Hard Disk Drives

Question 1

Hard Disk Drives

Answer 1

• main form of persistent data storage in computer systems for decades and much of file systems technology is predicated on their behavior

Question 2

Interface to a modern disk drive

Answer 2

• consists of a large number of sectors (512 bytes blocks), each off which can be read or written
• sectors are numbered from 0 to n -1 on a disk of n sectors
• can view disk as an array of sectors; 0 to n - 1 is thus the address space of the drive
• many file systems will read or write 4KB at a time, however only a single 512 byte write is atomic (complete in its entirety, or it won’t complete at all)
• if power loss occurs, only a portion of a larger write may complete (sometimes called a torn write)

Question 3

Track

Answer 3

• concentric circle of sectors where data is encoded onto the surface
• one concentric circle is called a track

Question 4

Disk Head

Answer 4

• to read and write from the surface we need a mechanism that allows us to read the magnetic patterns on the disk or to induce a change in them, which is accomplished by the disk hea

Question 5

Rotational Delay

Answer 5

• wait for the desired sector to rotate under the disk head

Question 6

Seek

Answer 6

• drive moving the disk arm to the correct track
• seek along with rotations are one of the most costly disk operations
• settling time: head is carefully positioned over the correct track, often quite significant

Question 7

Transfer

Answer 7

Data is read or written to the surface

Question 7

Transfer

Answer 7

Data is read or written to the surface

Question 8

Track Buffer

Answer 8

• modern disk drive’s cache
• small amounts of memory 8 or 16 MB, drive can use to hold data read from or written to the disk
• drive might read in all of the sectors on that track and cache them in its memory
• allows drive to quickly respond to any subsequent requests to the same track