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Transparency concerns association between path name of a file and location of the file File sharing semantics may differ between DFSs: Unix semantics Session semantics Transaction semantics (atomic transactions) Stateless server design provides high availability Notion of a hint used to improve performance DFS uses file caching to improve pe...

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Recovery from non-Byzantine faults can be performed by using two approaches: Backward recovery and forward recovery Fault tolerance implemented by maintaining logs E.g., undo or do logs Logs used to implement atomic transactions Two-phase commit protocol (2PC protocol) is used Nested transactions are a resiliency technique Used when transac...

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Actions of a distributed control algorithms are performed in many nodes of the system Two aspects of correctness are liveness and safety Distributed system models: physical and logical Examples of distributed control algorithms: Distributed mutual exclusion: e.g., token based Distributed deadlock detection: diffusion computation Distributed s...

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Using local clocks in processes Logical clocks Vector clocks Include process ids in timestamps for total ordering It is not possible to record the global state of a system Chandy-Lamport algorithm obtains consistent recording of process states using special messages called markers

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Security and protection measures are used to counter interference threats Use authentication and authorization techniques Threats launched using Trojan horses, viruses, worms, and exploitation of buffer overflows Encryption is an algorithmic transformation of data Block ciphers and stream ciphers Widely used encryption standards: DES and AES ...

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An access method improves the performance of a file processing activity within a process using buffering and blocking of records Caching reduces the number of I/O operations performed to access data stored in files Physical IOCS implements a disk cache An access method implements a file cache The virtual memory handler uses a page cache OSs us...

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Files are structured or unstructured (byte stream) File system provides: File organizations (sequential, direct, indexed) Directories for grouping of related files logically Sharing and protection of files Disk space allocation, typically indexed File map table (FMT) stores allocation information File control block (FCB) stores information a...

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Which page should VM manager remove from memory to make space for a new page? Page replacement algorithms exploit locality of reference LRU has stack property, but is expensive NRU algorithms are used in practice E.g., clock algorithms How much memory should manager allocate? Use working set model to avoid thrashing Copy-on-write can be used...

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Allocation/deallocation of memory can lead to fragmentation: internal or external First-fit, next-fit and best-fit strategies try to reduce fragmentation buddy systems and power-of-2 allocators eliminate external fragmentation Noncontiguous allocation reduces external fragmentation Requires use of the memory management unit (MMU) of CPU Kernel...

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Multiprocessor OS algorithms must be scalable Use of special kinds of locks: Spin locks and sleep locks Important scheduling concepts in multiprocessor OSs: Affinity scheduling Coscheduling Process shuffling

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