CS152 – Computer Architecture andEngineeringLecture 18.ppt

Review Buses are an important technique for building large-scale systems Their speed is critically dependent on factors such as length, number of devices, etc. Critically limited by capacitance Direct Memory Access (dma) allows fast, burst transfer into processor’s memory: Processor’s memory acts like a slave Probably requires some form of cache-coherence so that DMA’ed memory can be invalidated from cache. Networks and switches popular for LAN, WAN Networks and switches starting to replace buses on desktop, even inside chips Review: ATA cables Outline ECC RAID: Old School Update Latency vs. Bandwidth (if time permits) Hamming Codes How many parity-bits are needed? m parity-bits can code 2m-1-m info-bits Administrivia - HW 3, Lab 4 ECC Hamming Code. Hamming Coding is a coding method for detecting and correcting errors. ECC Hamming distance between 2 coded words must be ≥ 3 Number bits from right, starting with 1 All bits whose bit number is a power of 2 are parity bits We use EVEN PARITY in this example This example shows a 4 data bits Example: Hamming Code. Example: The message 1101 would be sent as 1100110, since: Example: Hamming Code. Will Hamming Code detect and correct errors on parity bits? Yes! RAID Beginnings We had worked on 3 generations of Reduced Instruction Set Computer (RISC) processors 1980 – 1987 Our expectation: I/O will become a performance bottleneck if doesn’t get faster Randy Katz gets Macintosh with disk along side “Use PC disks to build fast I/O to keep pace with RISC?” Redundant Array of Inexpensive Disks (1987-93) Hard to explain ideas, given past disk array efforts Paper to educate, differentiate? RAID paper spread like virus Products from Compaq, EMC, IBM, RAID I Sun 4/280, 128 MB of DRAM, 4 dual-string SCSI controllers, 28 5.25” 340 MB disks + SW RAID II Gbit/s net + 144 3.5” 320 MB disks 1st Network Attached Storage Ousterhout: Log Structured File Sys. widely used (NetAp) Today RAID ~ $25B industry; 80% of server di