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PartitionOffset.md
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PartitionOffset.md
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The partition offset feature of libisofs can produce ISO 9660 images which bear
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a quite conventional partition table if copied onto a USB stick. The first
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partition marks the size of the ISO image but starts at a non-zero address.
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Thus it marks a small part of the device as unclaimed by partitions and
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available for storing boot loader code.
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Nevertheless the USB stick is mountable via its overall device file as well as
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via the partition device file. E.g. on GNU/Linux: `/dev/sdb` and `/dev/sdb1`.
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This is achieved by two distinct sets of meta-data which refer to the same
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file content.
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The dual-mount feature supports Rock Ridge and Joliet too.
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It is capable of multi-session.
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Currently only offset 32 kB seems to make sense. Smaller offsets are prohibited
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by fundamental assumptions of libisofs and libisoburn. Larger offsets would
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extend the unclaimed area into vital blocks of the ISO image.
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--------------------------------------------------------------------------
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According to a
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[thread of march 2011](http://www.syslinux.org/archives/2011-March/016527.html)
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on Syslinux mailing list this enabled booting of a Kontron CG2100 server
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from USB stick, which otherwise failed.
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Regrettably the feature seems to prevent mounting of ISO 9660 images on
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Apple "Snow Leopard" systems.
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At least this is the outcome of a
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[debian-cd thread of april 2011](http://lists.debian.org/debian-cd/2011/04/msg00029.html).
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--------------------------------------------------------------------------
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Example:
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Testing mountability and ISOLINUX bootability from USB stick and CD.
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Overview:
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The test image was derived from one year old RIPLinux-9.3-non-X.iso which
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has an isohybrid MBR. Syslinux version seems to be 3.82. That MBR and the file
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tree from the mounted RIPLinux image was used to build a new ISO image
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with 16 \* 2kB partition offset. Isohybrid MBR patching was done by xorriso.
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Details:
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The first 32 kB of an ISO 9660 image are called System Area and may host any
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byte pattern. The first 512 bytes of RIPLinux-9.3-non-X.iso contain the
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isohybrid capable MBR, which will be re-used in this example.
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```
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dd if=RIPLinux-9.3-non-X.iso bs=512 count=1 of=RIPLinux-9.3-non-X.mbr
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```
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Normally the isohybrid MBR is provided by the Syslinux
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installation under the name `isohdp[fp]x*.bin` .
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E.g. `/usr/lib/syslinux/isohdpfx.bin`
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The files of the image are made accessible for reading
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```
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mount -o loop RIPLinux-9.3-non-X.iso /mnt
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```
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A new ISO image gets composed. The first three lines of arguments are taken
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from the prescriptions of ISOLINUX wiki and adapted to the names used in
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RIPLinux-9.3-non-X.iso.
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Option `-isohybrid-mbr` imports the copied MBR and patches it
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according to rules published by hpa on Syslinux mailing list.
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Option `-partition_offset 16` causes the first partition to start at 2 kB block
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number 16. It also prepares the image to be mountable by this partition, too.
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```
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xorriso -as mkisofs \
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-o new_image.iso \
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-b boot/isolinux/isolinux.bin -c boot/boot.cat \
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-no-emul-boot -boot-load-size 4 -boot-info-table \
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-isohybrid-mbr RIPLinux-9.3-non-X.mbr \
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-partition_offset 16 \
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/mnt
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```
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The image was copied onto a USB stick
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```
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dd if=new_image.iso of=/dev/sdc
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```
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and plugged into a Debian system.
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```
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fdisk -lu /dev/sdb
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```
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yields
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```
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Device Boot Start End Blocks Id System
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/dev/sdb1 * 64 120831 60384 17 Hidden HPFS/NTFS
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```
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I can mount `/dev/sdb` and `/dev/sdb1` alike:
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```
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mount /dev/sdb1 /mnt1
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mount -o loop /dev/sdb /mnt
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```
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`-o loop` avoids failure with "mount: /dev/sdb already mounted or /mnt busy".
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A comparison by
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```
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diff -r /mnt /mnt1
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```
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reports no difference.
