The good and bad of IPS (as I see it)
IPS (the 'Image Packaging System') is the new packaging system used in
Solaris 11 and (more importantly) many Illumos-derived distributions; it
replaces Solaris 10 packages and patches. I have previously described IPS as being more or less like
git; it puts all
files together in a hash-based content store and then has 'packages'
that are basically just indexes into the store. This contrasts with the
traditional Linux approach to packaging where each package is an archive
of some sort that contains all actual files in the package.
The attractive part of IPS is what the content store approach does for repositories and for package updates. If files are the same between two versions of a package (or between multiple packages), the repository only needs to store one copy and the package update or install process can detect that you already have the needed file installed already. This mimics the practical behavior of Solaris 10 patches, which only included changed files (as opposed to the Linux approach, where changing just one file in a package causes you to re-issue an entire second copy of the whole package).
(This also minimizes what needs to be digitally signed. Much as in git, you don't need to digitally sign the files themselves, just the package index data. The all-in-one Linux package format means that you generally need to sign and verify large blobs of data.)
The bad part of IPS is what it does to downloading and storing packages. As far as I know, files are downloaded from IPS repositories in the same way that they're stored; you ask for them one by one and they then dribble in bit by bit. As we've learned the hard way, this is not a great way to do things on the modern Internet (or in general) because each separate fetch requires a new connection (or at least a new request) and that has various consequences.
(IPS packages are normally fetched over HTTP or HTTPS but I don't know if the IPS client and server are smart enough to take advantage of HTTP connection reuse.)
I'm also not enthused about how this makes package repositories harder to manage and exposes them to subtle forms of breakage (such as a file that's listed in package manifests but not present in the repository). Pruning old packages is now necessarily a whole-repository operation, since you can't just remove their files without seeing if any other package uses them.
I suspect that Sun developed IPS this way to preserve the small sizes and small installation changes of Solaris 10 patches (which transfer and install only the changed files instead of the whole package). I prefer the simpler approach of Linux packages (and I note that Linux package updates themselves can optimize both transfer size and install time actions).