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Enhancing Disk Performance in Operating Systems: Techniques and Mechanisms

This document explores key concepts related to disk operations in operating systems, focusing on hardware support for interrupts and direct memory access (DMA). It discusses the role of I/O processors, buffering, scheduling, and the importance of disk access efficiency. Key mechanisms such as disk scheduling policies, RAID for redundancy, and caching strategies are analyzed alongside practical examples using C code. The challenges of contention for system bus cycles and the methods to optimize disk operations are highlighted, offering insights into system design for improved performance.

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Enhancing Disk Performance in Operating Systems: Techniques and Mechanisms

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  1. Operating Systems 11 - disk PIETER HARTEL

  2. Hardware • Intended for humans, local devices and communication (challenge?) • Hardware support for interrupts and DMA

  3. Direct Memory Access (DMA) • CPU sends commands to controller indicating: • what to do and • where in memory to get/put the data • Controller runs in parallel with CPU • Problem : Contention for system bus cycles • Solution : I/O processor or I/O bus

  4. Design issues • Efficiency • Buffering (how?) • Scheduling • Generality • Layering • Uniform access – Unix!

  5. #define M 1024*1024 /* One MByte */ int main(intargc, char *argv[]) { int out = open(argv[1], O_RDWR|O_CREAT|O_TRUNC, 0666); int n = atoi(argv[2]); lseek(out, n*M-1, SEEK_SET); write(out, "\0", 1); close(out); return 0; } Sparse files &Network vs disk • gccSparse.c • for d • in . /tmp • do • for n • in 1 2 4 8 16 32 64 128 256 512 1024 • do • time ./a.out $d/$n.dat $n • time cp $d/$n.dat $d/junk • done • done • ls –l /tmp/*dat /tmp.junk Time (s)

  6. Mechanisms • Disk with rotating platter and moving arm (alternative?) • Timing • Access time depends on the seek time + rotational delay • Transfer time depends on the rotational speed and the channel bandwidth • Example: • Small blocks? ??? 4ms 4ms 10ms/MB

  7. Disk scheduling policies • Example: head @ 100, queue contains 55,58,39,18,90,160,150,38,184 • Many requests • Known address + size • Which order? • SSTF has an issue…

  8. Redundant Array of Independent Disks • Several physical drives seen as one logical drive • Data is striped across the disks (why?) • Redundancy(why?)

  9. Linux design – uniform! • Block devicesfor fast peripherals • Character devices for slow peripherals • Policies • LRU block replacement policy (why?) • SCAN scheduler for block R/W • Devices are handled as special files: • df -l • ls –l /dev/cciss/c0d0p2 /dev/tty • cat /dev/tty >junk

  10. #define M 1024*256 #define N 1000 /* number of buffers written */ intmain(intargc, char *argv[]) { int out = open(argv[1], O_RDWR|O_CREAT|O_TRUNC, 0666); inti,k; intbuf[M]; /* One Mbyte */ for(i=0;i<N;i++) { for(k=0;k<M;k++) { buf[k] = i*N+k; } write(out,buf,sizeof(buf)); } if(argc>=3) { fdatasync(out); posix_fadvise(out,0,0, POSIX_FADV_DONTNEED); } close(out); return 0; } Disk cache • man free • gccFadvise.c

  11. Summary • Without I/O no communication with the outside world • Caching crucial for performance • Disks are slower than the CPU hence more scope for scheduling • RAID for more dependable storage

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