writing large files quickly

[Jens Theisen]

Donn wrote:

>> How the heck does that make a 400 MB file that fast? It literally takes
>> a second or two while every other solution takes at least 2 - 5 minutes.
>> Awesome... thanks for the tip!!!

> Because it isn't really writing the zeros.   You can make these
> files all day long and not run out of disk space, because this
> kind of file doesn't take very many blocks.   The blocks that
> were never written are virtual blocks, inasmuch as read() at
> that location will cause the filesystem to return a block of NULs.

Under which operating system/file system?

As far as I know this should be file system dependent at least under  
Linux, as the calls to open and seek are served by the file system driver.

Jens