> In the case of this old PC hard drive, files of varying sizes were written to disk then later moved or deleted, leaving a "hole" of available space between other used regions. As the disk starts to fill up, you might try to create a new file that doesn’t quite fit in one of those smaller areas, and you’d be out of luck. You’d need to "defrag" in order to reclaim those open blocks which are too small to hold the new file when they are split up, but could be big enough when they are contiguous.
While that is pretty much what heap fragmentation is about, the failure mode of disk fragmentation is less drastic. The file system will just split the file contents across multiple smaller free spots, making it possible to use the whole disk no matter your write pattern. The issue is that now the file is no longer contiguous, so reading the entire file takes longer. Much longer if we are talking about old HDDs.
No - heap fragmentation actually leaves memory unusable by the workload. It may or may not slow down allocation depending on the design of the allocator.
(The analogy to file system fragmentation for memory is that when physical pages are allocated in a discontinuous manner, it prevents some optimizations like coalescing them into hugepages, which for some workloads can help with TLB hit rate.)
I don't think either FAT32 or NTFS suffer this problem, they allow metadata to become fragmented instead. Certainly a different design philosophy to the unixy file systems.
NTFS most definitely suffers from this[1]. We ran into it with a customer who had a particularly fragmented database file.
IIRC it's due to a combination of relying on fixed-sized "pages" to hold fragment pointers and a limited number of page indirections. That is the root page can point to a sub-pages, which again can point to sub-pages, but those sub-sub-pages have to point to the actual fragments. Or something along those lines.
While that is pretty much what heap fragmentation is about, the failure mode of disk fragmentation is less drastic. The file system will just split the file contents across multiple smaller free spots, making it possible to use the whole disk no matter your write pattern. The issue is that now the file is no longer contiguous, so reading the entire file takes longer. Much longer if we are talking about old HDDs.