I'm experimenting with sparse files on an ext4 filesystem and using the `fallocate` command to create a file of size 10GB like this: `# fallocate -l 10G file.img`. The file appears to be created quickly without any issues, but I can't seem to determine if it is truly sparse. When I run `ls -ls` and `ls -lsh`, the commands show differing sizes, which is confusing. For instance, the first command indicates a size of 10GB while the second shows a human-readable size. I've also tried using `du` and `stat` to assess the file, but the outputs don't give me the clarity I need, especially regarding the blocks used. Can anyone help clarify how to detect if this file is actually sparse? Thanks!
3 Answers
It sounds like you're running into the common misconception about `fallocate`. By default, `fallocate` actually allocates file space on disk rather than creating a truly sparse file. If you want to create a sparse file from scratch, you might be better off using the `truncate` command instead. That way, you can reserve space without actually filling it with zeros or data that takes up space. Give that a try!
Not quite! While `fallocate` can reserve space, it doesn’t really create sparse files by definition. A true sparse file doesn't allocate disk blocks for empty space. If you use `fallocate -d` on an existing file, then you can make it sparse afterwards.
To check if a file is genuinely sparse, you're right to look at commands like `ls`, `stat`, and `du`. The key is comparing the logical size of the file to how many blocks it uses. If the blocks are fewer than what you'd expect, it’s sparse. Also, running the `find` command to check reported sizes can give you clarity. Just remember, what matters is whether the file occupies fewer blocks than its logical size. Give those checks a shot!
You can definitely detect if a file is sparse with some system calls. Using commands like `ls -A` alongside `stat` can help you understand the relationship between logical size and the number of blocks, which indicates whether it's sparse. If there are allocation discrepancies, then that’s a pretty solid sign!
Thanks for the tip! If `fallocate` reserves space but doesn’t fill it, isn't that considered effectively sparse? Just trying to wrap my head around the definition.