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Whenever I run apt update especially for those which have DEP 11 and are metadata never have pdiffs. Is this something which needs a feature in appstream (upstream) or it's a feature needed to be done in the apt implementation ?

Regular pdiff files (as an example) - 

Get:30 http://cdn-fastly.deb.debian.org/debian buster/main amd64 Contents (deb) 2019-02-01-0815.12.pdiff [91 B]         
Get:30 http://cdn-fastly.deb.debian.org/debian buster/main amd64 Contents (deb) 2019-02-01-0815.12.pdiff [91 B]

and Metadata DEP-11 files - 

Get:50 http://cdn-fastly.deb.debian.org/debian unstable/main DEP-11 48x48 Icons [3,923 kB]                              
Get:51 http://cdn-fastly.deb.debian.org/debian unstable/main DEP-11 64x64 Icons [7,876 kB]

Why do they need such big heavy updates, this is when I just ran an update few hours back ?

shirish
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    What is your `sources.list`? The examples you provide are of packages from buster/testing and sid/unstable. These different packages and repositories will have different types of metadata standards depending on who is the respective maintainer. According to [this Debian Administration article](https://debian-administration.org/article/439/Avoiding_slow_package_updates_with_package_diffs) not every package will be delivered via pdiff and pdiff is not always the most efficient way to deliver certain updates as well as the fact that with pdiffs you can have several updates per day. – kemotep Feb 01 '19 at 15:49
  • My sources.list is partially made of buster and sid. Although that article dates back to 2006. – shirish Feb 01 '19 at 16:15
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    I am not as familiar with Appstream however looking at their [documentation](https://www.freedesktop.org/software/appstream/docs/index.html) (last updated in 2018) Appsteam is a distro-agnostic meta data specification to make handling packaging easier. There is no specification that packages packaged using this standard uses pdiffs. Looking to the [Debian Policy](https://www.debian.org/doc/debian-policy/) (Also updated in 2018) on packaging also shows that there is no standard requiring the use of pdiffs. – kemotep Feb 01 '19 at 16:42
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    I have also found this [related APT backend](https://wiki.debian.org/BogdanPurcareata/PluggableAptBackend) information that says if a given package does not have the option to download as pdiff or fails to download as a pdiff the package will be downloaded in its entirety. – kemotep Feb 01 '19 at 16:50
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    I do not mean to bombard you with information but I think [this link](https://salsa.debian.org/apt-team/apt-file#optimising-contents-downloads) also provides useful information on why a given package may or may not use pdiffs. – kemotep Feb 01 '19 at 16:56

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