I am writing to find out benefit of delta upgrade/delta rpm on OpenSuse (particularly for slow internet connections).

I do want to know two things regarding this.

1) How much typically in % data is saved in downloading upgrade using delta rpm. Also does delta rpm is standard in OpenSuse means does it always works?

2) The OpenSuse site says that transition from let say e.g. 12.1 to 12.2 can be done by installing deltas of packages thus saving data. I also want to know whether transition from let say 12.2 to 13.1 e.g. i.e. major version transition can be done through delta upgrade or not?

they do save bandwidth BUT they a bit longer to install

for opensuse 12.1 to 12.2
forget it
only 12.3 is still supported in the old 12 series and that will be going END OF LIFE very soon in 78 DAYS ( Sept. 15 )

you can TRY to upgrade 12.3 to 13.1
BUT it is NOT!!!! recommended
do a new clean install using the 13.1 install dvd

In a very simple sense, the delta is only the bits that have changed. It generally works good. It could suffer from changes to the structure of the update. This is similar to other differential updates for iso images but this idea has fallen out of favor now that more people have fast unlimited connection.

Is this possible using delta packages, I mean are delta for 12.3-13.1 available?

Well, that looks like two misunderstandings, right there.

• Give the existence of 12.3, that's not a jump of one version to the subsequent one, and it is only an upgrade from one version to the immediately subsequent one that is supported.
• The concept that 12.x is one major version and 13.x is some other major version is one that doesn't really have any meaning. The new numbering system (the one with no Y.0 releases) was specifically selected to get away from this idea of there being some kind of difference in a Y.0 release, actually, because you can find, all over the web, the spurious information that Y.0 releases are 'known to be buggy and the series is worth avoiding, except for the adventurous, until the better-debugged Y.1 release comes along'. (This is just junk info - all releases are likely to have minor problems, and if you were expecting a Y.0 to be buggy, you probably put every minor thing that you see down to it being a Y.0 release, whether that means anything worthwhile, or not.)

I know of nothing that always works, but I may be overlooking something, having had a bad day. But, I don't have any experience with Delta RPMs being notably unreliable, but then I haven't tried an infinite number of times.

Typically, a delta rpm for a package, will be of the order of 33 to 66 % of the size of the plain rpm. There is, of course, no 'delta rpm for the upgrade...'

I've never tried (and I probably won't ever try it, except in a slightly perverse bid to see if something breaks). Most people who have any experience with this will do a semi-clean install from, eg, a DVD or a memory stick. This method of using the release DVD (etc), and leaving, say, the home directory structure in place is quite reliable and well tested. The other method can easily leave you with more time spent cleaning up minor niggles than you have saved on the download time (in fact, I'd say it probably does, but that based on the late 7.x series, or something). And if anything goes wrong, with the DVD method you still have the DVD and don't need to download it again, where the other puts you in a less-than-desirable position, if bandwidth is your problem.

If download bandwidth is a problem, why not get a DVD from a commercial seller of such things, or from a magazine cover, if they exist conveniently where you are.