In Solaris 10, a major design choice is whether to use sparse root zones or full root zones. Remember, a zone is a lightweight virtual server running within a shared Solaris instance.

With sparse root zones, the standard Solaris software packages are shared read-only from the global zone into the non-global zones. With whole root zones, the same software packages are instead copied into the non-global zones.

Note: In Solaris 11, sparse root zones are no longer available -- the benefits are now provided by ZFS and the new Image Packaging System.

The main benefits of sparse root zones are:

• They use less disk space
• They are faster to install and upgrade
• They are more difficult to break because large parts of the filesystem are read-only
• They enforce consistency between Solaris packages and patch versions across zones

The main benefits of whole root zones are:

• They provide more flexibility around which Solaris packages and patches are installed
• They may be supported by more third-party software

In my opinion the benefits of whole root zones are not particularly great. The main problem is that whole root zones create the illusion of flexibility, but if you exercise this flexibility you tend to run into problems with the Solaris 10 packaging system due to dependencies between non-global and global zones. In addition, flexibility around package and patch levels creates administrative overhead and leads to confusion when adding, removing, or patching packages, or migrating zones between physical servers.

Any third-party software which relies on whole root zones is probably not putting its files in the best possible place. As a rule, it is better for third-party software to be kept separate from native software in order to make upgrading and future migration work easier and less risky. In most cases it is possible to get an application to work in a sparse root zone through the use of well-placed symbolic links or mountpoints, and this can make the separation between application and OS much cleaner. However, there may be supportability issues if you go down this path.