Runtime filesystem consistency checking

We do handle "unallocated" writes due to shadow paging (e.g., copy-on-write on btrfs) where metadata is written to unallocated regions and then linked to the file-system on commit. We find this linkage at commit point and will not report an error. We haven't worked with online resizing but I suspect the handling should be similar on commit. Writing to unallocated data to which there is no linkage at commit seems suspect: how would the file system know that data is useful in anyway after a crash?