week11_friday - CSCC 69H3 Operating Systems Spring 2010...

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CSCC 69H3 Operating Systems Spring 2010 Professor Bianca Schroeder U of T
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Summary so far Files Operations, access methods Directories Operations, using directories to do path searches Sharing Protection ACLs vs. capabilities File System Layouts Unix inodes FFS (Fast File System)
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Data and Inode Placement Original Unix FS had two placement problems: 1. Data blocks allocated randomly in aging file systems Blocks for the same file allocated sequentially when FS is new As FS “ages” and fills, need to allocate into blocks freed up when other files are deleted Problem: Deleted files essentially randomly placed So, blocks for new files become scattered across the disk 2. Inodes allocated far from blocks All inodes at beginning of disk, far from data Traversing file name paths, manipulating files, directories requires going back and forth from inodes to data blocks Both of these problems generate many long seeks Inodes Data Blocks Default usage of LBN space Superblock
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4/4/10 CSC369 Cylinder Groups BSD FFS addressed placement problems using the notion of a cylinder group (aka allocation groups in lots of modern FS’s) Disk partitioned into groups of cylinders Data blocks in same file allocated in same cylinder group Files in same directory allocated in same cylinder group Inodes for files allocated in same cylinder group as file data blocks Superblock Cylinder group organization Cylinder Group
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4/4/10 CSC369 More FFS solutions Small blocks (1K) in orig. Unix FS caused 2 problems:
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week11_friday - CSCC 69H3 Operating Systems Spring 2010...

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