> There are very good reasons NOT to use DD mode if you use certain
> types of Adaptec SCSI controllers - they simply won't boot from DD.

Never seen.  All my SCSI controllers so far booted from my disks
(obviously :).

I figure from Peter's comment in that piece of code that the original
(386BSD 0.0 inherited) DD mode fake fdisk table apparently had some
poor (faked) values inside that could upset some BIOSes.  That's bad,
and IMHO we should fix what could be fixed, but without dropping that
feature entirely (see below).

<personal opinion>
Still, it's my opinion that these BIOSes are simply broken:
interpretation of the fdisk table has always been in the realm of the
boot block itself.  The BIOS should decide whether a disk is bootable
or not by looking at the 0x55aa signature at the end, nothing else.
Think of the old OnTrack Disk Manager that extended the fdisk table to
16 slots -- nothing the BIOS could ever even handle.  It was in the
realm of the boot block to interpret it.
</personal opinion>

> Aside from that, FreeBSD needs to have *one* recommendation for
> disks, anything else creates too much confusion.

DD mode has never been a recommendation.  It is for those who know
what it means.  I'm only against the idea to silently drop support for
it...  fdisk tables are something that has been designed in the
previous millenium, and i think nobody is going to argue about it that
they are rather a mis-design from the beginning (or even no design at
all, but an ad-hoc implementation).  Two different values for the same
(which could become conflicting, thus making disks unportable between
different controllers), not enough value space to even remotely cover
the disks of our millenium, enforcement of something they call
`geometry' which isn't even remotely related to the disks' geometry
anymore at all, far too few possible entries anyway, ...  FreeBSD is
in a position where it doesn't strictly require the existence of such
an obsoleted implementation detail, so we should users leave the
freedom of decision.

Again, it has never been the recommendation (well, at least not after
386BSD 0.0 :), and i normally wouldn't recommend it to the innocent
user.  But we should not break it either.

> (The other day a coworker of mine wanted to use DD for some IBM DTLA
> disks, because he'd heard that the disks performed better that way -
> something to do with scatter-gather not working right unless you
> used DD. [...])

As much as i personally prefer DD mode: that's an urban legend.

cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)

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