Hard disk drives (HDDs) are data storage devices that use rotating magnetic disks to store and retrieve digital information. They are a key component in most computers because they allow for non-volatile data storage, meaning the data persists even when the power is turned off. The speed at which data can be read from or written to the rotating disks is determined by factors like disk rotation speed, data density, and disk interface. This article will examine the key specifications that determine HDD read and write speeds.
HDD speeds are measured in megabits per second (Mb/s) for the rate at which data is transferred, and access time in milliseconds (ms) for the delay before data can be read or written. Faster disk rotation and increased data density allow modern HDDs to achieve sequential transfer speeds of 100-200Mb/s. But their average access times of 10-15ms are still much slower than flash-based solid state drives (SSDs). While HDDs cannot compete with SSDs in speed, their much lower cost per gigabyte makes them popular for secondary storage.
Measuring HDD Speeds
There are several common metrics used to measure hard disk drive (HDD) speeds:[1]
- MB/s – Measures the drive’s maximum sustained sequential transfer speed in megabytes per second for reading and writing data.
- IOPS – Stands for input/output operations per second. Measures the number of random read and write operations a drive can handle per second.
- Latency – The time delay between requesting data and receiving it, measured in milliseconds.
An important distinction is between sequential and random access speeds. Sequential access is reading/writing data in sequential order, like playing a video file. This allows the HDD head to optimize movement. Random access is reading/writing from random locations on the disk, resulting in more head movement. HDDs are much faster at sequential access than random.
For example, a HDD might have a sequential read speed of 160 MB/s but a random 4k read speed of just 0.9 MB/s.[1] SSDs do not have this discrepancy between sequential and random access.
Internal HDD Speeds
Internal hard drives typically have higher performance than external drives since they connect directly to the computer’s motherboard via SATA or IDE cables rather than relying on USB or Firewire. However, speed can vary greatly depending on the drive’s specifications.
Most modern internal 3.5″ hard drives have spindle speeds between 5,400 and 10,000 RPM. 7,200 RPM is most common for desktop PCs, while laptop HDDs tend to be 5,400 RPM. Faster spindle speeds allow the drive heads to access data quicker.[1]
In terms of interface, SATA III offers faster potential bandwidth than SATA II or SATA I, with transfer rates up to 600MB/s. Most current internal drives use SATA III.
Access times for internal HDDs are typically in the 10-15ms range.[2] Some server-focused drives can have access times under 10ms.
2.5″ internal HDDs spin more slowly and have slightly higher access times than 3.5″ drives, but the performance difference is usually not substantial. The smaller form factor saves space for laptops and compact PCs.
Overall, current 3.5″ internal hard drives can sustain sequential reads/writes over 200MB/s and random 4K read/writes up to 2MB/s on average. 2.5″ drives may be 10-30% slower.[3]
[1] https://www.seagate.com/blog/everything-you-wanted-to-know-about-hard-drives-master-dm/
[2] https://superuser.com/questions/56977/whats-a-normal-copy-speed-between-two-internal-hard-disks
[3] https://www.partitionwizard.com/clone-disk/internal-vs-external-hard-drive.html
External HDD Speeds
External hard drives connect to a computer through ports like USB, Thunderbolt, eSATA, or FireWire. This means their speeds are limited by the throughput of the connection interface. For example, USB 2.0 has a maximum transfer rate of 60 MB/s, while USB 3.0 can reach 625 MB/s (source: https://www.seagate.com/support/kb/how-fast-should-an-external-drive-be-172213en/).
In general, external HDD speeds are slower compared to internal drives using SATA or NVMe connections. However, new interfaces like USB 3.2 and Thunderbolt 3 have helped improve external drive speeds. For example, a Thunderbolt 3 external SSD can reach over 2,000 MB/s read speeds, rivaling many internal NVMe drives (source: https://www.cloudwards.net/fastest-external-hard-drive/).
When shopping for an external drive, it’s important to consider both the drive’s rated speed and interface throughput. USB 3.2 Gen 2×2 and Thunderbolt 3 provide the fastest speeds for external storage.
SSD vs HDD Speeds
SSDs generally have much faster read/write speeds compared to HDDs. According to Avast, HDDs can copy data at speeds of 30-150 MB/s, while SATA SSDs can reach 500 MB/s performing the same task. The main reason SSDs are faster is because they are flash-based with no moving mechanical parts, allowing very fast access times. HDDs rely on spinning magnetic platters and moving read/write heads which limits their speed.
As explained by Tekie, SSDs don’t have to physically move parts to locate and access data like HDDs do. The lack of moving parts allows SSDs to access data almost instantly. HDDs require time to spin up, move read-heads, and wait for the disk to spin to the correct position to access data. This gives SSDs a major speed advantage over traditional HDDs.
Average HDD Access Times
The access time or latency of a hard drive refers to the delay between when a request for data is made and when the data is available to read from or write to the disk. This access time has a major impact on overall HDD performance.
