Updated partition offset wiki about cylinder alignment
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@ -131,7 +131,7 @@ was edited to replace in the options of the xorriso command:
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}}}
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by
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{{{
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-partition_offset 16 -no-pad --no-emul-toc
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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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@ -142,10 +142,24 @@ The resulting image from
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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 options -no-pad --no-emul-toc the image is about 300 kB smaller than
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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 130 kB. The mkisofs emulation of newer versions of xorriso has
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--no-emul-toc as default. Thus the overhead would only be about 50 kB.
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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 KiB.
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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 MiB.
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E.g. by 402 KiB to 2 MiB.
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--------------------------------------------------------------------------
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@ -162,12 +176,15 @@ preparations.
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Application:
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The partition offset feature can be controlled by libisofs API call
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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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resp. by libisoburn calls
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{{{
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@ -178,13 +195,24 @@ int isoburn_igopt_set_part_offset(struct isoburn_imgen_opts *opts,
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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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resp. 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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-as mkisofs ... -partition_offset (2kb_block_adr) ...
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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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