MassStorageSystems().pptVIP

Chapter 12: Mass-Storage Systems Overview of Mass Storage Structure: Tape Magnetic tape Early secondary-storage medium Relatively permanent and holds large quantities of data Access time slow Random access ~1000 times slower than disk Mainly used for backup, storage of infrequently-used data, transfer medium between systems Kept in spool and wound or rewound past read-write head Once data under head, transfer rates comparable to disk 20-200GB typical storage Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT Overview of Mass Storage Structure: Disk Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 200 times per second 3,600 -12,000 RPM Transfer rate is the rate at which data flows between drive and computer Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency) Head crash results from disk head making contact with the disk surface That’s bad Overview of Mass Storage Structure: Disk Disks can be removable, mounted as needed Drive attached to computer via I/O bus Busses vary, including EIDE, ATA, SATA, USB, Fiber Channel, SCSI, and FireWire Host controller in computer uses bus to talk to disk controller built into drive or storage array Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks The logical block is the smallest unit of transfer, usually 512 bytes The array of logical blocks is mapped into the sectors of the disk sequentially Sector 0 is the first sector of the first track on the outermost cylinder Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost Disk Attachment Host-attached storage accessed through I/O ports talking to I/O busses SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests operation and SCSI targets perform task