Sequential IO means that the access addresses for read and write operations are consecutive. In sequential IO access, the HDD requires significantly less track search time because the read/write heads can access the next block with minimal movement. Operations such as data backup and logging are sequential IO operations.

Random IO is when read and write operations are sequential in time, but access addresses are not sequential and are randomly distributed across the address space of the disk. Businesses that generate random IO are OLTP services, SQL, instant messaging services, and so on.

Mysql data will eventually be brushed to disk, brushing the disk is divided into random IO and sequential IO, the performance difference between the two is very large, most of the time we will change the design to make Mysql's random IO into sequential IO to improve the performance

Add a cache to the random IO to have a greater benefit:

IOPS (Input/OutputOperations Per Second). OutputOperations Per Second) is a measurement used for performance testing of computer storage devices such as hard disk drives (HDDs), solid state drives (SSDs), or storage area networks (SANs), and can be thought of as the number of reads and writes per second.

For mechanical hard drives or other similar electromechanical storage devices, the random access IOPS is primarily related to the addressing time of the storage device, while for SSDs and other solid-state electronic devices, the random access IOPS is primarily related to the speed of the storage device's internal controller and memory interface. The sequential access IOPS for both devices (especially for accessing large blocks of data) are generally related to the maximum bandwidth that can be sustained by the included storage device.

There are a number of reasons why this is the case, one being that SSDs are typically master chips that contain a number of channels, each connected to a number of flash chips, and random IO cannot be shared across multiple chips and transferred in parallel for higher performance as with sequential IO, two being that SSDs have mechanisms such as erase/garbage collection, and random IO will obviously make these mechanisms more difficult and more frequent, and three. Mechanisms such as pre-reading and caching work better for sequential IO. However, the random IO throughput of SSDs nowadays under multi-threading is very close to sequential IO, but there is still a gap.