Access time and data transfer rate are key performance indicators that directly affect how quickly your system can retrieve and process data. For users, particularly in environments where speed is paramount—such as in gaming, vlogging, or enterprise IT—understanding these two concepts is crucial.
- Access time: Hard disk drives have moving mechanical parts. After a read/write instruction is received, the read/write head has to move to the desired track and wait for the desired sector to rotate under it. The time it takes to do so is termed access or response time. A shorter access time means faster data retrieval, making it ideal for applications requiring quick random access to files.
- Data transfer rate: It refers to the speed at which data is actually transferred once it has been accessed. This affects how quickly files are read from or written to the disk, and can make a significant difference in overall system performance, especially when working with large files.
To truly appreciate why these factors matter, it’s important to first understand hard disks’ internal components and their mechanics. Here, we’ll explain a bit more about what happens when a read or write request is made, to help you understand the core concepts that determine performance.
The Internal Mechanics of a Hard Drive
When a request to read or write data is made, the hard drive undergoes a series of actions. These can be broken down into two main phases:
- Data Access: In this phase, the read/write head moves to the correct track (seek time) and waits for the desired sector to rotate into position under the head (rotational latency).
- Data Transfer: Once the data access phase is completed, the data is transferred to or from the hard drive at a certain rate. This is where the drive’s data transfer rate becomes critical, as it defines how fast data can move between the hard drive and the system.
Together, these two phases determine how efficiently your system can read from or write to the hard drive. But before diving into data transfer rates, it’s essential to first understand access time—because the drive can’t transfer anything until it locates the relevant sector.
Access Time
Access time combines all the small durations involved in each step of the data access phase. For users, access time is a key metric for assessing how quickly a drive can access random data.
The major contributors to access time are seek time and rotational latency.
It is important to understand that manufacturers often calculate access time differently, making direct comparisons between drives difficult. To compare drives effectively, it’s better to focus individually on seek time and latency, as these are the primary components of access time.
Seek time
Seek time is the time it takes for the read/write head to move between tracks on the disk platter. A key concept to understand here is positioning performance. This refers to a hard drive’s ability to quickly and accurately position its read/write head over the correct track during a read or write operation.
When a file is requested, the hard drive must identify which track it resides on, then physically move the actuator arm to align the read/write head with that track. This mechanical movement introduces a delay, especially if the head has to travel a relatively long distance across the platter surface.
A specifications sheet of an HDD may list three types of seek times:
- Average Seek Time: The time to move between random tracks, typically 8–10 ms.
- Track-to-Track Seek Time: The time to move between adjacent tracks, usually around 1 ms.
- Full Stroke Seek Time: The time to move across the entire disk, generally 15–20 ms.
Seek time is critical because even a small improvement can significantly boost system performance, especially in server environments that run multi-user applications.
The actuator assembly and read/write head design play a major role in determining the seek time of a hard drive.
For instance, drives that use voice coil actuators can accelerate and decelerate the read/write head more precisely than those using older stepper motor designs. Additionally, short-stroking—where only a portion of the disk is designated for storage to reduce travel distance—can significantly lower average seek times, although at the cost of usable storage capacity.
Once the read/write head aligns itself with the right track, another movement needs to happen before data can be read or written. This is where rotational latency becomes important.
Rotation Latency
As explained previously, data is stored in concentric circles on the platter, and these circles are called tracks.
Every track is divided into smaller units, each of which is called a sector, and this is where the actual data resides.
When a computer requests data access or transfer, read/write heads in the hard drive move to the correct track. The time they take to do so is seek time, as we previously understood.
Now, the platter of a hard drive keeps spinning at a constant speed. So, the specific sector that has the requested data might not be directly under the read/write head when it arrives at the track. So, there is a delay accounting for the time it takes for the platter to spin and align the correct sector under the head, and this delay is called rotational latency.
So, for platters that spin fast, the rotational latency is low, which means the higher the rotational speed (measured in RPM), the lower the rotational latency. For example:
- At 5,400 RPM, average latency is ~5.6 ms.
- At 7,200 RPM, average latency drops to ~4.2 ms.
- At 15,000 RPM, it can be as low as ~2 ms.
In certain cases, when the performance demands from storage are high, such as in servers, you should select a hard drive that has a high spindle speed so that its rotational latency is low.
Access time has two more components, but these are negligible as compared to seek time and rotational latency.
