Renich Bon Ciric <renich <at> woralelandia.com> writes:
Didn't know about this. Should this be published somewhere on
front/download page? This is great. Thanks!
The following issues should be noted:
1) In F15, the xz compression used changed, and unfortunately that isn't handled
automatically, so to use delta ISOs between versions after that, you need to
rebuild the ISOs on a machine running F15 or later. I do have semi-static
binaries in http://robatino.fedorapeople.org/deltaiso_executables/
that can be
used to rebuild recent ISOs on older OSes (F14 or below, or RHEL/CentOS, for
example), but as long as you're running F15 or later, just install the deltaiso
package from the Fedora repo and use the applydeltaiso command from that.
2) Since this involves delta rpms, there's the same bandwidth/CPU speed tradeoff
that's involved in deciding whether or not to use yum-presto for updates. If you
have a very fast download and a not-so-fast CPU, downloading full updated
packages becomes faster than downloading and applying delta rpms. In this case,
you could use rsync instead, which is available on at least some of the mirrors.
The yum-presto code for F18 and above will be threaded and make use of multiple
cores for rebuilding, which will make yum-presto more favorable, but the
deltaiso code isn't threaded yet (although it should be possible).
3) If you have the old ISO burned to optical disc, and you want to read it off
and verify the checksum before downloading the delta ISO (which is a good idea),
you need to know the exact size to read off. This is tricky. You can't just do
something like "cat /dev/sr0 > file.iso" and expect to get the right size.
need to find the exact size, and then either use a dd command to read off
exactly that much, or use the truncate command on a padded image to reduce it to
the right size. To get the right size, you can look up the image at
, then start a download with wget,
which shows the exact size in bytes at the beginning, then Ctrl-C it. With
standard ISOs, you would be able to look in the ISO header to find the size, but
all recent Fedora ISOs are "hybrid" which means they have extra padding in
addition to the ISO header size. Currently the extra padding consists of zeros
up to the next largest multiple of 1 MiB, so it's possible to infer the size,
but that may change in the future. I avoid all this myself by writing the exact
size of the image on my discs itself right after burning them (i.e.,
"3,834,642,432 bytes" on the Fedora-17-x86_64-DVD.iso disc).