DeltaISOs

Andre Robatino robatino at fedoraproject.org
Sun Aug 29 19:14:26 UTC 2010


David <dgboles <at> gmail.com> writes:

> I think that I understand the principal behind this. Disk space nor
> bandwidth are not problems for me. I was thinking of others that might
> have a bandwidth/byte counter problem.
> 
> Explain to me just how downloading only a part of the new ISO, the
> changes, and then creating the new one with parts of the old ISO and the
> new parts. How can that be the same as downloading the whole new ISO?
> 
> Or am I not understanding how this works.

If what you're asking is, is there a way of getting the bandwidth savings of
deltaisos without someone else having to provide them, then no, I don't think
so. Deltaisos basically work the same way as deltarpms, just on entire ISOs
instead of individual RPMs. If there was a way to do that, then it wouldn't be
necessary for deltarpms to be generated in order for yum-presto to work, either.
The delta compression needed to express the changes between the old and new
files is expensive, and if it wasn't done once and stored in a file (a deltaiso
or a deltarpm), then the server would have to do it separately for each
download, which would be prohibitive.

The processing that rsync/zsync does is much simpler, but generally doesn't
provide as much bandwidth savings as deltaisos. Unfortunately, even rsync is
expensive on the server, so not all servers provide it. And zsync, which puts
the load on the client instead of the server, isn't available in Fedora yet
(though you can install it from other sources), and like deltaisos, someone else
has to provide zsync control files before it can be used. I've been doing this
for recent test releases at

https://fedoraproject.org/wiki/User:Robatino/Downloads

so people can experiment with it and maybe create some more pressure to get it
into Fedora. See

https://bugzilla.redhat.com/show_bug.cgi?id=490140
https://bugzilla.redhat.com/show_bug.cgi?id=495310

The performance of zsync vs. deltaisos varies from almost as good to much worse,
depending on how many packages have changed between old and new ISOs. (For
example, going from Fedora (N-1) to Fedora N Alpha TC1, zsync is almost useless,
since almost all packages are updated. Between later iterations, it helps a
lot.) Even if zsync becomes available, deltaisos should still be provided for
people with very slow connections.



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