Chapter 7 Flashcards
External memory
Define
Magnetic disk
A circular ‘platter’ constructed of nonmagnetic material, called the ‘substrate,’ and coated with a magnetizable material. Traditionally the substrate has been an aluminium or aluminium alloy, recently glass been introduced
Describe
Benefits of glass substrate
for magnetic disks
Improvement in uniformity of the magnetic film surface to increase disk reliability.
A significant reduction in overall surface defects to help reduce read/write errors.
Ability to support lower fly heights.
Better stiffness to reduce disk dynamics.
Greater ability to withstand shock and damage.
Describe
Magnetic read/write mechanisms
R/w done via a conducting coil named the ‘head.’
In many systems, separte r/w heads.
During the r or w operation, the head is stationary while the platter rotates.
Write mechanism exploits fact that electricity flowing through a coil produces a magnetic field.
List
Physical characteristics of Disk systems
- Head motion: fixed or moveable
- Disk portability: removable or not
- Sides: single or double sided
- Platters: single or multiple
- Head mechanism: contact (floppy), fixed gap, aerodynamic gap (Winchester)
List and define
The three types of disk classification
1st: traditional – r/w head positioned a fixed distance above platter, allowing air gap
2nd: floppy disk – head comes in physical contact with platter during r/w operations
3rd: Winchester heads – used in sealed drive assemblies that are almost free of contaminants. Designed to operate closer to the disks surface than conventional rigid disk heads, thus allowing greater data density. Is actually an aerodynamic foil that rests lightly on the platter’s surface when the disk is motionless. Air pressure generated by spinning disk is enough to make foil rise above the surface.
List
Disk performance parameters
When disk drive is operational, disk is rotating at constant speed.
To r/w, head must be positioned at desired track and beginning of desired sector on track.
Seek time
Rotational delay (latency time)
Bloc access time
Transfer time
bloc is not typo
Define
Seek time
magnetic disk
For moveable head, time to position at the right track
Define
Rotational delay
magnetic disk
Latency time. Time for beginning of sector to reach head
Define
Bloc access time
magnetic disk
Sum of seek, latency, and transfer time
Define
RAID
Redundant array of independent disks
List
Three common characteristics of all RAID levels
- Set of physical disk drives viewed by the operating system as a single logical drive
- Data are distributed across the physical drives of an array in a scheme known as striping
- Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure
Define
RIAD level 0
Addresses the issues of request patterns of the host system and layout of the data. Impact of redundancy does not interfere with analysis
List
RAID level 0 applications
Video production and editing, image editing, prepress applications, anything that requires high bandwidth
Define
RAID level 1 characteristics
Redundancy handled differently than levels 2-6, just duplicates all the data.
Data striping used, but each logical strip mapped to 2 separate physical disks so each disk has mirror
Principle disadvantage is cost
List
Applications of RAID level 1
Accounting, payroll, financial, high availability stuff
Define
RAID level 2 characteristics
Makes use of parallel access technique.
In a parallel access array, all member disks participate in the execution of every I/O request.
Spindles of independent drives are synched so each disk head is in the same position on each disk at same time.
Number of redundant disks is proportional to the log of the number of data disks.
Error correcting code is calculated across corresponding bits on each data disk and the bits of code are stored in corresponding bit positions on each data disk.
List
Applications of RAID level 2
No commercial implementations exist / is not commercially viable.
List
RAID level 3 characteristics
Requires only a single redundant disk, no matter how large the disk array.
Employs parallel access but with a single parity bit calculated for set of individual bits in same position on all of the data disks.
Can achieve very high data transfer rates.
In event of of a disk failure, all of the data are still available in what is referred to as ‘reduced mode’
List
Applications of RAID level 3
Video production, live streaming, image and video editing, prepress, high throughput stuff.
Define
Characteristics of RAID level 4
Makes use of independent access technique, separate I/O requests can be satisfied in parallel.
Data strips are relatively large.
To calculate new parity, the array management software must read data drive user strip and old parity strip.
Each time a write occurs, the array management software must update user data and parity bits.
List
Applications of RAID level 4
No commercial implementations exist / is not commercially viable.
Define
Characteristics of RAID level 5
Organization similar to L4, except parity bits distributed across all disks.
Typical allocation is round-robin.
Avoids potential I/O bottleneck in L4.
List
Applications of RAID level 5
File application servers
Database servers
Web, email, or news servers
Intranet servers
Most versatile RAID level
Define
Characteristics of RAID level 6
2 different parity calculations carried out and stored in separate blocks on different disks.
Advantage: provides extremely high data availability. Three disks would have to fail before one is repaired to cause loss of data
Disadvantage: incurs substantial write penalty because each write affects two parity blocks
