1. File Management Systems
Chapter 12
File Management Systems

2. Introduction to Computer Science
Chapter Goals
Describe the components and functions of a file management system
Compare the logical and physical organization of files and directories
Explain how secondary storage locations are allocated to files and describe the data structures used to record those allocations
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3. Chapter Goals (continued)
Describe file manipulation operations, including open, close, read, delete, and undelete operations
List access controls that can be applied to files and directories
Describe security, backup, recovery, and fault tolerance methods and procedures
Compare and contrast storage area networks and network-attached storage
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5. File Management Systems
Collection of system software that manages all aspects of user and program access to secondary storage
Usually part of the operating system
Translates operations into commands to physical storage devices
Implemented in four layers (command layer, file control, storage I/O control, and secondary storage devices)
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Bridges between logical and physical views of secondary storage
Allocates secondary storage locations to individual files and directories
Includes software modules for device drivers for each storage device or device controller, interrupt handlers, buffers and cache managers
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7. Logical and Physical Storage Views
Logical view
Collection of files organized within directories and storage volumes
Physical view
Collection of physical storage locations organized as a linear address space
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The file is subdivided into multiple records and each record is composed of multiple fields.
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File Content and Type
FMS supports limited number of file types:
Executable programs
Operating system commands
Textual or unformatted binary data
Modern FMSs can define new file types and install utility programs to manipulate them (file association)
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File Types
Normally declared when a file is created and:
Stored within a directory, or
Declared through a filename convention
Determine:
Physical organization of data items and data structures within secondary storage
Operations that may be performed upon the file
Filename restrictions
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14. Directory Content and Structure
Contain information about files and other directories, typically name, file type, location, size, ownership, access controls, and time stamps
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15. Hierarchical Directory Structure
Directories can contain other directories, creating a tree structure, but cannot be contained within more than one parent
Ways that names of access paths can be specified:
Complete path (fully qualified reference)
Relative path
Each storage device has a root directory
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16. Active (working) directory
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17. Graph Directory Structure
More flexible than hierarchical directory structure
Files and subdirectories can be contained within multiple directories
Directory links can form a cycle
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Storage Allocation
Secondary storage devices
Large number of storage locations; low frequency of allocation changes
Divided into allocation units
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Allocation Units
Smallest number of secondary storage bytes that can be allocated to a file; cannot be smaller than unit of data transfer between storage device and controller (block)
Assigned/reclaimed by FMS as files and directories are created or expanded/shrink or are deleted
Size difficult to change once set
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Allocation Unit Size
Tradeoffs
Efficient use of secondary storage space for files
Size of storage allocation data structures
Efficiency of storage allocation procedures
Smaller units: More efficient use of storage space
Larger units: Allow smaller storage allocation data structures
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22. Storage Allocation Tables
Data structures that record which allocation units are free and which belong to files
Format and content vary across FMSs
Can contain linked lists in simpler FMSs or indices or other complex data structures in more complex FMSs
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Free allocation units are assigned to a hidden system file called SysFree.
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All of a file allocation’s units are “chained” together in sequential order by a series of pointers.
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Blocking
Logical record grouping within physical records
Described by a numeric ratio of logical records to physical records (blocking factor)
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Blocking factor = 4:3
Blocking factor = 2:3
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Buffering
Temporary storage of data as it moves between programs and secondary storage devices
Physical records are stored in the buffer as they are read from secondary storage
FMS extracts logical records from buffers and copies them to data area of the application program
Each buffer is the size of one allocation unit
Improves I/O performance if enough are used
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File Manipulation
Exact set of service layer functions varies among FMSs, but typically includes create, copy, move, delete, read, and write
Application programs interact directly with FMS through OS service layer
Users interact indirectly with FMS through command layer
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31. File Open and Close Operations
Causes FMS to find the file, verify access privileges, allocate buffers, and update internal table of open files
File close
Causes FMS to flush buffer content to the storage device, release buffers, update file time stamps, and update table of open files
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32. Delete and Undelete Operations
Does not immediately remove files; some content remains on secondary storage unit all allocation units have been reassigned and overwritten
File content can be visible to intruders
Undelete
Can be used to reconstruct directory and storage allocation table contents
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Access Controls
Granted by file owners and system administrators for reading, writing, and executing files
Provide security at the expense of additional FMS overhead
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34. File Migration, Backup, and Recovery
Provided by most FMSs to protect files against damage or loss
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35. File Migration (Version Control)
Automatic storage and backup of old file versions
Balances storage cost of each file version with anticipated user demand for that version
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Original
Copy that has been updated to reflect new data
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File Backup
Protects against data loss (file content, directory content, and storage allocation tables)
Store backup copies on a different storage device in a different physical location
Manual or automatic
Full or incremental
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Transaction Logging
Automatically records all changes to file content and attributes in a separate storage area; also writes them to the file’s I/O buffer
Provides high degree of protection against data loss due to program or hardware failure
Imposes a performance penalty; used only when costs of data loss are high
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File Recovery
Automated and manual components
Can search backup logs for copies of lost or damaged files
Can perform consistency checking and repair procedures for crashed system or physically damaged storage device
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Fault Tolerance
Methods of securing file content against hardware failure
File backup
Recovery
Transaction logging
Mirroring
RAID (Redundant Array of Inexpensive Disks)
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Mirroring
All disk write operations are made concurrently to two different storage devices
Provides high degree of protection against data loss with no performance penalty if implemented in hardware
Disadvantages
Cost of redundant disk drives
Higher cost of disk controllers that implement mirroring
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RAID
Disk storage technique that improves performance and fault tolerance
All levels except RAID 1 use data striping
Breaks a unit of data into smaller segments and stores them on multiple disks
Multiple levels can be layered to combine their best features (e.g. RAID 10)
Can be implemented in hardware or software
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Data striping: Each segment is written in parallel to a separate disk.
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If the parity disk fails, the other disks still retain their original data bits.
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RAID 10: Mirrors individual disks (RAID 1), then stripes data (RAID 0) across multiple mirrored pairs.
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47. Storage Consolidation
Overcomes inefficiencies of direct-attached storage (DAS) in multiple-server environments
Common approaches
Storage area network (SAN)
Network-attached storage (NAS)
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48. Storage Consolidation
Storage Area Network (SAN)
Network-Attached Storage (NAS)
High-speed interconnection among general-purpose servers and one or more storage servers
Block-oriented access
Common in multi-server environments with mainframes or supercomputers and substantial overlap among server storage needs
Expensive to purchase and administer, but avoid costs of duplicate storage and storage administration
Dedicated to managing one or more file systems
Accessed by other servers and clients over a local or wide area network
File-oriented access
Common when geographically dispersed servers need access to a common file system
Cheaper to acquire than SAN, but at the price of lower performance
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Summary
File management systems
Directory content and structure
Storage allocation
File manipulation
Access controls
File migration, backup, and recovery
Storage consolidation
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