Lesson plan
Block allocation map
In computer file systems, a block allocation map is a data structure used to track disk blocks that are considered "in use". Blocks may also be referred to as allocation units or clusters. CP/M used a block allocation map in its directory. Each directory entry could list 8 or 16 blocks (depending on disk format) that were allocated to a file. If a file used more blocks, additional directory entries would be needed. Thus, a single file could have multiple directory entries. A benefit of this method is the possibility to use sparse files by declaring a large file size but only allocating blocks that are actually used. A detriment of this method is the disk may have free space (unallocated blocks) but data cannot be appended to a file because all directory entries are used.
Article summary
In computer file systems, a block allocation map is a data structure used to track disk blocks that are considered "in use". Blocks may also be referred to as allocation units or clusters. CP/M used a block allocation map in its directory. Each directory entry could list 8 or 16 blocks (depending on disk format) that were allocated to a file. If a file used more blocks, additional directory entries would be needed. Thus, a single file could have multiple directory entries. A benefit of this method is the possibility to use sparse files by declaring a large file size but only allocating blocks that are actually used. A detriment of this method is the disk may have free space (unallocated blocks) but data cannot be appended to a file because all directory entries are used.
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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