HDD vs SSD: Read/Write Methods Explained

When describing the performance of products such as solid-state drives, U disks, microSD cards (TF cards), and SD cards, parameters such as sequential read/write speed, random read/write/4K read/write performance, and IOPS are often mentioned. Often the test results of related evaluation articles will be expressed in the form of screenshots about the performance of a certain flash memory product. So what do these indicators mean?

Basic Spec

How Does a Aisk Store Data?

The information is stored on the hard disk, and when you take it apart you can't see anything in it, just some platters. If you use a microscope to magnify the disk, you will see that the surface is uneven, the raised place is magnetized, and the concave place is not magnetized; the raised place represents the number 1 (after magnetization), and the concave place represents number 0. Therefore, the hard disk can store information such as text and pictures in binary.

HDD vs SSD

1) Hard Disk Drive (HDD)

The read and write speed of a mechanical hard disk is determined by the rotation of the disk, the moving speed of the magnetic head, and the moving distance of the magnetic head. Reading and writing are addressed by swinging the magnetic head. And sequential reading and writing of HDDs mainly takes time in transmission.

This reading method is very slow, directly restricting the efficiency of the entire computer. When the hard disk is produced, the disk speed and the moving distance of the head are fixed and cannot be changed. The feasible solution to increase the read speed is to reduce the moving distance of the head. The ideal state is that the data accessed is on a magnetic column or adjacent on the magnetic column, which makes it easier to manage and control the system and improve efficiency. Because related files and directories are placed in one partition, when the system reads and writes the disk, the moving distance is shortened.

2) Solid State Drive (SSD)

The solid-state hard disk is composed of a control unit and a solid-state storage unit (DRAM or FLASH chip). The storage unit is responsible for storing data, and the control unit is responsible for reading and writing data. Because the actual writing speed of flash memory is not high, it will use a cache mechanism. Now the mainstream is triple-level cell (TLC). For example, a product with a nominal 128GB will generally be designed with an single-level cell (SLC) of about 8GB~16GB. Therefore, if the total amount of data written by the test software is within the capacity of the SLC cache, the test result is only the performance of it. When data is written into TLC flash memory, the subsequent writing speed will drop to the actual writing speed of TLC cache, which is generally 100MB/s. This is the unavoidable problem of mainstream TLC SSDs.

In addition, the larger the capacity of the hard disk, the larger the capacity allocated to the SLC cache. However, if the TLC flash memory is used as an SLC, the negative impact is that the service life of the hard disk will be damaged. The larger the capacity of SSD, the larger the internal particles, the better the read and write performance, and the longer the service life. However, the larger the capacity, the less the impact on the read speed, the greater the impact on the write speed. In addition, unlike HDD, SSD do not have seek problems.

Read/Write Methods

Sequential Read/Write

Sequential reading and writing is to collect the data to be processed and line them up, and perform continuous operations at the optimal speed. Usually, when reading and writing large files, an ideal speed can be obtained. In a nutshell, sequential reading and writing means that the solid-state hard disk works in the order of data in continuous state. When we use SSDs to watch videos or copy files of more than tens of GB, we are using the sequential way.

For flash memory application products such as solid-state drives, the sequential read and write speed will be marked in the key parameters of the product. The higher the speed value, the stronger the sequential read and write performance, and the speed unit of the sequential read and write value is MB/s. This performance is critical for large file copies.

Also this way may be limited by interfaces and transmission protocols, resulting in differences in read/write speeds when using different interfaces and transmission protocols. For example, the speed of SATA or M.2 SATA interfaces can theoretically reach 600MB/s, while using the M.2 interface that supports the NVMe protocol and the solid-state hard disk that uses PCIE4.0×4 channels, its speed can reach 7G/s.

Random Read/Write

Random read and write is relative to the above mentioned. It means that the pointer to access the file can be pointed to any position of the file content at any time.

When a SSD has been used for a period of time, data has been written and deleted continuously before. After a long time, it will naturally generate a lot of scattered storage space of the solid-state drive or the data storage address is not concentrated in a certain place. At this time, the method of reading and writing data is not sequential reading and writing. Under this situation, the speed is usually lower than the sequential reading and writing speed, or even much lower. Now we will use the random read and write, and it aims to the small and scattered data. So the randomness is strong, and the read and write time is long.

The performance of random read and write is a key indicator of solid state drives, and its unit is IOPS (input/output operations per second). The stronger the performance, the faster the task of processing fragmentary files (such as startup programs, virus scans, etc.).

Applications such as small file storage (pictures), mail servers, and OLTP databases all have frequent random reads and writes. So more attention should be paid to random reads and writes performance when selecting SSDs, and the key measurement indicator is IOPS.

4k Read/Write

To understand what 4k read and write is, we must first understand what 4k stands for. It refers to the minimum occupation of files is 4kb, which is the smallest occupation space under windows. Usually, when copying a complete large file with a SSD, the writing speed is very fast, which means that the sequential read and write speed is very fast. But this speed will drop when copying non-contiguous files, at this time, it is the random read and write speed, which is often closely related to the 4K read and write performance. Therefore the 4K read and write performance directly affects the efficiency of processing small files.

In addition, 4k random read and write is closer to our daily habits. For example, starting the computer, updating system files, writing web pages, copying a large number of pictures, loading and responding to game programs, etc. all require it. So it is crucial to the operating experience of the computer.

IOPS

IOPS has been introduced above, a high IOPS value means that the 4k read and write speed will be faster.

Parameter Lists

The following table is the performance parameters of the Samsung 990 PRO, which lists the?parameters of sequential read speed, sequential write speed, random read, and random write.

From the above chart, the sequential read and write speed is represented by how many MB/s, and the random read and write speed is measured by how many IOPS, why they are different? MB/s and IOPS are both units that can be used to express the speed, but their focus are different. MB/s is more related to the data throughput, while IOPS is closely to the number of operations on SSDs per second.

In short, both are very important performances, and each has its own advantages. When choosing a good solid state drive, you should not only focus on sequential read and write performance, but also need to pay attention to random read and write. In addition, the SAS mechanical hard disk mainly depends on the sequential read and write performance, and the hard drives mainly depends on the random read and write performance. In addition, it is very good if the random read and write speed reaches more than 1000K IOPS.

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