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Chapter 12

Chapter 12. File and Secondary Storage Management. Chapter 12 File and Secondary Storage Management. Chapter Outline File-Management Systems Directory Content and Structure Storage Allocation File Manipulation Access Controls Focus – Windows NTFS File Migration, Backup, and Recovery

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Chapter 12

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  1. Chapter 12 File and Secondary Storage Management

  2. Chapter 12File and Secondary Storage Management Chapter Outline File-Management Systems Directory Content and Structure Storage Allocation File Manipulation Access Controls Focus – Windows NTFS File Migration, Backup, and Recovery Focus – RAID Storage Consolidation

  3. 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 • 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

  4. Chapter 12Topics

  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)

  6. Operating System Layers vs.File Management System Layers 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

  7. Logical and PhysicalStorage 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

  8. Typical Logical and PhysicalSecondary Storage

  9. Logical Structure of a Data File The file is subdivided into multiple records and each record is composed of multiple fields.

  10. 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)

  11. 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

  12. Registered File TypesWindows XP

  13. Association Detailsof a File Type in Windows XP

  14. Directory Content and Structure A directory contains information about files and subdirectories • name • file type • location • size • ownership • access controls • time stamps

  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

  16. Hierarchical Directory Structure Active (working) directory

  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

  18. Graph Directory Structure

  19. Storage Allocation • Secondary storage devices • Large number of storage locations • Low frequency of allocation changes • Divided into allocation units

  20. 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

  21. 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

  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

  23. Storage Blocks Allocated to Three Files

  24. Directory Contentfor the Three Files Free allocation units are assigned to a hidden system file called SysFree

  25. Storage Allocation Tablefor the Three Files All of a file allocation’s units are “chained” together in sequential order by a series of pointers.

  26. Blocking • Logical record grouping within physical records • Described by a numeric ratio of logical records to physical records (blocking factor)

  27. Blocking of Logical Recordsinto Physical Records Blocking factor = 4:3 Blocking factor = 2:3

  28. 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

  29. Input from Secondary Storage to an Application Using a Buffer

  30. 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

  31. NTFS MFT Records

  32. File Open and Close Operations • File open • 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

  33. Delete and Undelete Operations • Delete • 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

  34. Access Controls • Granted by file owners and system administrators for reading, writing, and executing files • Provide security at the expense of additional FMS overhead

  35. File Migration,Backup, and Recovery • Provided by most FMSs to protect files against damage or loss

  36. 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

  37. Batch AccountTransaction Update Original Updated Copy

  38. 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

  39. 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

  40. 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

  41. Fault Tolerance • Methods of securing file content against hardware failure • File backup • Recovery • Transaction logging • Mirroring • RAID • “Redundant Array of Inexpensive Disks”

  42. 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

  43. 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

  44. RAID Levels

  45. Data Striping Each segment is written in parallel to a separate disk.

  46. RAID Write If the parity disk fails, the other disks still retain their original data bits.

  47. RAID 10 Mirrors individual disks (RAID 1), then stripes data (RAID 0) across multiple mirrored pairs.

  48. Storage Consolidation • Overcomes inefficiencies of direct-attached storage (DAS) in multiple-server environments • Common approaches • Storage area network (SAN) • Network-attached storage (NAS)

  49. Storage Consolidation

  50. Server Cluster with SAN

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