Chapter goals

Speed cont.

• Speed is also an issue for secondary storage
• Called “access time” or “seek time”
• Access time is defined as time to complete one read or write operation
• Access time for disk or tape storage can vary depending on location of information, therefore access time is expressed as an average

Access times

• Primary storage – expressed in nanoseconds (billionths of a second)
• Secondary storage – expressed in milliseconds (thousandths of a second)

Data transfer rate

• Complete measure of data access speed consists of access time and the unit of data transfer to/from the storage device
• Access time plus how much data is transferred
• Data transfer unit for primary storage is based on word size (usually 32 bit)

Data transfer unit

• Data transfer unit (amount of data moved at a time) for secondary storage varies depending on the device
• Unit is called a “block”
• Block size is stated in bytes
• “Sector” is data transfer unit for magnetic and optical devices
• Common sector/block size is 512 bytes