How are 'files with holes' stored?

[Carlo Wood <carlo alinoe com>]

Hi, I don't know how to call them, but it seems
that ext3 grep allows files to be stored that
have a very large size (when doing an 'ls -l')
but do not actually allocate all blocks.

I assume this is achieved by using 0 as blocknumber
for indirect blocks.

What are the exact requirements for such files?
Is it allowed to have a double indirect block
that exists entirely of zeroes? Is it possible
there is are 0 entries in the tripple indirect
block? Etc.

-- 
Carlo Wood <carlo alinoe com>