Delta ISOs

A delta ISO is a file that contains the differences between two ISO files, and can be used to convert one into the other. It makes use of delta RPMs between RPM files in the old and new ISOs, and so is only useful if these ISOs mainly consist of RPM files (as is the case for Fedora install images, but not live images).

Fedora

 * Fedora 11 or later: RPM packages in Fedora 12 and later use xz compression (RPMs in older versions use gzip compression). In order to either create or use delta ISOs between ISOs containing xz-compressed RPMs, Fedora 11 or later is required. (Fedora 10 or below does not work because there is not a   package with xz support.)
 * In Fedora 11, the  package must be installed, and it should be the updated version, not the   release version that does not contain xz support.
 * In Fedora 12 or later, the  package must be installed.  (In Fedora 11, delta ISO support is contained in the   package, and there is no separate   package.)

RHEL/CentOS

 * RHEL/CentOS 4 or later. In RHEL/CentOS 4 and 5, the EPEL repository must be enabled.
 * In RHEL/CentOS 4, the  package must be installed.  Note that the available version  does not include xz support which is required for working with delta ISOs between recent versions of Fedora (12 or later).
 * In RHEL/CentOS 5 or later, the  package must be installed.  (In RHEL/CentOS 4,   support is contained in the   package, and there is no separate   package.)

Using Delta ISOs
The  command is used. The syntax is:

applydeltaiso oldiso deltaiso newiso

If  is on a mounted CD/DVD disc, the following also works, but runs more slowly due to the optical drive's greater access time:

applydeltaiso /dev/dvd deltaiso newiso

Creating Delta ISOs
The  command is used. The syntax is

makedeltaiso oldiso newiso deltaiso

Here, unlike the  command,   should always refer to an ISO file, never a CD/DVD disc.

If the delta ISO is intended for a large audience, then run and time the  command and verify that   is correctly generated. There are two reasons for this.


 * 1) Telling users the estimated run time for   lets them determine whether the download time saved by using a delta ISO is likely to be greater than the time required to run.
 * 2) Around the time of   compression changes, certain delta ISOs may be nonfunctional as a result of not all RPMs in   using the same compression. Hence the delta ISO should be checked. When   finishes running, there should be a message similar to this:

iso successfully re-created, md5sum: f90d7a6d19a2cc5428a95892d5e2ca84

The md5sum is that of, which is stored in the delta ISO for later verification. On the other hand, a failed reconstruction shows this message:

md5sum mismatch, iso is corrupt

and in this case the delta ISO should not be used.