CHAPTER 11: File System Implementation (Part 2)

CHAPTER 11: File System Implementation (Part 2)

1. File-System Structure

   • File structure
     • Logical storage unit
     • Collection of related information
   • File system resides on secondary storage (disks)
   • Disk provides in-place rewrite and random access
   • Device driver controls the physical device
   • File control block (FCB)
   • Device driver controls the physical device

2. Allocation Methods

   • Contiguous
     • Best performance in most cases
     • Simple – only starting location (block #) and length (number of blocks) are required
     • Problems include finding space for file, knowing file size, external fragmentation, need for compaction off-line (downtime) or on-line
   • Linked
     • File ends at nil pointer
     • No external fragmentation
     • Each block contains pointer to next block
     • No compaction, external fragmentation

3. Free-Space Management

   • File system maintains free-space list to track available blocks/clusters
   • Bit vector or bit map (n blocks)
   • (Using term “block” for simplicity)

4. Efficiency and Performance

   • Efficiency dependent on:
     • Disk allocation and directory algorithms
     • Types of data kept in file’s directory entry
     • Fixed-size or varying-size data structures
   • Performance
     • Keeping data and metadata close together
     • Buffer cache – separate section of main memory for frequently used blocks
     • Free-behind and read-ahead – techniques to optimize sequential access

5. File-System Operations

   • Directory structure organizes the files
   • Volume control block (superblock, master file table) contains volume details
   • Boot control block contains info needed by system to boot OS from that volume

6. Recovery

   • Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies
   • Use system programs to back up data from disk to another storage device (magnetic tape, other magnetic disk, optical)
   • Recover lost file or disk by restoring data from backup