IO units of the disk

For a disk, a continuous read or write of a disk is called an IO.

"one IO" at all levels of the computer

The application program requests to the operating system: "read / etc/passwd to my buffer". The operating system reads it and returns a signal from the application. This time the IO is completed. This is an IO made by the application.

The file system asks the disk controller driver to "read the 128sectors after the start of the LBA10000", and then "read the last 64 sectors of the LBA50000", which is the two IO made by the file system down, assuming that it corresponds to the request of the application in the first step.

The disk controller driver uses signals to drive the disk controller to send SCSI instructions and data to the disk. For SCSI protocol, reading and writing a continuous LBA address sector is an IO, but in order to complete this read or write, it may need to send several SCSI instruction frames. From the bottom point of view, one SCSI frame is sent to the disk at a time, even once IO, which is the finest IO. But generally speaking, disk IO refers to the completion of an entire SCSI read or write.

If you insert another layer of volume manager between the file system and the disk, or another layer of RAID virtualization layer between the disk controller and the disk, the upper layer of IO will often evolve into multiple IO of the lower layer.

For disk, each IO refers to an interactive round of SCSI instructions, which may contain several SCSI instructions, but only one IO can be completed in this round, such as "reading the last 128sectors starting from LBA10000"