writing large files quickly

[rbt]

Donn Cave wrote:
> In article <drduvt$m6r$1 at solaris.cc.vt.edu>,
>  rbt <rbt at athop1.ath.vt.edu> wrote:
> 
>>Won't work!? It's absolutely fabulous! I just need something big, quick 
>>and zeros work great.
>>
>>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. 

Hmmm... when I copy the file to a different drive, it takes up 
409,600,000 bytes. Also, an md5 checksum on the generated file and on 
copies placed on other drives are the same. It looks like a regular, big 
file... I don't get it.


> 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.
> 
>    Donn Cave, donn at u.washington.edu