Disk Performance Parameters

Disk Performance Parameters

• The actual details of disk I/O operation depend on the :

1. Computer system

2. Operating system

3. Nature of the I/O channel and disk controller hardware.

• Currently, disks are at least four orders of magnitude slower than main memory. Fig. 5.3.1 shows a general timing diagram of disk I/O transfer.

• The disk is rotating at constant speed when it perform read or write operation. To read or write the head must be positioned at the desired track and at the beginning of the desired sector on that track. Head is moving from one track to other track for selecting proper track.

• Track selection involves moving the head in a electronically selecting one head  fixed-head system.movable-head system

Following parameters are used for disk performance :

1. Seek time   2. Rotational delay   3. Access time

1. Seek time: The time it takes to position the head at the track is known as seek time.

Seek time = Number of track traversed × Disk drive constant + Startup time Seek time do not apply to device with fixed read/write heads.

2. Rotational delay: The time it takes for the beginning of the sector to reach the head is known as rotational delay. It is also known as search time.

3. Access time: The sum of the seek time and the rotational delay equals the access time.

Access time = Seek time + Rotational latency

• Transfer time:

• Disk capacity:

Disk capacity = Number of cylinders × Number of heads × Number of sectors per track × Number of bytes per track

Example 5.3.1 What is the maximum size of disk which contains following parameters ?

Cylinder = 1024, heads = 16 and sectors per track is 63.

Solution : Total size of disk = Number of cylinders × Number of heads × Number of sectors/track × Number of bytes/sector.

= 1024 ×16 × 63 × 512

= 528 48

Example 5.3.2 A disk has 19456 cylinders, 16 heads and 63 sectors/tracks. The disk spins at 5400 r.p.m. Seek time between adjacent tracks is 2 ms. Assuming the read/write head is already positioned at track 0. How long does it take to read the entire disk?

Solution :

 1) Each track can be read in one revolution.

2) 11.11 ms is required to read one track.

3) For reading all 19456 × 16 tracks, approximately 3459 seconds is required.

4) Seek time = 19456-1 × 2 = 39 sec.

5) Total time is 3498 sec.