Hard Disk (Hard Drive) Performance – transfer rates, latency and seek times

The performance of a hard disk is very important to the overall speed of the system – a slow hard disk having the potential to hinder a fast processor like no other system component – and the effective speed of a hard disk is determined by a number of factors.

Generally speaking, Disk RPM is a critical component of hard drive performance because it directly impacts the latency and the disk transfer rate. The faster the disk spins, the more data passes under the magnetic heads that read the data; the slower the RPM, the higher the mechanical latencies.

In meantime other compoments impact hard drive performance too.

The time it takes for a hard drive to respond to and complete an I/O request is dependent on two things:

  • The hard drive's mechanical and electrical limitations
  • The I/O load imposed by the system

The following sections explore these aspects of hard drive performance in more depth.

Hard Drives

Hard drives occupy a special place in the storage spectrum. Their non-volatile nature makes them ideal for storing programs and data for longer-term use. Another unique aspect to hard drives is that, unlike RAM and cache memory, it is not possible to execute programs directly when they are stored on hard drives; instead, they must first be read into RAM. Also different from cache and RAM is the speed of data storage and retrieval; hard drives are at least an order of magnitude slower than the all-electronic technologies used for cache and RAM. The difference in speed is due mainly to their electromechanical nature. There are four distinct phases taking place during each data transfer to or from a hard drive. The following list illustrates these phases, along with the time it would take a typical high-performance drive, on average, to complete each:

  • Access arm movement (5.5 milliseconds)
  • Disk rotation (.1 milliseconds)
  • Heads reading/writing data (.00014 milliseconds)
  • Data transfer to/from the drive's electronics (.003 Milliseconds)

Of these, only the last phase is not dependent on any mechanical operation.

Access Arm Movement

If there is one component in hard drives that can be considered its Achilles' Heel, it is the access arm. The reason for this is that the access arm must move very quickly and accurately over relatively long distances. In addition, the access arm movement is not continuous -- it must rapidly accelerate as it approaches the desired cylinder and then just as rapidly decelerate to avoid overshooting. Therefore, the access arm must be strong (to survive the violent forces caused by the need for quick movement) but also light (so that there is less mass to accelerate/decelerate). Achieving these conflicting goals is difficult, a fact that is shown by how relatively much time the access arm movement takes when compared to the time taken by the other components. Therefore, the movement of the access arm is the primary determinant of a hard drive's overall performance, averaging 5.5 milliseconds. Hard Drive Latency

Mechanical latencies, measured in milliseconds, include both seek time and rotational latency.

Seek Time is measured defines the amount of time it takes a hard drive’s read/write head to find the physical location of a piece of data on the disk.

Rotation latency is the average time for the sector being accessed to rotate into position under a head, after a completed seek. It is easily calculated from the spindle speed, being the time for half a rotation. A drive’s average access time is the interval between the time a request for data is made by the system and the time the data is available from the drive.

This is the reason why higher-performance hard drives typically rotate their disk platters at higher speeds. Today, speeds of 15,000 RPM are reserved for the highest-performing drives, while 5,400 RPM is considered adequate only for entry-level drives. This averages approximately 3 milliseconds for a 10,000 RPM drive.

Access time includes the actual seek time, rotational latency, and command processing overhead time.

Hard Drive Transfer Rates

The disk transfer rate (sometimes called media rate) is the speed at which data is transferred to and from the disk media (actual disk platter) and is a function of the recording frequency. It is generally described in megabytes per second (MBps). Modern hard disks have an increasing range of disk transfer rates from the inner diameter to the outer diameter of the disk. This is called a zoned recording technique. The key media recording parameters relating to density per platter are Tracks Per Inch (TPI) and Bits Per Inch (BPI). A track is a circular ring around the disk. TPI is the number of these tracks that can fit in a given area (inch). BPI defines how many bits can be written onto one inch of a track on a disk surface.

The hard drive's read/write heads only work when the disk platters over which they "fly" are spinning. The time that it takes for media containing the desired sector to pass completely underneath the head is the sole determinant of the head's contribution to total access time. This averages .0086 milliseconds for a 10,000 RPM drive with 700 sectors per track.

IO load imposed by host

Host transfer rate

The host transfer rate is the speed at which the host computer can transfer data across the IDE/EIDE or SCSI interface to the CPU. It is more generally referred to as the data transfer rate, or DTR, and can be the source of some confusion. Some vendors list the internal transfer rate, the rate at which the disk moves data from the head to its internal buffers. Others cite the burst data transfer rate, the maximum transfer rate the disk can attain under ideal circumstances and for a short duration. More important for the real world is the external data transfer rate.

Command Processing Time

All hard drives produced today have sophisticated embedded computer systems controlling their operation. These computer systems perform the following tasks:

  • Interacting with the outside world via hard drive's interface
  • Controlling the operation of the rest of the hard drive's components, recovering from any error conditions that might arise
  • Processing the raw data read from and written to the actual storage media

Even though the microprocessors used in hard drives are relatively powerful, the tasks assigned to them take time to perform. On average, this time is in the range of .003 milliseconds.

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