[Pkg-libburnia-devel] Large size penalty from -isohybrid-mbr and -partition_offset 16?

Thomas Schmitt scdbackup at gmx.net
Wed Jun 15 13:53:46 UTC 2011


> The 1MB penalty for -isohybrid-mbr is larger than I expected,

Alignment to full cylinder size was prescribed by hpa at SYSLINUX
mailing list, when he defined the constraints of isohybrid.

The alignment was not performed by mistake with the earlier
point releases of Debian 6. The only negative effect was that
fdisk -lu complained about differing "physical" and "logical"
cylinder sizes.

You may disable this by xorriso -as mkisofs option
  -partition_cyl_align  off

> but the
> last size is particularly weird.  Why is there a 3MB size penalty for
> having a 32KB partition offset?

The partition offset creates a second directory tree, which can be
mounted as /dev/sdX1. Its size depends on the number of files in
the image.

ISOLINUX works with the tree for /dev/sdX, but at some point of
the boot process, the other tree might get into effect.

See also http://libburnia-project.org/wiki/PartitionOffset

Option -J caused another pair of directory trees with Joliet names.

You can get an estimation of the size of metadata by inquiring the
start address of the first data file.
The run:
  xorriso -indev oneiric-server-i386.raw \
          -find / -sort_lba -exec report_lba -- 2>/dev/null | head -2

will report something like
  Report layout: xt , Startlba ,   Blocks , Filesize , ISO image path
  File data lba:  0 ,       67 ,      370 ,   757248 , ...some.path...
which tells that the directory trees end before block 67 * 2 kB.


I am a bit puzzled by the fact that xorriso -isohybrid-mbr isohdpfx.bin
yields a size of 658 MB + 300 kB.
300 kB is the usual default padding, but it should be counted as part
of the image.

Does fdisk complain about cylinder size if you run
  fdisk -lu oneiric-server-i386.raw


Have a nice day :)


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