Storage Management - File Systems

Storage Management - File Systems

1. Concept + Structure

   • File
     • A logical storage unit; a named collection of related information that is recorded on secondary storage
     • Types
       • Data (data, binary)
       • Program
     • Structure
       • None - Sequence of words, bytes
       • Simple Record Structure - Lines, Fixed/Variable Length
       • Complex structures - Formatted documents
     • Attributes
       • Name
       • Identifier (unique tag)
       • Type
       • Location (pointer to file location)
       • Size
       • Protection
       • Time, data, user identification
   • Mounting
     • Combine multiple file systems into one name space w/ one root

2. File Operations

   • Create
     • Allocate space, enter name, location
   • Write
     • Access file, update pointers
   • Read
     • Access file, update current position pointers
   • Reposition within file
     • Set pointer position to a certain value
   • Delete
     • De-allocate space & remove directory entry
   • Truncate
     • Erase contents but keep all other attributes

3. File Access Methods

   • Sequential
     • File 'blocks' processed in order
     • Read operation reads next block + updates pointer
     • Write appends to end of file
   • Direct/Random Access
     • File viewed as array of blocks
     • Can read/write blocks in random order
   • Sequential Indexing
     • Records arranged in logical sequence - According to key in each record
     • Maintain index from key
     • Can access sequentially OR randomly
       • Allows searching large files with little I/O

4. Directories

   • Role: Map symbolic file names to disk locations
   • Goal - Organise Directory logically
     • Efficiency
     • Naming
   • Operations: Search for file, create/delete/rename file, traverse file system
   • Single-Level Directory
     • Single directory for all users
     • Simplest file system
     • Stores all files using one directory
     • Two files cannot have same name
     • File System performs linear search
       • Poor performance
   • Tree-Structured Directories
     • Root indicates where on storage device the root directory begins
     • Root directory points to various directories
     • File names only unique within directory
     • Name of file is usually pathname
   • Acyclic-Graph Directories (Aliasing)
     • Shared subdirectories + files
     • Aliasing - Multiple paths for same data location
     • Backpointers
       • Remove dangling pointers (when one parent deletes a child)
     • Links
       • Reference pointer to other part of tree

5. Disk Allocation

   • Contiguous Allocation
     • Each file occupies set of contiguous blocks on disk
       • +Simple, sequential and random access supported
       • -External fragmentation, result in poor performance if files shrink/grow
     • Extent-based Systems
       • Modified contiguous allocation scheme
         • Allocate disk blocks in extents - contiguous block of disks
           • Allocation recorded as location and block count + link to first block of next extent
   • Linked Allocation
     • Each file is a linked list of disk blocks - Blocks may be scattered anywhere
       • Advantages - Simple, No external Fragmentation, a file can continue to grow
       • Disadvantages - No random access, large space required
   • Indexed Allocation
     • Brings all pointers together in index block

Images and files in the map