Delta ISOs

A Delta ISO (deltaiso) 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 DeltaRPMs (deltarpm) 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).

If you like DeltaRPMs, you will like Delta ISOs
Using deltaisos saves bandwidth and time.

System Requirements

Fedora

xz compression change in Fedora 15
Only DVD->netinst disos are currently available for F15 due to RHBZ #677578.

• 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 deltaisos between ISOs containing xz-compressed RPMs, Fedora 11 or later is required. (Fedora 10 or below does not work because there is not a deltarpm package with xz support.)
• In Fedora 11, the deltarpm package must be installed, and it should be the updated version (deltarpm-3.5-0.4.20090913git.fc11.1), not the deltarpm-3.4-16.fc11 release version that does not contain xz support.
• In Fedora 12 or later, the deltaiso package must be installed. (In Fedora 11, deltaiso support is contained in the deltarpm package, and there is no separate deltaiso package.)

RHEL/CentOS

• RHEL/CentOS 4 or later with the EPEL repository enabled: The necessary packages for deltaiso support in RHEL/CentOS 4 or 5 are available in the EPEL repository.
• In RHEL/CentOS 4, the deltarpm package must be installed. Note that the available version (deltarpm-3.4) does not include xz support which is required for working with deltaisos between recent versions of Fedora (12 or later).
• In RHEL/CentOS 5, the deltaiso package must be installed. (In RHEL/CentOS 4, deltaiso support is contained in the deltarpm package, and there is no separate deltaiso package.)

Using Delta ISOs

The applydeltaiso command is used. The syntax is:

applydeltaiso oldiso deltaiso newiso

If oldiso 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

Alternatively, the rawread script (download) can be used to read the disc image into a file, as follows:

rawread /dev/dvd > oldiso

This can be done while the deltaiso is downloading, and the resulting ISO file can be verified (both in terms of integrity, and to make sure the correct image is being used for oldiso). When the download is finished, using applydeltaiso on the ISO file will save time compared to using the optical disc.

Creating Delta ISOs

The makedeltaiso command is used. The syntax is

makedeltaiso oldiso newiso deltaiso

Here, unlike the applydeltaiso command, oldiso should always refer to an ISO file, never a CD/DVD disc (RHBZ #569499). If necessary, the rawread script (download) can be used to read a CD/DVD disc image into an ISO file.

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

1. Telling users the estimated run time for applydeltaiso lets them determine whether the download time saved by using a deltaiso is likely to be greater than the time required to run applydeltaiso.
2. Around the time of xz compression changes, certain deltaisos may be nonfunctional as a result of not all RPMs in newiso using the same compression. To be sure the deltaiso should be checked. When applydeltaiso finishes running, there should be a message similar to this:

iso successfully re-created, md5sum: f90d7a6d19a2cc5428a95892d5e2ca84

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

md5sum mismatch, iso is corrupt

and in this case the deltaiso should not be used.

External Links

• Wikipedia's Delta ISO page
• Deltaisos (openSUSE)
• applydeltaiso man page
• makedeltaiso man page