Size of metadata required for GBDE

[Ulrich Spoerlein]

On Sun, 30.01.2005 at 23:24:37 +0100, Poul-Henning Kamp wrote:
> The amount of metadata in GBDE is pretty straight forward:
> 
> 1.	If do not use off-line keyfiles:  deduct one sector.
> 
> 2.	Deduct the key sectors (1 to 4)
> 
> 3.	Find zone size:
> 
> 		nsect = sectorsize / 16
> 		nzone = nsect + 1
> 
> 4.	Find number of zones:
> 
> 		z = remaining_sectors / nzone
> 
> 5.	Find usable size:
> 
> 		size = z * nsect
> 
> 6.	Find overhead/metadata as:
> 
> 		total_sectors - size

This translates to:

overhead = sectors - (sectors-4-1)/((sectorsize/16) + 1) * sectorsize/16

So, if I understand that correctly, I need one additional sector every
16 sectors and 5 sectors for the keys (using all the gbde init
defaults).

What's wrong with the attached program then? It gives wrong results :(

% cc -Wall gbde.c -o gbde && ./gbde $((31744*512))
Wanted size: 16252928 bytes in 31744 sectors (blocksize 512)
Needed size: 16763392 bytes in 32741 sectors (blocksize 512)

This should be 32768 sectors, as can be seen from my earlier mail.

Ulrich Spoerlein
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didn't you understand?
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#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char **argv) {
  int okeysec = 1;
  int nkeysec = 4;
  int sectorsize = 512;
  long long size, wanted_sec, needed_sec;

  if (argc < 2)
    exit(1);
  
  size = atoll(argv[1]);
  if (argc == 3)
    sectorsize = atoi(argv[2]);
  
  /* Round up to nearest multiple of sectorsize */
  if (size % sectorsize)
    size += sectorsize - size % sectorsize;

  wanted_sec = size / sectorsize;
  printf("Wanted size: %lld bytes in %lld sectors (blocksize %d)\n",
      wanted_sec*sectorsize, wanted_sec, sectorsize);

  needed_sec = wanted_sec + (wanted_sec*16)/sectorsize + okeysec + nkeysec;
  printf("Needed size: %lld bytes in %lld sectors (blocksize %d)\n",
      needed_sec*sectorsize, needed_sec, sectorsize);

  return (0);
}
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