Daily builds now hybrid CD/USB images

[Colin Watson]

As of tomorrow's daily builds, all Oneiric amd64 and i386 CD images on
cdimage.ubuntu.com can also be written directly to a USB device.  You
can still use usb-creator if you need to enable persistent storage on
the USB stick, but if all you need is to install from the stick then
this simplifies the process.

There is a small size cost to this feature; part of this is partition
alignment to 1MB "cylinders" (since the image now has to simultaneously
look like a CD and a partitioned USB disk), and part of it is a second
directory tree whose size is roughly proportional to the number of files
in the image.  On the server CD (and probably the alternate CD too,
though I haven't measured that), this came out to about 5MB, but as luck
would have it I found a way to save about the same amount by hardlinking
some files together, so the size there stays roughly the same.  On the
desktop CD, unfortunately, we lose about 1MB (although I consider myself
to have "paid" for this by way of the 2MB or so we gained by switching
to live-build ...).  If this becomes a problem close to release time,
then we may be able to shave off a bit at the possible cost of some
USB-booting compatibility on a few machines by dropping the 1MB
alignment.  See the recent thread on pkg-libburnia-devel for details.

I realise that we're behind a number of other major distributions on
this.  The delay was because (like Debian) we couldn't simply use
isohybrid because that would break jigdo downloads, so we had to switch
to xorriso as the CD image generator on these architectures for its new
JTE support, and by the time all that landed in Debian I didn't really
want to cram it into 11.04.  See
http://blog.einval.com/2011/01/07#isohybrid_CDs for the gory details,
and thanks a lot to Steve, Thomas, and George for their hard work on
that.

Please file bugs on https://bugs.launchpad.net/ubuntu-cdimage if this
change causes the image to stop working on machines where it previously
worked.

Cheers,

-- 
Colin Watson                                       [cjwatson at ubuntu.com]