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Human readable files look ok.
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Test-reading all content by
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```
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tar cf - /mnt | wc
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```
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yields a reasonable byte count of 60743680 and no errors.
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The machine boots RIPLinux from this USB stick with no visible problems.
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It can then mount `/dev/sdb` as well as `/dev/sdb1`.
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The ISO image boots from CD too.
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Mounting the partition can be simulated with an image file on hard disk by
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cutting off the first partition_offset blocks of 2 KB:
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```
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dd if=new_image.iso of=partition_image.iso bs=2048 skip=16
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mount -o loop partition_image.iso /mnt1
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```
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--------------------------------------------------------------------------
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Another test was made with GRUB 2 by downloading
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```
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bzr branch http://bzr.savannah.gnu.org/r/grub/trunk/grub
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```
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Before building GRUB 2, the file
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```
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util/grub-mkrescue.in
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```
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was edited to replace in the options of the xorriso command:
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```
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--protective-msdos-label
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```
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by
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```
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-partition_offset 16 -no-pad
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```
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Then GRUB 2 was built and installed.
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The resulting image from
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```
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./grub-mkrescue -o image.iso
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```
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was put onto USB stick. It passed the same tests on Debian
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as above RIPLinux example. It boots to a GRUB prompt.
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Due to option `-no-pad` the image is about 250 kB smaller than
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the image produced by original `grub-mkrescue`. Else it would have grown by
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about 50 kB.
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Unpadded ISO images are safe except for burning on CD in TAO mode.
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In this case problems may occur with reading the last few data blocks.
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So when burning onto CD make sure to require SAO mode and/or to
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require padding by 300 kB.
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Burning on DVD or BD needs no such caution. Neither does copying
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on USB stick or hard disk.
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Program `fdisk` will complain about "different physical/logical" addresses.
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This can be silenced by adding option
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```
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-partition_cyl_align on
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```
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at the cost of image padding up to the next full MB.
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E.g. by 402 kB to 2 MB.
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--------------------------------------------------------------------------
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Open questions:
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- Shall the partition of an isohybrid image be marked bootable ?
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Currently xorriso keeps the 0x80 mark of an imported MBR
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and the 0x80 mark which xorriso sets by its own MBR
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preparations.
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- If not to be marked bootable:
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What equipment would the partition need to justify having the mark ?
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------------------------------------------------------------------------
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Application:
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The partition offset feature can be controlled by libisofs API calls
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```
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int iso_write_opts_set_part_offset(IsoWriteOpts *opts,
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uint32_t block_offset_2k,
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int secs_512_per_head,
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int heads_per_cyl);
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int iso_write_opts_set_system_area(IsoWriteOpts *opts, char data[32768],
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int options, int flag);
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```
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or by libisoburn calls
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```
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int isoburn_igopt_set_part_offset(struct isoburn_imgen_opts *opts,
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uint32_t block_offset_2k,
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int secs_512_per_head, int heads_per_cyl);
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int isoburn_igopt_get_part_offset(struct isoburn_imgen_opts *opts,
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uint32_t *block_offset_2k,
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int *secs_512_per_head, int *heads_per_cyl);
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int isoburn_igopt_set_system_area(struct isoburn_imgen_opts *o,
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char data[32768], int options);
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int isoburn_igopt_get_system_area(struct isoburn_imgen_opts *o,
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char data[32768], int *options);
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```
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or by xorriso options
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```
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-boot_image any partition_offset=(2kb_block_adr)
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-boot_image any partition_sec_hd=(number)
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-boot_image any partition_hd_cyl=(number)
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-boot_image any partition_cyl_align(on|auto|off)
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-as mkisofs ... -partition_offset (2kb_block_adr) \
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-partition_hd_cyl (number) \
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-partition_sec_hd (number) \
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-partition_cyl_align (on|auto|off) ...
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```
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As stated above, an offset larger than 16 would expose vital parts of the
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ISO image as unclaimed space. Values smaller than 16 are not accepted.
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So use either an offset of 16 blocks or keep the feature disabled by
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offset 0.
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Loading…
Reference in New Issue
Block a user