Command Processing Time
This component—also known as command overhead—is the first step of the data access phase. It is the time that the internal electronics of the drive take to interpret a read/write command and then set up the communication between the system and the components—before any physical movement happens.
This time is extremely short in duration and is typically of the order of three microseconds, so it is negligible in comparison to the mechanical delays like seek time or rotational latency. That is why it is typically ignored when we measure drive performance.
Settle Time
Once the read/write head reaches the right track, it needs to stabilize before it can begin to read or write data. This brief pause is known as the settle time. In modern hard disks, the settle time is less than 100 microseconds. So, in most HDDs, this time is internally accounted for in the overall seek time itself. That is why settle time is not considered a performance bottleneck.
Now that we have covered access time in detail, the next factor we need to explore is how quickly the data moves once it is located—this is the data transfer rate. So let us look at what influences the data transfer rate and why it matters.
Data Transfer Rate
Once the read/write head has positioned itself correctly, the next phase is data transfer. The data transfer rate depends on the factors discussed below.
Media Rate
Media rate is the speed at which a hard drive can read bits directly from the surface of a disk platter. This rate reflects the raw speed at which magnetic transitions (which represent data) can be detected and processed by the read/write head while the platter spins beneath it.
The media rate is the foundation of a hard drive’s internal throughput. It indicates how fast the hard drive can retrieve data from its physical media before any buffering or interface-related transfer happens. This is why, even in drives with the same RPM, the performance can be different depending on their areal density and internal design.
Sector Overhead Time
To understand this, we need to understand how a sector works in a bit more detail.
Every sector contains not just data but also additional information that helps the hard drive manage and protect that data. So, when a read/write head is accessing a sector, the hard drive performs many tasks.
Some of these tasks are managing the organization and flow of data, ensuring the data being read or written is accurate and error-free, and handling other internal processes essential to maintaining the drive's reliability.
The sector overhead time is the small extra time the hard drive needs to manage and process the information around the data stored in a sector. It is a very small part of the total time required to transfer data.
It is generally much less compared to the seek time we understood earlier.
Head Switch Time
To understand the head switch time, we first need to understand the meaning of a cylinder.
A hard drive usually comprises multiple platters. A cylinder is formed by stacking a set of tracks vertically across multiple platters (as shown below).
Each platter has a dedicated read/write head.
Now, imagine a sequential read or write operation. The drive will aim to read or write all the tracks in a cylinder before it moves to the next cylinder. It does so to save time.
To access data on a different track in the same cylinder, which is on a different platter, obviously, the drive has to switch from one head to the other. Although this is an electronic process, not a mechanical movement, it still takes some time—called the head switch time.
This time generally ranges between 1 and 2 milliseconds.
Head switch time is a critical factor in determining data transfer rates because head switching occurs frequently during large sequential read or write operations that span multiple tracks.
The head switch time is mainly determined by the controller inside the hard disk, and it does not vary much between different models or manufacturers of hard drives.
Cylinder Switch Time
An understanding of head switch time simplifies the concept of cylinder switch time. This is the time it takes for the hard drive to switch from one cylinder to the next.
When the drive has to read all the tracks in a cylinder and then switch cylinders, then this time becomes relevant. This time is more than the head switch time, as the process is mechanical—the actuator has to move from one cylinder to the next. Cylinder switch time is generally between 2 and 3 milliseconds.
Conclusion
Access time and data transfer rate are among the two most pivotal parameters of a hard drive.
- In internal hard disk drives with an RPM of 7200, average access time ranges between 12 to 13 milliseconds—particularly in popular models like WD Blue (WD10EZEX) or Seagate Barracuda.
- In external HDDs with 5400 RPM, access time can be between 15 and 17 milliseconds, such as in WD My Passport or Toshiba Canvio.
- Typically, the data transfer rate in consumer-grade desktop drives is between 140 to 280 MB/s, as found in Toshiba P300 or WD Blue.
- There are high-performance HDDs such as WD Black Performance Mobile or Toshiba N300 NAS, where the data transfer speed can be between 227 to 248 MB/s.
- In external drives connected by USB 3.2, the transfer speed can theoretically hit 625 MB/s, though real-world speeds are lower than that.
Please note that access time and data transfer rates are important, but they are part of a bigger picture—so you should also consider other HDD specifications such as areal density, power consumption, reliability, and more before you choose which one to buy.
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