sparse files

[Guy Harris]

>UNIX treats the "holes" as 0's when read. In fact, UNIX has only
>minimal support for sparse files.  Backing up sparse files often
>involves copying large amounts of nulls.  Once an area of a file is
>written, it cannot be returned to its previous sparse state.

Not in general, anyway.  At least the first version of AIX for the RT PC
claimed, in its documentation, that it had an "fclear()" call to punch
holes in files; I think this may show up in future releases of other
UNIXes as well.

>In arguments that UNIX is not suitable for DP applications, sparse
>files usually come up if the conversation goes on long enough between
>knowledgeable people.

Umm, what other operating systems support sparse files *and* return a
"there's a hole there" indication?  For instance, are there any OSes
with extent-based file systems (VMS, OS/360 and successors as I
remember, IRIX with SGI's Extent File System) that support sparse files?