how can i limit system resources for a particular process?

[Jordi Moles Blanco]

hi everyone,

let's say i want to create an .img file of 50GB with "dd" command. After 
that, i will give it a format with mkfs.ext3.

The problem is that while "dd" is running I'm not able to do anything 
else on the machine, not even ssh in, it just consumes all the available 
resources, and as I'm creating a 50GB image, i can't access the machine 
for several minutes.

I've tried to use "nice" command, giving the "dd" process the lowest 
possible priority, 19. The thing is that the whole thing performs 
better, i can establish ssh connection. However, i can't do much when 
I'm in it, everything is so slow.

I've also read some documentation about "limit/ulimit" command, but i 
fail to see how i can use it successfully for this matter.

I would like to run dd and let it use, for example, only 10% of the CPU 
time or 30% of the total amount of memory. Is that possible?  I'm not 
looking for a "general" process limit for the whole system, only for a 
particular process.

Thanks in advance for your help.