sparse files

[Doug Gwyn]

One problem with sparse files is that simply updating (i.e., reading a
record, modifying it, and writing it back on top of the original record)
can result in an I/O failure if the original record overlapped a hole
and the file system doesn't have any more free data blocks.

I don't offer a solution for this, just a warning that even I/O
operations that "can't" fail might, so applications should always
check and be prepared to recover.