Disk Performance

Access Time

Access time is the amount of time between the start of a request and the start of data transfer. It contains * Seek Time - time taken to move the arm over the correct track. Typically 4~10 ms. * Rotational Latency - time taken for the sector to be accessed by the head. Typically 4~11 ms.

Data Transfer Rate

The rate at which data is retrieved from or stored to the disk. Typically 25~200 MBps, slower for inner tracks.

Access Patterns

Sequential Access Pattern

• In this process, data is accessed in successive disk blocks on the same track. This means that disk seek is only performed for the first block.

Random Access Pattern

• In this process, successive requests can be anywhere on the disk. So, each request needs to perform its own seek.

IOPS

• IOPS is the number of random block reads supported per second. It is 50~200 on current Magnetic Disks.

MTTF

• Mean Time To Failure is the expected time that the disk will run theoretically without any failure. It's typically 3~5 years.