EXTERNAL MEMORIES Flashcards
What are the types of external memory
• Magnetic Disk —RAID —Removable • Optical —CD-ROM —CD-Recordable (CD-R) —CD-R/W —DVD • Magnetic Tape
Describe a magnetic disk
- Disk substrate coated with magnetizable material
* Substrate used to be aluminum but is now glass
What is the advantage of using glass over aluminum as disk substrate
—Improved surface uniformity: Increases reliability
—Reduction in surface defects: Reduced read/write errors
—Lower flight heights
—Better stiffness
—Better shock/damage resistance
Describe the read/write mechanisms of magnetic disks
- Recording and retrieval via conductive coil called a head
- May be single read/write head or separate ones
- During read/write, head is stationary, platter rotates
Describe specifically what happens in the write mechanism of a magnetic disk
—Current through coil produces magnetic field
—Pulses sent to head
—Magnetic pattern recorded on surface below
Describe both the traditional and contemporary approaches to the read mechanism of magnetic disks
• Read (traditional)
—Magnetic field moving relative to coil produces current
—Coil is the same for read and write
• Read (contemporary)
—Separate read head, close to write head
—Partially shielded magneto resistive (MR) sensor
—Electrical resistance depends on direction of magnetic field
—High frequency operation
What is the advantage of the contemporary read mechanism over the traditional one
Higher storage density and speed
Describe the data organization and formatting in magnetic disks
• Concentric rings or tracks —Gaps between tracks —Reduce gap to increase capacity —Same number of bits per track (variable packing density) —Constant angular velocity • Tracks divided into sectors • Minimum block size is one sector • May have more than one sector per block
*See notes for picture
Give examples of magnetic disks
Winchester Hard disks • Sealed unit • One or more platters (disks) • Heads fly on boundary layer of air as disk spins • Very small head to disk gap • Getting more robust • Universal • Cheap • Fastest external storage • Getting larger all the time
floppy disk • Small capacity • Slow • Universal • Cheap • Obsolete
zip disks
Describe CD-ROM
- Polycarbonate coated with highly reflective coat, usually aluminum
- Data stored as pits
- Read by reflecting laser
- Constant packing density
- Constant linear velocity
Give examples of optical storage devices
- CD-Recordable (CD-R)
- CD-RW
- Data can be stored only once in CD-R and it cannot be erased while data can be stored multiple times in CD-RW and it can be erased
DVD
• Multi-layer
• Very high capacity (Full length movie on single disk using MPEG compression)
• Standardized
What are the characteristics of Magnetic tape
- Serial access
- Slow
- Very cheap
- Backup and archive
What are Magneto-optical disks
The magneto-optical disk is a special removable disk. The design of the drive allows the inserted disk to be exposed to the magnetic head on one side and to the laser on the other
What is RAID
- Redundant Array of Inexpensive Disks is a set of physical disks viewed as single logical drive by O/S
- Data distributed across physical drives
- It has 6 levels that are in common use. The levels are not arranged in a hierarchy
- Can use redundant capacity to store parity information
Briefly describe the different types of raid
RAID 0 – striping (system data are split up into blocks that get written across all the drives in the array.)
RAID 1 – mirroring (Data are stored twice by writing them to both the data drive (or set of data drives) and a mirror drive (or set of drives).)
RAID 5 – striping with parity
RAID 6 – striping with double parity
RAID 10 – combining mirroring and striping
