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COMP1321 Digital Infrastructures

Learn about file systems, file attributes, and how data is organized and stored on secondary storage devices. Explore the concepts of partitions, boot sectors, fragmentation, and file deletion.

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COMP1321 Digital Infrastructures

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  1. COMP1321Digital Infrastructures Richard Henson November2017

  2. Week 6: Hard & Solid State Disk Storage • Objectives: • explain what a file is and why it is such a useful way to store instructions and data • name the commonly used file systems used to store digital data off the motherboard • explain how data is organised into files by the CPU • explain how files are saved to secondary storage for fast retrieval

  3. Storing Data • Overview: • https://www.youtube.com/watch?v=TQCr9RV7twk

  4. Files • “file” ~ conventional name for a package of bytes of data • Created and Used in Primary Storage : • set of stored data usually in consecutive memory locations • controlled directly by CPU instructions

  5. Secondary Storage Devices • Connected to the Motherboard • directly (SATA connection) • may need to be configured with operating system on ROM (CMOS settings) • allocates a drive letter (e.g. D, E, F) • indirectly (USB connection) • drive letter allocated through “plug and play”

  6. Finding data on Secondary Storage (1) • Secondary storage: controlled on CPU via disk controller programs & file system manager • storage locations given addresses when the media is “formatted” • works just like memory but has to be loaded into memory to be CPU accessible • need an EOF (end of file) marker

  7. File Attributes • When a file is saved, and data is transferred from RAM to disk, a number of “attributes” are also saved: • File name & suffix: • File starting address on disk: • File size: • Date/time saved: • Whether: read-write/read-only; hidden/visible

  8. Overview of File Organisation • https://www.youtube.com/watch?v=KN8YgJnShPM&t=30s • Secondary storage devices need to organise files for quick access via starting address… • tapes fine for writing, but too slow for reading!

  9. Primary/Secondary Storageof data as files • If Windows = operating system, each drive/partition allocated a letter (e.g. C:, D:, etc.

  10. File Organisation on Disk(1) • Partition: area earmarked for formatting to meet the needs of a particular file system • Could be several partitions on a single disk.. • Each has its own drive letter • Each could be formatted for a different file system

  11. File Organisation on Disk (2) • Disk surface physically divided up into sectors • size of sectors depends on formatting type • 512 locations (i.e. bytes) • 2048 locations • files laid down in locations within sectors

  12. Finding data on Secondary Storage • Each partition creates a table/index/catalogue for files that are written to it • includes file attributes otherwise file very difficult to retrieve… • Method depends on filing system chosen when partition formatted… • as well as formatting the surface to receive data, each filing system formatting type structures the media to receive data in its own unique way…

  13. “Boot Sector” • Important part of PC boot up involves secondary storage • Starts from “boot sector” (first sector)… • provides configuration information for effective communication with CPU • if damaged, boot up halted! • should have a backup… needs to be copied to boot sector to overwrite corrupted data

  14. Partitions and Boot Up • Whilst a disk can have multiple partitions… • Only one of these contains the boot sector • Boot sector configuration options… • Selectable via screen menu • Can point to different operating systems

  15. Booting up: loading an Operating System… • Needs to be loaded into RAM • some operating systems load everything from ROM (e.g. smartphone) • others (e.g. PC) use a combination… • some loaded first from ROM • rest from hard disk or other source • disk needs a bootable partition or can’t load rest of operating system into RAM

  16. Data Storage on Disk Partition • Sectors numbered • Files stored in specified sector address ranges

  17. Disk Index/Catalogue • Stores location for start of file, and size of file • Also, organises files into directories/folders • top folder (C:) = root • rest of folders link hierarchically from the root • Index/Catalogue logically allocates each file to a folder for ease of retrieval

  18. “Fragmentation” (Data in Secondary Storage) • General problem with hierarchical data storage… • deleted data items leave holes in the structure • New data items saved try to fill the gaps • large files can be broken into fragments • fragments linked by address pointers • slows down retrieval

  19. Removing Fragmentation • If disk only partly fragmented… • defragmented files copied into memory • remaining files moved around to close up holes • Previously defragmented files copied back to disk as complete files • If disk >75% fragmented • most effective solution is to copy all files to another partition • can copy back later once original partition has all data deleted

  20. Is it true that deleted files aren’t really deleted? • Absolutely! • Two things happen when a file is deleted: • the first data item stored in the file (first character of filename) is changed to “?” • the catalogue entry ceases to recognise & display the filename • starting address shows “?” Character • file system is programmed to ignore ? at such a location • Rest of the data is untouched… • easily demonstrated through use of a Hex editor program: can show file contents “before” and “after”

  21. “Normal” Loading of a File from Secondary Media • File catalogue essential for data retrieval • application reads file catalogue • displays folders and files • user chooses file, application uses disk addresses to load into memory • File catalogue corruption? • can’t find files! • backup copy on disk, in separate sector

  22. Direct access by address on Secondary Media • If both file catalogues are damaged… • file (and its data) cannot be located • ideally, needs to be a backup catalogue elsewhere • “Hex editors” available to do the equivalent of debug –d (peek) and –e (poke) • enables full search of all addresses for particular ASCII string(s) • essential for recovery of data…

  23. “Undelete” • Tools also available for restoring recently deleted files • “?” character poked, and restored to a real character • then picked up & shown on catalogue display

  24. WinHex • Probably the most popular tool to examine hard disks • readout quite similar to debug –e • data presented byte-by-byte according to catalogue address • range of options for extracting, overwriting data, and (like debug) writing consecutive raw data items to a file

  25. WinHex, Disk Investigation Tools, and Forensics • Essential for • police: collecting evidence • data recovery: • to rebuild a damaged or accidentally deleted catalogue • to repair a boot sector so that computer can reboot

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