Often I find myself running tests (such as transfer speed) where I need large files as testers. Doesnt matter whats in them... just use them for the data transfer or whatever.

Normally I do a:

head -c SIZE /dev/urandom > filename

where SIZE is the size of the file I need in bytes.

The problem is that for files over a few megs this method is slow because you have to wait on the machine to fill it with random information.

So my question... Is there a way to create files of arbitrary sizes instantly. Like... some way to manipulate the file descriptor so that a file is allocated a large amount of space. ???

Someone told me there is a way.. but I have no idea.

Thanks

[root@jebus root]# time head -c 100000000 /dev/zero >filename

real 0m2.174s
user 0m0.090s
sys 0m0.830s
[root@jebus root]#

P3 1ghz machine w/256 MB ram.

awesome!

thats fast enough...

what is /dev/zero?

It's one of those "black hole" special files. If you use it to make a ten meg file, then try to cat that file, you'll see that there's not a damn thing in the file.

Think of it as a filesize BS machine.

takes about 30 secons for a gig file on my dual processor machine.

I wonder if there is a way to just manipulate a file so this process can be instant?

If you want the process to be instant, why don't you just create the huge file(s) once, then not delete it? Sure, you'll waste a chunk a space, but since it sounds like this activity is something you do frequently, why not save yourself the trouble of having to create these bogus files each time. -- J.W.

because thats not an interesting solution

And a sysadmin told me it was possible but not obvious. (and he couldnt remember either)

A fair enough assessment. At the same time though, which is better: A non-interesting solution that actually works exactly the way you want it to, or spending a lot of time trying to track down rumors about a potentially faster solution, which few people, if any, seem to be able to recall? -- J.W.

they have actually done it for some project.. ill make him look it up and ill post the solution

for now doing a head on /dev/zero is pretty darn good

FWIW, I believe /dev/zero is actually the writing of zero's to the HD.

Cool