feature request: output the free size of a block device

[hannesm]

Please excuse me if I'm wrong, but I was not able to find a way to output the bytes / blocks that are free on a given volume (also not how many are used, how many are there all together).

It would be great to have these statistics to figure out when care should be taken to clean the file system (or add another disk).

Thanks for providing this excellent library :)

[yomimono]

I agree that this would be useful - currently the only way to know the disk is full is to try to write things and see them fail for lack of space.

Free bytes is tricky in LittleFS, but free blocks are fairly easy to report on. Unfortunately there are potentially surprising results there too, like two previously-unused blocks being potentially necessary for a very small file write.

Would answers to the following questions be useful? Both are based on the number of free blocks, but try to handle some FS-specific structure knowledge for the user.

• "What's the largest file I can currently write without running out of space?"
• "How many more directories can I make without running out of space?"

Currently, getting the number of free blocks would require a full filesystem traversal each time. This does have to be determined at mount time anyway and I think it can be subsequently kept updated by the block allocator, but I need to think on it a bit more.

This general idea seems like something users of any block-based kv would want to know about, so I wonder whether it'd be better put in RO or RW of Mirage-kv, but the semantics there are even more confusing.

[hannesm]

I'd be fine with an approximation: the number of used blocks, and free blocks. If the API is only in bytes, then multiply that with the block size. I don't think we need accurate numbers of how many bytes will fit onto the disk. ;)