Difference between Seek Time and Disk Access Time in Disk Scheduling

Seek Time:
A disk is divided into many circular tracks. Seek Time is defined as the time required by the read/write head to move from one track to another.

Example,
Consider the following diagram, the read/write head is currently on track 1.

Now, on the next read/write request, we may want to read data from Track 4, in this case, our read/write head will move to track 4. The time it will take to reach track 4 is the seek time.



Disk Access Time:
Disk Access Time is defined as the total time required by the computer to process a read/write request and then retrieve the required data from the disk storage.

Disk Access Time is divided into 2 parts:

  1. Access Time
  2. Data Transfer Time
Disk Access Time = Access Time + Data Transfer Time 

1. Access Time:
Access Time is defined as the setup time before the actual data transfer takes place.
For example, the read/write head is on track 1 but we need to read data from another track or segment. Thus, the read/write head will move to the data block location before the actual transfer can take place. This delay is called Access Time.

Access Time is calculated by summation of the following:

(a). Seek Time
(b). Rotational Latency
(c). Command Processing Time
(d). Settle Time 

These are explained as following below in brief.

  • (a). Seek Time –
    It is the time required by the read/write head to move from the current track to the requested track.

    Seek Time 
    = (Number of tracks/cylinders crossed) * (Time to cross one track/cylinder) 
  • (b). Rotational Latency –
    It is the time required by the read/write head to move from the current sector to the requested sector.

    Rotational Latency 
    = (Angle by which disk is rotated) / (Angular Frequency) 
  • (c). Command Processing Time –
    It is the time required by the disk device to process the command and establish a connection between the various components of the disk device to read/write data. It is due to the internal circuitry.
  • (d). Settle Time –
    Settle Time is the time required by read/write head to stop vibrating.

Note: Command Processing Time and Settle Time are not normally mentioned in numerical question. We take them as zero.



2. Data Transfer Time:
Data Transfer Time is defined as the time required to transfer data between the system and the disk.
Data Transfer Time is of two types:

(a). Internal Transfer Rate
(b). External Transfer Rate 

These are explained as following below in brief.

  • (a). Internal Transfer Rate –
    It is defined as the time required to move data between the disk surface and hard disk cache.
  • (b). External Transfer Rate –
    It is defined as the time required to move data between the hard disk cache and the system.

Let’s see the difference between Seek Time and Disk Access Time:

S.NO. Seek Time Disk Access Time
1 It is the time required by read/write head to move from one track to other. It is the time required by the computer to process a read/write request and retrieve the required data.
2 It is always less than Disk Access Time. Since, it is a sub part of Disk Access time. It is very large compared to Seek time.
3 It doesn’t consider transfer of data.

It considers the time required to transfer data.c

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