[ros-dev] HDD Geometry switch

Love Nystrom love.nystrom at gmail.com
Wed Nov 30 04:25:21 UTC 2011

Olaf, please don't leave, the project needs you just
as much as it needs the developers.

Eric, Olaf's regression testing can not entail patching/rebuilding
the system to accomodate disk structure (as you surely realize).
That said, your reasons for change are sound, but perhaps you
can make provisions to temporarily revert this if it causes massive
problems. The timing _is_ not good if indeed a release is close
at hand.. Can it wait until after the release?

Alex, calling Olaf "not an OS tester" is a bit harsh, don't you think?
Not everyone can be as weather hardened and experienced as you are.

Regarding the reasons for change:

Max CHS capacity: 10 bit C, 6 bit S, 8 bit H
2^10 * 2^6 * 255 == 16711680 sect(512) == 8.5 GB

This is the geometry Eric wants to use, which makes sense
because it can utilize the full CHS addressing capacity,
and reflects the absolute limit of BIOS int 13h CHS calls.

Previous 32S 64H CHS limit:
2^10 * 32 * 64 == 2097152 sect(512) == 1 GB

This is an artificial and unnecessary limit, imposed by some
strange design decision in Redmond a long long time ago.

For non-devs: In CHS mode BIOS has an absolute disk limit of 8.5 GB,
due to the available address bits, a 10/6 bit Cylinder/Sector composite,
and an 8 bit head number. Any disk bigger than that can only be addressed
in LBA mode (logical sector nr), which has a limit of 2^32 sectors == 2 TB.
And, if anyone didn't know, the geometry parameters are stored
in the boot sector, not in the MBR.
There's more to it, but I stop at that.

So.. Eric's change _should_ not cause any problem on disks
larger than 8.5 GB, since they run in LBA mode by necessity.
Concordingly, there could be forseeable problems on virtual disks
which will often be smaller than 8 GB and may be running CHS mode.

That'll be my penny to the pot for now.

PS. A standard sector is 512 bytes. Bigger sectors, e.g 1kB, have been used
in the past to increase capacity, but are _very_ unusual since they require
proportionately higher bit frequencies on the media and are error-prone.

Best Regards
// Love
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