Chapter 9 Peripheral Devices Magnetic Disks Basic Disk Operation Performance Parameters and History of Improvement Example disks RAID Redundant Arrays of Inexpensive Disks Improving Reliability Improving Performance Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Magnetic Disk Drives High density and non volatile Motion of medium supplies power for sensing More random access than tape direct access Densities similar to semiconductor RAM on an inexpensive medium No power required to retain stored information Different platters selected electronically Track on platter selected by head movement Cyclic sequential access to data on a track Structured address of data on disk Drive Platter Track Sector Byte Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Multi Platter Hard Disk Drive Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Simplified View of Disk Track and Sector Organization Comp Sys Design Arch 2nd Ed An integral number of sectors are recorded around a track A sector is the unit of data transfer to or from the disk 2004 Prentice Hall Simplified View of Individual Bits Encoded on a Disk Track Inside tracks are shorter thus have higher densities or fewer words All sectors contain the same number of bytes Inner portions of a platter may have fewer sectors per track Small areas of the disk are magnetized in different directions Change in magnetization direction is what is detected on read Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Typical Hard Disk Sector Organization Serial bit stream has header data error code Header synchronizes sector read and records sector address Data length is usually power of 2 bytes Error detection correction code needed at end Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Disk Formatting Disks are pre formatted with track and sector address written in headers Disk surface defects may cause some sectors to be marked unusable for the software Comp Sys Design Arch 2nd Ed 2004 Prentice Hall The PC AT Block Address for Disk Access Head number determines platter surface Cylinder is track number for all heads Count sectors up to a full track can be accessed in one operation Comp Sys Design Arch 2nd Ed 2004 Prentice Hall The Disk Access Process 1 OS Communicates LBA to the disk interface and issues a READ command 2 Drive seeks to the correct track by moving heads to correct position and enabling the appropriate head 3 Sector data and ECC stream into buffer ECC is done on the fly 4 When correct sector is found data is streamed into a buffer 5 Drive communicates data ready to the OS 6 OS reads data byte by byte or by using DMA Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Static Disk Characteristics Areal density of bits on surface density 1 bit spacing track spacing Maximum density density on innermost track Unformatted capacity includes header and error control bits Formatted capacity capacity bytes sectors tracks of surfaces sector track surface Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Dynamic Disk Characteristics Seek time time to move heads to cylinder Track to track access time to adjacent track Rotational latency time for correct sector to come under read write head Average access time seek time rotational latency Burst rate maximum transfer bandwidth burst rate Comp Sys Design Arch 2nd Ed revs sectors bytes sector sec rev 2004 Prentice Hall Improving Disk Reliability Performance RAID Redundant Array of Inexpensive Disks Raid LEVEL 0 Speed improvement only by striping data across several disks so they can be accessed in parallel RAID Level 1 Mirroring writing the exact same data to two different disk drives If one drive fails the other can be used RAID Level 2 Data is striped at the bit level across several disks with additional ECC bits to recover data if one drive fails Comp Sys Design Arch 2nd Ed 2004 Prentice Hall Improving Disk Reliability Performance RAID Redundant Array of Inexpensive Disks RAID Level 3 Striped as in level 0 but at the byte level and ECC data is written to a separate drive RAID Level 4 Same as level 3 but blocks are used instead of bytes and data are written asynchronously Often used in transaction based systems such as in airline reservation systems RAID Level 5 Similar to level 4 but both data and ECC bits are both striped across 3 or more drives RAID Level 6 Similar to level 5 but extra correction bits are present to permit recovery from multiple errors Comp Sys Design Arch 2nd Ed 2004 Prentice Hall