track} / 2

= {0 + (k-1) t} / 2

= (k-1) t / 2.

The average rotational latency is calculated as = 1 / 2 x time taken for one full rotation.

The capacity of a disk pack is calculated as = Total number of surfaces x Number of tracks per surface x Number of sectors per track x Storage capacity of one sector.

Formatting overhead is calculated as = Number of sectors x Overhead per sector.

Formatted disk space or usable disk space is calculated as = Total disk space or capacity – Formatting overhead.

Recording density or storage density of a track is calculated as = Capacity of the track / Circumference of the track.

Capacity of a track is calculated as = Recording density of the track x Circumference of the track.

Data transfer rate is calculated as = Number of heads x Bytes that can be read in one full rotation x Number of rotations in one second.

It can also be calculated alternatively as = Number of heads x Capacity of one track x Number of rotations in one second.

Total number of tracks on each surface is calculated as = (Outer radius – Inner radius) / Inter track gap.

The controller time is equal to the overhead levied by the disk controller.

Disk Access Time Example

The disk access time can range between 12 milliseconds to 20 milliseconds usually, depending on the parameters and type of hard disks. However, it will be different for the enterprise drives, desktop drives, and the Solid State Drives (SSDs).

For example, assume that a hard disk comes with the following parameters:

Further, assume that there is no queuing delay since the disk of the drive is supposed to be idle and the advertised seek time undertakes no locality.

With all these things considered, the average disk access time for a given sector on it will be as follows:

12 ms + 0.5/ (7200 RPM/60) + 8 KB/4 MB/s + 2 ms

= 12 + 4.15 + 2 + 2

= 20 ms

Now, this access time of the disks may also vary depending on the type of the hard drive in the first place, as said earlier.

What are the Components of Access Time in a Disk?

The main components of disk access time are the seek time, data transfer time, processing time of the commands, and rotational latency. All these components are considered and added together to obtain the access time of a disk.

Seek time:

The seek time is the time taken to move the read and write heads from one location to another location on the disk to read data or locate the desired track. The higher the seek time is, the greater the data access time will be.

Rotational latency:

Rotational latency or wait time refers to the time the read and write head has to wait for the disk to spin around so that the desired sector comes under it and it can read the data stored on that particular sector.

Data transfer speed:

The data transfer speed typically involves two separate components. These are:

The slower the speed, the higher will be the disk access time, especially for making large data block transfers.

Command processing time:

And the processing time taken by the computer to deduce the commands and instructions also adds to the total disk access time and data transfer. Though it mainly depends on the Central Processing Unit, all other components of the computer also play a significant role in it.

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What is the Average Disk Access Time for a Sector?

Typically, in most cases, the average disk access time is about 12 milliseconds to 20 milliseconds.

However, it may vary according to the type of the hard drive as well as other parameters such as:

In addition to that, it is also quite likely that the working process of the disk controller will affect the disk access time. This is because it may need the controller to perform two separate operations in order to read a sector such as:

In such cases, the average disk access time will be higher, even higher than 20 ms.

However, at this point it is good to keep in mind that the type and working process of the hard disk and its controller will affect the data access times quite significantly.

Disk Access Time vs Seek Time

Conclusion

Just as the article points out, disk access time is the time taken by the computer to process and retrieve the results of the read and write requests.

It is usually measured in milliseconds but is relatively higher than data seek time, the two being completely different things, as this article has also pointed out.