Falk Joensson's
Learning Programming with Eas
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How the Computer Works:11. The File System

Each storage device's data starts at address "0" with a table of contents, listing all files and directories that are stored on it. For each item, the table of contents lists: - the ID of its parent directory ("0" = root) - the directory ID (starting at "1"), if not a file - its name (with an optional type extension) - the start address of a file's data - the length (size) of a file in bytes and, depending on the operating system: - a read-only protection flag - user-group access flags (or "system", "hidden", etc) - date/time records for "last changed" and maybe also "created", "last opened", and so on. To minimize access times, energy consumption and mechanical stress for physical drives, the table of contents may be segmented and contain fast-lookup data, depending on the operating system. It also records the overall sum of all the bytes used by all files together, so it is always known how much free memory spae is still available. When your computer lists the contents of the root, it reads the table of contents and scans it for all items that have the parent ID set to "0". When it lists the contents of another directory, it scans the table of contents for all items that have its directory ID set as their parent ID. When you delete a file, the operating system will typically just remove its entry in the table of contents, while the file data remains untouched on the storage device until overwritten by a new file. This is why lost files can sometimes still be recovered by software that scans for file data that has no table-of-contents entry. When you move a file within the same storage device, only its parent ID gets changed in the table of contents, which is why moving happens instantly in this case. But when you copy a file, the whole file data needs to be scanned and copied, which may take a bit for huge files. Moving a file from one storage device to another first copies the file (slow) and then deletes it (fast) from the original location. The operating system may partition storage devices into various "virtual" storage devices which then each get their own table of contents. Moving files from one partition to another then is as slow as with physical drives, as it likewise has to copy and delete. When files get deleted, unused "holes" of storage space are the result. A smaller hole is created when a file is updated (save changes) with a reduced size. And when a file is updated with an increased size, the operating system may write it all new to an unused region of the storage device and set its start address to that, leaving the old file data location as a hole. The user can run a utility program to defragment the file system, which re-arranges all file data so that no holes are left after — which with all the copying of file data and updating tthe table of contents takes a very long time, of course. The file system may even fragment individual files by storing them as part-blocks that each may rest in different regions on the storage device (adding another layer of structure information on top of the table of contents), which is also unwoven by defragmenting.
11. The File System
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