Re: [Xen-users] Clean deleted space in linux diks?

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Niels Dettenbach (Syndicat IT&Internet) wrote:

>  >a=0 ; while [ $a -lt X ]
> > do
>  >   dd if=/dev/zero of=null-file-$a bs=1024k count=1024
> > done
> > rm null-file*
> hmmm, just to understand: this is writing onto a file system and not 
> to a device directly, or not?
This particular command sequence will create a number of files called 
null-file-0, null-file-1, and so on. With these figures, each file 
will be 1GB in size.
If you do :
  dd if=/dev/zero of=big-null-file
then you will create a file called big-null-file which will grow 
until either the filesystem is full, or the file reaches the file 
size limit for your combination of OS and filesystem.
The same effect can be had with
  cat /dev/zero > big-null-file


In case you don't quite get what is happening. "dd" is a command that 
copies input to output, while applying conversions (none in this 
case). "if" specifies infile, "of" specifies outfile, "bs" specifies 
block size, and "count" specifies how many blocks to copy.
So "dd if=/dev/zero of=null-file-$a bs=1024k count=1024" means : copy 
/dev/zero to null-file-<something>, use 1MByte blocks, and copy 1k of 
blocks (to get 1GByte of file).
/dev/zero is a "magic file" that just produces an infinite number of 
zeros (nulls) for as long as you read it.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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