The Disk: Some Sample Problems
When a track on a hard disk is selected, the head must physically move to that track. The further apart the tracks are, the longer the access time. The SSD has no moving parts and uses random access semiconductor memory. Therefore, track order is unimportant.
There are two reasons. First, as a disk fills up, the clusters occupied by files become scattered and the file is defragmented or spread across many tracks. An undue amount of head movement is required to access a fragmented file. This problem can be reduced by defragmenting files (regrouping the clusters to occupy fewer tracks). A second reason that nearly empty disks are efficient is due to the surface linear bit density. The outer tracks of a disk are longer than the inner tracks. Consequently, the number of bits that can be stored along a track is greater at the periphery. If a disk rotates at a constant angular velocity, the bits written to or read from an outer track are transmitted at a greater rate than those on inner tracks. So, if data is stored only on outer tracks, data rates are improved. Note that this approach also increases the chance of failure because it requires more disk drives. It also uses more energy.
The bit cells of flash memory that store 1s and 0s are very small indeed. In order to store data in the cell, a charge has to be moved through a conductor. To erase a cell, the charge must be removed which also involves crossing the insulator. There are various means of moving a charge across the insulator. However, the movement of the charge can physically damage the insulator. Eventually it breaks down and fails.
Flash memory stored data as a charge on a floating gate embedded within an insulator in a field effect transistor. These transistors can be arranged in NOR gate arrays or NAND gate arrays. A NOR gate array arranges the individual transistors in parallel and a NAND array arranges transistors in series. The NOR array has a better endurance (a factor of 10) and its write cycle is about 100 ns. Its read cycle is very slow at 10 ms. NOR flash is used in specialized applications such as program store in mobile electronics.
The NAND cell is smaller than the NOR cell and has approximately equal read and write
cycles of 1 ms. The read access time of a NAND cell is to slow to allow the memory
to be used directly. NAND memory is used for bulk data storage in solid state drives,
Because of the storage mechanism in a flash memory cell, repeated writes and erases
gradually break down the insulation of a cell’s embedded gate. When this happens
the cell can behave erratically or become stuck-
If you executed one write cycle to each cell , it would take capacity x write time = 256 GB/(256 x 106) s to access all cells. Since we can perform 20,000 write cycles/cell, the total endurance is 20,000 x 256 GB/(256 x 106) s = 2 x 107 s. This corresponds to about 230 days.
Remember the memory hierarchy: registers, cache, DRAM, disk. The disk comes well
down the memory hierarchy; that is, few accesses come from disk. If the hit rate
of the cache is 95%, its miss rate is 5%; that is, only 5% of accesses are to main
store and disk combined. Since most of the remaining accesses are to main store,
the actual load on disks is low. Even if all misses went to disk, the actual life
of the SSD would be 25 times greater than the value we calculated; that is, 25 x
230 days = 15.75 years. In practice, the actual value would be reduced because our
assumption about perfect wear-