writing large files quickly

[Bengt Richter]

On Fri, 27 Jan 2006 12:30:49 -0800, Donn Cave <donn at u.washington.edu> 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.   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.
>
I wonder if it will also "write" virtual blocks when it gets real
zero blocks to write from a user, or even with file system copy utils?

Regards,
Bengt Richter