freebsd stable - Jan 2005 - Size of metadata required for GBDE

Hi all,

I just fiddled around with using GBDE for ISO images (it works) and
stumbled across the need to "guess" the size required for the GBDE
container.

Looks like the size is not increasing linearly. Here are the numbers of
512 byte blocks available in md0 and md0.bde
  md0 | md0.bde | diff.
 2048    1952       96
 4096    3936      160
 8192    7936      256
16384   15872      512
32768   31744     1024
65536   63520     2016

So, what's the correct formula?

Ulrich Spoerlein
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Ok, which part of "Ph'nglui mglw'nafh Cthulhu R'lyeh
wgah'nagl fhtagn."
didn't you understand?
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In message <20050130221803.GB2600@galgenberg.net>, Ulrich Spoerlein
writes:
>Hi all,
>
>I just fiddled around with using GBDE for ISO images (it works) and
>stumbled across the need to "guess" the size required for the GBDE
>container.
>
>Looks like the size is not increasing linearly. Here are the numbers of
>512 byte blocks available in md0 and md0.bde
>  md0 | md0.bde | diff.
> 2048    1952       96
> 4096    3936      160
> 8192    7936      256
>16384   15872      512
>32768   31744     1024
>65536   63520     2016
>
>So, what's the correct formula?
First off, if you want to use gbde on a CDROM you should use a sectorsize
of 2048 througout (-S 2048 argument to mdconfig).

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

-- 
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FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.