Disk Memory
I think these are out of date nowadays so we use SSD Hard drives no??
Disks operate mechanically, so they wear out? They are great because they are cheaper per bit because they have very high storage capacity at a modest cost.
- flash latency is 1000 times as fast and DRAM is 100,000 times as fast
- Disks are 6 to 300 times cheaper
To read and write information on a hard disk, a movable arm containing a small electromagnetic coil called a read-write head is located just above each surface.
- Disks rotate at 5400 RPM to 15,000 RPM
- The entire drive is permanently sealed to control the environment inside the drive, which, in turn, allows the disk heads to be much closer to the drive surface
Each disk surface is divided into concentric circles, called tracks:
- ~ 10,000 tracks per surface
Each track is divided into sectors:
- a sector = the smallest amount of information that is read or written on a disk
- each track may have thousands of sectors.
- Sectors are typically 512 to 4096 bytes in size
For the production of disks, manufacturers report
- minimum seek time
- maximum seek time
- average seek time
The average latency to get to a desired information is rotations. So you divide that by the RPM of the disc to get your rotational latency / delay.
For example, if you have a disk rotating at 5400RPM, you have a rotational latency of