1CSC 4103 - Operating SystemsSpring 2007Tevfik KoşarLouisiana State UniversityApril 10th, 2007Lecture - XIXStorage and I/O - IIRAID Structure• As disks get cheaper, adding multiple disks to the samesystem provides increased storage space, as well asincreased reliability and performance.• RAID: Redundant Array of Inexpensive Disks– multiple disk drives provides reliability via redundancy.• RAID is arranged into six different levels.RAID (cont)• RAID schemes improve performance and improve thereliability of the storage system by storing redundantdata.– Mirroring (shadowing): duplicate each disk• Simplest but most expensive approach– Block interleaved parity uses much less redundancy.– Data Striping: splitting each bit (or block) of a file acrossmultiple disks.RAID LevelsRAID (0 + 1) and (1 + 0)Hierarchical Storage Management (HSM)• A hierarchical storage system extends the storagehierarchy beyond primary memory and secondarystorage to incorporate tertiary storage — usuallyimplemented as a jukebox of tapes or removable disks.• Usually incorporate tertiary storage by extending thefile system.– Small and frequently used files remain on disk.– Large, old, inactive files are archived to the jukebox.• HSM is usually found in supercomputing centers andother large installations that have enormous volumes ofdata.Hierarchical Storage ManagementI/O HardwareMapping I/O Ports to MemoryPolling• Determines state of device– command-ready– busy– Error• Busy-wait cycle to wait for I/O from deviceInterrupts• CPU Interrupt-request line triggered by I/O device• Interrupt handler receives interrupts• Maskable to ignore or delay some interrupts• Interrupt vector to dispatch interrupt to correcthandler– Based on priority– Some nonmaskable• Interrupt mechanism also used for exceptionsInterrupt-Driven I/O CycleIntel Pentium Processor Event-Vector TableDirect Memory Access• Used to avoid programmed I/O for large datamovement• Requires DMA controller• Bypasses CPU to transfer data directly between I/Odevice and memorySix Step Process to Perform DMA TransferKernel I/O Subsystem• Scheduling– Some I/O request ordering via per-device queue– Some OSs try fairness• Buffering - store data in memory while transferringbetween devices– To cope with device speed mismatch– To cope with device transfer size mismatch– To maintain “copy semantics”Device-status TableSun Enterprise 6000 Device-Transfer RatesKernel I/O Subsystem• Caching - fast memory holding copy of data– Always just a copy– Key to performance• Spooling - hold output for a device– If device can serve only one request at a time– i.e., Printing• Device reservation - provides exclusive access to adevice– System calls for allocation and deallocation– Watch out for deadlockError Handling• OS can recover from disk read, device unavailable,transient write failures• Most return an error number or code when I/O requestfails• System error logs hold problem reportsI/O Protection• User process may accidentally or purposefully attemptto disrupt normal operation via illegal I/O instructions– All I/O instructions defined to be privileged--> I/O must be performed via system calls• Memory-mapped and I/O port memory locationsmust be protected tooI/O Requests to Hardware Operations• Consider reading a file from disk for a process:– Determine device holding file– Translate name to device representation– Physically read data from disk into buffer– Make data available to requesting process– Return control to processLife Cycle of An I/O RequestPerformance• I/O a major factor in system performance:– Demands CPU to execute device driver, kernel I/O code– Context switches due to interrupts– Data copying– Network traffic especially stressfulImproving Performance• Reduce number of context switches• Reduce data copying• Reduce interrupts by using large transfers, smartcontrollers, polling• Use DMA• Balance CPU, memory, bus, and I/O performance forhighest throughput26Any Questions?Hmm..27Reading Assignment• Read chapter 13 from Silberschatz.28Acknowledgements• “Operating Systems Concepts” book and supplementarymaterial by Silberschatz, Galvin and