Seek Time

What is Seek Time?

Seek time refers to the time taken by the data or information stored on the disk of a hard drive to be positioned under the read/write head.

Technically, it is the time taken by the read/write head assembly to have its actuator arm reach the desired data stored on a different track on the same side of the disk.

Understanding Seek Time

What is Seek Time

Seek time refers to the amount of time required by the disk controller of a hard drive to find out a particular type of data stored on the disk.

Therefore, in simple words, it is also called disk seeking and refers to the actual physical placement of the read/write heads of the disk.

Typically, the larger disk drives tend to have much slower seek times as compared with the smaller disk drives. This is primarily due to the larger construction of the heads in these larger disk drives which include:

The enterprise storage devices usually come with hard disk drives that are intentionally made to be smaller in their storage capacity.

This means that the actuator arm will have a fewer number of tracks to move through and a much shorter distance to travel. This is referred to as short stroking.

As for the Solid State Drives (SSDs), sometimes the delay of a signal relay from the hardware and from buffering is also referred to as seek time. However, this is not actually a seek time in the truest sense.

The main reason for it is that data on an SSD is typically retrieved without using any moving parts since these non-volatile microchips do not need any such moving parts to store data.

The seek time of a hard disk does not include other delays that may be encountered while accessing the data such as:

Read Also:  What is Zero Filling? (Explained)

The seek time of the disk of a hard drive may vary due to the different lengths of distance the read/write head has to travel from the place it is positioned currently to where it is instructed to move.

It is for these variables that the seek time of the hard disk drive is often measured as an average seek time instead of an individual seek time.

There is another reason for the seek time of the hard drives to be measured as an average seek time.

It is the fact that there is no industry standard for the method of measuring seek time. This means that there will be no single value for the seek time of an entire hard disk drive.

Moreover, there are quite a few hard disk drive manufacturers who even include the track to track and full stroke specifications along with the average seek time.

Evaluation Methods

The seek time of a hard disk drive is usually measured in two different ways such as:

In the track to track methods, it is the amount of time taken by the disk head to seek or search between two adjacent tracks that is considered.

It is also called the minimum seek time and is typically measured in milliseconds. It can be as low as 1 ms and as high as 4 ms.

On the other hand, in the full stroke or the maximum seek time method, it is the amount of time taken by the head to search the entire disk for a particular piece of data that is considered. However. this is also measured in milliseconds.

Significance

Seek time is one of the most important factors that affects the performance of the hard disk drive since it determines the overall speed of the disk, along with other important components such as:

Typically, there should not be any intervals in the seek times of a hard disk because that will have adverse effects on the performance of the drive.

This means that seek time is inversely proportional to the overall performance of the hard drive, and therefore, the lower it is, the better the performance will be.

However, the seek time is not important for all the tasks you do on your computer.

For example, the process of downloading a video file from the internet may not have anything at all to do with the seek time of the hard disk drive, but saving it on the drive surely does.

Read Also:  HPFS (High Performance File System) Explained

The time taken to download the file will largely depend on the bandwidth of the network.

The same concept is applicable to other tasks performed on a computer such as:

Seek Time Example

Typically, a seek time lower than 10 ms is considered to be suitable for a hard disk drive. It has been around 9 ms maximum since 2004, but there have been a few instances when it has been seen to range anywhere between 3 ms and 15 ms as well, especially for the drives used particularly in the mobile devices.

For example, if the read/write head of the hard drive is currently positioned on track 1 and now receives a request to read data stored on track 4, it will need to move to track 4.

The time taken by the head to travel the distance is called the seek time.

This seek time, however, does not include the wait time of the head in case the data is stored on the opposite side of the track. This delay is known as Rotational Latency.

The seek time for the hard drives has been improving slowly over time. For example:

Typically, most common hard drives have a seek time ranging between 8 ms and 10 ms. However, the maximum seek time may vary according to the type of the hard drive. For example:

As said earlier, the SSDs do not have any moving parts in them and therefore their seek time is measured in a different way and typically ranges between 0.08 ms and 0.16 ms.

Also, the seek times of some common larger hard disk drives are found to be as follows:

Seek Time Formula

The seek time of a hard drive is typically calculated by multiplying the number of tracks or cylinders crossed by the time taken.

Mathematically, it will be:

Seek Time = (Number of tracks/cylinders crossed) * (Time to cross one track/cylinder)

If you want to calculate the average seek time, the formula for it is:

Average Seek Time = 1 / 3 x Time taken for one full stroke

Alternatively:

Average seek time = {Time taken to move from track 1 to track 1 + Time taken to move from track 1 to last track} / 2.

Read Also:  What is File Allocation Table (FAT)? Types, Uses & More

Therefore, if ‘t’ is the time taken by the read/write head to move from one track to the adjoining track, and if ‘k’ is the total number of tracks on the disk, the formula will be:

Average seek time =

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

What is the Maximum Seek Time?

The maximum seek time is a condition when the read/write head of a hard drive has to move across all of the tracks on the disk. It is also called the full stroke time.

For example, the maximum seek time for 10000 tracks, which is actually 9999, will be 11 milliseconds according to the formula 1+ 0.001n, where n is substituted by 10000.

The seek time, average or maximum, is one of the physical properties of a hard disk drive, and therefore, you can do little to lower it.

However, as said earlier, the overall performance of the computer and the hard drive depends on several other factors, which can be controlled a bit to make things better and a bit faster.

You can use a defrag tool to reduce the fragmentation of files while storing them on the disk.

This will consolidate the fragments of a file and store them in a contiguous location, which, in turn, will reduce the seek time to some extent.

You may also try doing a few other things such as:

All these efforts will ensure that the actuator arm of the hard disk drive does not have to move and search through a pile of less-used or unused data unnecessarily, every time it needs to read from or write something to the disk.

In turn, it will reduce the maximum seek time of the hard disk drive.

Conclusion

Seek time is one of the components of the disk access time, the other being rotational latency.

It indicates the time taken by the read/write head to reach the desired location of data storage.

It is a very vital factor for the performance of the hard drive because there should be no gaps in it that may lower the performance.