For hard disk drives, there are two main types of access times to consider – random access time and sequential access time. Random access refers to jumping to random locations on the disk platter to read or write data. This requires physically moving the read/write head and waiting for the platter to spin around to the correct position. Sequential access refers to reading or writing data in contiguous blocks sequentially. This results in much faster access times since less physical movement of components is required.
According to Wikipedia, the average random access time for consumer HDDs is typically between 10-15 ms. The average sequential access time is much faster at around 1 ms. Server-focused HDDs sacrifice capacity for faster spindle speeds and can achieve under 10ms average latency.
To improve access times, HDDs utilize on-board caching in the form of RAM memory buffers. Frequently accessed data like file tables and directory structures are cached so subsequent lookups don’t require accessing the physical platters. Modern HDDs typically have between 16-256MB of cache memory which can dramatically improve read/write speeds for repeated operations.
Improving HDD Speeds
There are several ways to optimize HDD performance and increase read/write speeds. Some tips include:
Using a defragmentation tool to rearrange files and consolidate free space on the HDD. This allows the hard drive head to access files faster with less movement across the disk platters.
Enabling write caching on the HDD to allow data writes to be queued and written in larger batches rather than small individual writes. This reduces overhead and improves performance.
Setting up a RAID array using multiple HDDs. RAID 0 can provide faster read/write speeds by striping data across drives. RAID 1 provides redundancy through disk mirroring.
Upgrading to a faster RPM HDD, as higher rotational speeds allow data to be accessed more quickly. 10,000 RPM HDDs offer better performance than 7,200 RPM models.
Using an SSD cache drive to store frequently accessed data, taking load off the HDD. The SSD provides faster access times for cached data.
Disabling unnecessary services and startup programs to reduce resource load on the HDD subsystem.
Overall, optimizing HDDs involves managing fragmentation, utilizing caching, configuring RAID arrays, upgrading hardware, and reducing unnecessary load to get the best performance.
HDD Interface Speeds
The interface between a hard disk drive and the rest of the computer system plays a major role in determining the drive’s maximum theoretical transfer speed. Common interfaces for HDDs include:
- SATA – Serial ATA is the most widely used interface for HDDs. SATA revisions allow for theoretical maximum speeds of 1.5 Gbit/s (SATA I), 3 Gbit/s (SATA II), 6 Gbit/s (SATA III), and up to 16 Gbit/s for the latest SATA 4.0 specification.
- SAS – Serial Attached SCSI is a high-speed serial interface used primarily in servers and enterprise systems. SAS offers theoretical maximum speeds of 3, 6, 12, and 24 Gbit/s for increasing revisions of the specification.
- NVMe – Non-Volatile Memory Express is a high-performance interface designed for SSDs. NVMe enables blistering speeds, with a PCIe 3.0 x4 interface reaching nearly 4 GB/s.
However, real-world HDD speeds are impacted by overheads such as command processing time, seek times, rotational delays, and the need to transfer data between disk platters and the interface. So while a SATA III interface supports 6 Gbit/s, actual HDD speeds typically max out around 160 MB/s for today’s high performance drives.
Future HDD Speed Improvements
While HDD speeds have steadily increased over the decades, we are starting to reach the physical limits of mechanical hard drives. However, new technologies aim to continue pushing HDD capacities and speeds further.
One technology is HAMR (heat-assisted magnetic recording), which uses laser thermal assistance to enable higher density drives. Seagate expects to release 20TB HAMR drives by 2020 (Source).
MAMR (microwave-assisted magnetic recording) is another technology, similar to HAMR, that allows increasing areal densities. Western Digital expects to launch MAMR-enabled HDDs in 2019 (Source).
SMR (shingled magnetic recording) is a way to increase density by overlapping tracks, but it comes with performance tradeoffs. While technologies like HAMR and MAMR aim to push HDD capacities and speeds higher, SSDs continue to offer superior performance.
SSDs have faster seek times measured in microseconds versus milliseconds for HDDs. And SSDs continue to improve in speed, durability, and capacity. So while new technologies will push HDDs forward, their mechanical and seek limitations mean SSDs will continue dominishing performance-critical roles.
Summary
In this article, we reviewed key points on HDD read and write speeds:
– HDD speeds are measured in megabytes per second (MB/s) for both reads and writes. Typical speeds range from 50-160 MB/s for internal HDDs and 80-150 MB/s for external HDDs.1
– Interface types like SATA, USB, and Thunderbolt impact maximum HDD speeds. SATA 3.0 supports up to 600 MB/s, USB 3.0 up to 640 MB/s, and Thunderbolt 3 up to 2800 MB/s.2
– Average HDD access times range from 8-15 milliseconds for high performance drives. Access times impact overall responsiveness.3
– SSDs are much faster than HDDs, with typical speeds of 200-550 MB/s. However, HDDs have higher storage capacities per dollar.
In conclusion, HDD speeds continue to increase slowly but steadily. Emerging interface technologies like SATA Express and NVMe allow higher transfer rates. But for massive storage capacities, HDDs remain essential components of computer systems and data centers.
