Exam 2 short answer concepts

Question 1

Seek time

Answer 1

The time it takes the read/write head to reach the desired cylinder. This has nothing to do with disk rotation, but rather with the arm moving in/out. This number would be given.

Question 2

How are RPM and latency related.

Answer 2

Increasing RPM = decreasing latency. Disk moves faster so it takes less time to find the desired location.

Question 3

Access latency(aka average access time) = ?

Answer 3

Average seek time + average latency.

Question 4

Rotational latency.

Answer 4

Time it takes the read/write head to reach the desired sector on a track. This is how fast the disk rotates under the head.

Question 5

Access time = ?

Answer 5

Seek time + rotational latency.

Question 6

Average latency = ?

Answer 6

First you find latency. You divide RPM/60. This gives you the fraction of a second it takes rotate once (in milliseconds). Then you divide this number by 2 to find the average latency.

Question 7

Average I/O time.

Answer 7

Average access time + (amount to transfer/transfer rate) + controller overhead

Question 8

RAID 0

Answer 8

Non-redundant striping. Increases chance for data loss. Data is spread across 2 separate disks. Used when speed matters, since it allows faster access to data.

Question 9

RAID 1

Answer 9

Mirroring. Data copied on a second disk.

Question 10

RAID 5

Answer 10

Striping with parity. Most common. Requires as least 3 disks. Data spread across many disks, and each disk uses parity information to rebuild the data if a disk fails. Downside - reduces total amount of data that can be stored since an entire disk worth of data is used for parity. Example, 4 1GB disks are used, but one whole disk is used for parity so there’s only 3GB available to store info.