3 ms·

I'm pretty sure you can change various file formats without rewriting the entire file and without using "incremental updates".

• deleted 10mo ago
  [deleted]
• mattzito 10mo ago
  No, if you are going to change the structure of a structured document that has been saved to disk, your options are: 1) Rewrite the file to disk 2) Append the new data/metadata to the end of the existing file I suppose you could pre-pad documents with empty blocks and then go modify those in situ by binary editing the file, but that sounds like a nightmare.
  • cubefox 10mo ago
    Aren't there file systems that support data structures which allow editing just part of the data, like linked lists?
    • layer8 10mo ago
      Look at the C file API which most software is based on, it simply doesn’t allow it. Writing at a given file position just overwrites existing content. There is no way to insert or remove bytes in the middle. Apart from that, file systems manage storage in larger fixed-size blocks (commonly 4 KB). One block typically links to the next block (if any) of the same file, but that’s about the extent of it.
      • anthk 10mo ago
        DD should.
    • formerly_proven 10mo ago
      No. Well yes. On mainframes. This is why “table of contents at the end” is such an exceedingly common design choice.
    • PhilipRoman 10mo ago
      Yeah there are, Linux supports parameters FALLOC_FL_INSERT_RANGE and FALLOC_FL_COLLAPSE_RANGE for fallocate(2). Like most fancy filesystem features, they are not used by the vast majority of software because it has to run on any filesystem so you'd always need to maintain two implementations (and extensive test cases).
      • cubefox 10mo ago
        Interesting that after decades of file system history, this is still considered a "fancy feature", considering that editing files is a pretty basic operation for a file system. Though I assume there are reasons why this hasn't become standard long ago.
        • layer8 10mo ago
          File systems aren’t databases; they manage flat files, not structured data. You also can’t just insert/remove random amounts of bytes in RAM. The considerations here are actually quite similar, like fragmentation. If you make a hundred small edits to a file, you might end up with the file taking up ten times as much space due to fragmentation, and then you’d need the file system to do some sort of defragmentation pass to rewrite the file more contiguously again. In addition, it’s generally nontrivial for a program to map changes to an in-memory object structure back to surgical edits of a flat file. It’s much easier to always just serialize the whole thing, or if the file format allows it, appending the serialized changes to the file.
          • PhilipRoman 10mo ago
            Indeed, also userspace-level atomicity is important, so you probably want to save a backup in case power goes out at an unfortunate moment. And since you already need to have a backup, might as well go for a full rewrite + rename combo.
          • ogurechny 10mo ago
            File systems aren't databases, but journaling file systems use journals just like databases. It can theoretically define any granularity for something that might happen to a file to become an irreversible transaction. I suppose that file systems have to remain “general purpose enough” to be useful (otherwise they become part of the specific program or library), and that's why complex features which might become a pitfall for the regular users who expect “just regular files” rarely become the main focus.
• layer8 10mo ago
  You can’t insert data into the middle of a file (or remove portions from the middle of a file) without either rewriting it completely, or at least rewriting everything after the insertion point; the latter requires holding everything after the insertion point in memory (or writing it out to another file first, then reading it in and writing it out again). PDF is designed to not require holding the complete file in memory. (PDF viewers can display PDFs larger than available memory, as long as the currently displayed page and associated metadata fits in memory. Similar for editing.)
  • maweki 10mo ago
    While tedious, you can do the rewrite block-wise from the insertion point and only store a an additional block's worth of the rest (or twice as much as you inserted) ABCDE, to insert 1 after C: store D, overwrite D with 1, store E, overwrite E with D, write E.
• deleted 10mo ago
  [deleted]
• bahmboo 10mo ago
  This was 1996. A typical computer had tens of megabytes of memory with throughput a fraction of what we have today. Appending an element instead of reading, parsing, inserting and validating the entire document is a better solution in so many ways. That people doing redactions don't understand the technology is a separate problem. The context matters.