Lec21 - Today: Coda, xFS Case Study: Coda File System Brief...

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Computer Science Lecture 21, page Today: Coda, xFS Case Study: Coda File System Brief overview of other file systems xFS Log structured file systems CS677: Distributed OS Computer Science Lecture 21, page Distributed File System Requirements Transparency Access, location, mobility, performance, scaling Concurrent file updates File replication Hardware and OS heterogeniety Fault tolerance Consistency Security Efficiency
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Computer Science Lecture 21, page AFS Background Alternative to NFS Originally a research project at CMU Then, defunct commercial product from Transarc (IBM) Public versions now available Goal: Single namespace for global files (sound familiar? DNS? Web?) Designed for wide-area file sharing and scalability Example of stateful file system design Server keeps track of clients accessing files Uses callback-based invalidation Eliminates need for timestamp checking, increases scalability Complicates design Computer Science Lecture 21, page Coda Overview Offshoot of AFS designed for mobile clients Observation: AFS does the work of migrating popular/ necessary files to your machine on access Nice model for mobile clients who are often disconnected Use file cache to make disconnection transparent At home, on the road, away from network connection Coda supplements AFS file cache with user preferences E.g., always keep this file in the cache Supplement with system learning user behavior Consistency issues?
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Computer Science Lecture 21, page Coda Consistency How to keep cached copies on disjoint hosts consistent? In mobile environment, “simultaneous” writes can be separated by hours/days/weeks Callbacks cannot work since no network connection is available Coda approach (in order): Assume that write sharing the rare case Attempt automatic patch Fallback to manual (user) intervention Computer Science Lecture 21, page File Identifers Each file in Coda belongs to exactly one volume Volume may be replicated across several servers Multiple logical (replicated) volumes map to the same physical volume 96 bit file identifier = 32 bit RVID + 64 bit file handle CS677: Distributed OS
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Computer Science Lecture 21, page Sharing Files in Coda Transactional behavior for sharing files: similar to share reservations in NFS File open: transfer entire file to client machine [similar to delegation] Uses session semantics: each session is like a transaction Updates are sent back to the server only when the file is closed CS677: Distributed OS Computer Science Lecture 21, page Transactional Semantics Network partition: part of network isolated from rest Allow conflicting operations on replicas across file partitions Reconcile upon reconnection Transactional semantics => operations must be serializable Ensure that operations were serializable after thay have executed Conflict => force manual reconciliation CS677: Distributed OS
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Lec21 - Today: Coda, xFS Case Study: Coda File System Brief...

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