Re: Script to set up sparse device for testing

On Thu, Jul 06, 2006 at 01:05:14AM +0100, Stephen C. Tweedie wrote:
> Attached is a script from Heinz Mauelshagen to set up a sparse device
> for testing purposes, allowing a given amount of storage to act as a
> test bed for a much larger filesystem --- as long as you don't actually
> write to all that data. :)   It basically creates a snapshot of the
> empty zero device, and fills it in using the backing store you give it
> for snapshot storage.

Here's another way I always use: create a file, truncate it to the
desired size and use it as a loop mount. Requires a small program
called "truncate":

/* truncate.c: truncate files
 * compile with: gcc -Wall -O2 -D_GNU_SOURCE -o truncate truncate.c
 */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main (int argc, char **argv)
{
  int i;
  long long l;

  if(argc < 3) {
          printf("Truncates one or more files.\n");
          printf("Usage: %s size file [file ...]\n", argv[0]);
          return 0;
  }
  sscanf (argv[1], "%lli", &l);

  for (i=2;i < argc;i++) {
    if (truncate64 (argv[i], l) < 0) {
      perror (argv[i]);
    }
  }
  exit (0);
}

(IIRC truncate() isn't required to work this way according to
POSIX/SUSv3, but it works under Linux).

Example: create a 1 GB ext3 filesystem:

touch foobar
truncate 1073741824 foobar
mke2fs -j foobar
losetup /dev/loop0 foobar
mount -t ext3 /dev/loop0 /mnt


Erik

-- 
+-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 --
| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands

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