Computer Science 322 Operating Systems Mount Holyoke College Spring 2008

Agenda

• Announcements
• Lecture assignment recap
• Disks and partitions
  • a system may have multiple disks, each of which contains multiple partitions
  • we create a file system (more later) in each partition
  • mounting: drive letters in Windows, paths in Unix
  • In Unix, a virtual file system layer translates generic paths to the correct partitions (local of varying types, network-accessible)
  • partitioning: why and how?
• File System Implementations
  • want to provide: convenience, protection, efficiency
  • issues:
    • allocation of disk blocks to files/directories
    • track available blocks
    • do this efficiently
• Disk block allocation methods
  • contiguous allocation
    
  • extents
  • linked allocation
    
  • variation on linked: file allocation table (FAT)
    
  • indexed allocation
    • straightforward implementation:
      
    • linked indexed allocation
    • two-level index
  • Unix inodes
    • a combination of the best features of many of the above
    • set aside special disk blocks called inodes (index nodes)
    • inode has entries pointing to direct file blocks, a single indirect block, a double indirect block, and a triple indirect block
    • a superblock keeps track of inodes and data blocks
    • see /usr/src/sys/ufs/ufs/dinode.h in FreeBSD
  • Concerns: file size limits, fragmentation, overhead, performance

Lecture Assignment

Due at the start of class, Monday, April 28.

Turn in short answers to these questions. Please turn in a hard copy (typeset or handwritten are OK). We will discuss these questions during class, so no late submissions are accepted.

1. SG&G 11.2
2. What is the largest file that can be represented in the usual Unix inode scheme when the inode includes 12 direct blocks, 1 single indirect block, 1 double indirect block, and 1 triple indirect block? Assume a data block is 4KB and a block is identified and located on the disk using a 4-byte address.

Examples

• ls2