COT 4600 Operating Systems Fall 2009Lecture 5Names and the three basic abstractionsLatency and BandwidthMemoryAttributes of the storage medium/systemCritical properties of a storage medium/systemWhy it is hard to guarantee the critical properties? Access type; access timePhysical memory organizationNames and physical addressesRAID – Redundant Array of Inexpensive Disks RAID (cont’d)RAID (cont’d)Lecture 51COT 4600 Operating Systems Fall 2009Dan C. MarinescuOffice: HEC 439 BOffice hours: Tu-Th 3:00-4:00 PMLecture 522Lecture 5Lecture 5 Last time:  Project. Today: Names and the basic abstractions Storage Next Time Naming in computing systemsLecture 533Names and the three basic abstractions Memory Î stores named objects write(name, value) value Å READ(name) file system: /dcm/classes/Fall09/Lectures/Lecture5.ppt  Interpreters Î manipulates named objects machine instructions ADD R1,R2 modules Î Variables call sort(table)  Communication Links Îconnect named objects HTTP protocol used by the Web and file systemsHost: boticelli.cs.ucf.eduput /dcm/classes/Fall09/Lectures/Lecture5.pptget /dcm/classes/Fall09/Lectures/Lecture5.pptLecture 54Latency and Bandwidth Important concepts for physical characterization. Applies to all three abstractions. Informal Bandwidth Î number of operations per second! Latency Î to get there  The bandwidth of the CPU, Memory, and I/O sbsystemsmust be balanced.4Lecture 555Communication latency- time it takes the first bit sent to reach the receiverLatencyBandwidthBandwidth- number of bits/bytes transmitted per unit of timet1t2t3t4t5t6Time Time Sender Receiver ProgramStoragedevicet2t3t1Operation latency- time it takes the command to read the deviceLatencyBandwidth- number of bits/bytes transmitted per unit of timeBandwidtht4t5t6Lecture 566Memory Hardware memory: Devices RAM (Random Access Memory) chip Flash memory Î non-volatile memory that can be erased and reprogrammed Magnetic tape Magnetic Disk CD and DVD Systems RAID File systems DBMS (Data Base management Systems)Lecture 577Attributes of the storage medium/system Durability Î the time it remembers  Stability Î whether or not the data is changed during the storage Persistence Î property of data storage system, it keeps trying to preserve the dataLecture 588Critical properties of a storage medium/system Read/Write Coherence Î the result of a READ of a memory cell should be the same as the most recent WRITE to that cell. Before-or-after atomicity Îthe result of every READ or WRITE is as if that READ or WRITE occurred either completely before or completely after any other READ or WRITELecture 599timeWRITE item A in cell MAMREAD from cell MAMA ACurrent READ/WRITEPrevious READ/WRITENext READ/WRITELecture 51010Why it is hard to guarantee the critical properties?  Concurrency Î multiple threads could READ/WRITE to the same cell Remote storage Î The delay to reach the physical storage may not guarantee FIFO operation Optimizations Î data may be buffered to increase I/O efficiency Cell size may be different than data size Î data may be written to multiple cells. Replicated storage Î difficult to maintain consistency.Lecture 51111Access type; access time Sequential access  Tapes CD/DVD Random access devices Disk Seek Search time Read/Write time RAMLecture 512Physical memory organization RAM Î two dimensional array. To select a flip-flop provide the x and y coordinates. Tapes Î blocks of a given length and gaps (special combination of bits. Disk: Multiple platters Cylinders correspond to a particular position of the moving arm Track Î circular pattern of bits on a given platter and cylinder Record Î multiple records on a track12Lecture 513Names and physical addresses Location addressed memory Î the hardware maps the physical coordinates to consecutive integers, addresses Associative memory Î unrestricted mapping; the hardware does not impose any constraints in mapping the physical coordinates 13Figure 2.2 from textrbookLecture 514RAID – Redundant Array of Inexpensive Disks  The abstraction put to work to increase performance and durability. Raid 0 Î allows concurrent reading and writing. Increases performance but does not improve reliability. Raid 1 Î increases durability by replication the block of data on multiple disks (mirroring) Raid 2 Î Disks are synchronized and striped in very small stripes, often in single bytes/words.  Error correction calculated across corresponding bits on disks, and is stored on multiple parity disks (Hamming codes).14Lecture 51515Lecture 516RAID (cont’d) Raid 3 ÎStriped set with dedicated parity or bit interleaved parity or byte level parity. Raid 4 Î improves reliability, it adds error correction16Lecture 517RAID (cont’d) Raid 5 Î striped disks with parity combines three or more disks to protect data against loss of any one disk. The storage capacity of the array is reduced by one disk Raid 6 Î striped disks with dual parity combines four or more disks to protect against loss of any two disks. Makes larger RAID groups more practical.  Large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity gives time to rebuild the array without the data being at risk if a (single) additional drive fails before the rebuild is complete.17Lecture 51818Figure 2.3 from