The terms sequential and nonsequential (random) have been used to refer to hard disk drive operations. A single hard disk drive consists of a set of drive platters, each of which provides services for read and write operations with a set of arms with read and write heads that can move across the platters and read information from the drive platter or write data onto the platters. Remember these points about hard disk drives and SQL Server:
The read/write heads and associated disk arms must move to find and operate on the location of the hard disk drive platter that SQL Server and the Windows operating system requested. If the data is located on nonsequential locations on the hard disk drive platter, it takes significantly more time for the hard disk drive to move the disk arm and read/write head to all of the necessary hard disk drive platter locations. This contrasts with the sequential case, in which all of the required data is located on one continuous physical section of the hard disk drive platter, so that the disk arm and read/write heads move a minimal amount to perform the necessary disk I/O operations. The time difference between the nonsequential versus sequential case is significant, about 50 milliseconds per nonsequential seek versus approximately 2 to 3 milliseconds for sequential seeks. These times are rough estimates and will vary based on how far apart the nonsequential data is spread on the disk, how fast the hard disk platters can spin (RPM), and other physical attributes of the hard disk drive.
A typical hard disk drive supports about 75 nonsequential and 150 sequential I/Os per second. It takes almost as much time to read or write 8 KB as it does to read or write 64 KB. Within the range of 8 KB to about 64 KB the disk arm and read/write head movement account for most of the time of a single disk I/O transfer operation. Perform 64-KB disk transfers as often as possible when more than 64 KB of SQL data must be transferred, because a 64-KB transfer is fast as an 8-KB transfer and eight times the amount of SQL Server data is processed for each transfer. Read Ahead Manager does disk operations in 64-KB chunks (referred to as a SQL Server extent). Log manager also performs sequential writes in larger I/O sizes.