2.1.10 The pool directory

Historically, packages were kept in the subdirectory of dists corresponding to the distribution that contained them. This turned out to cause various problems, such as large bandwidth consumption on mirrors when major changes were made.

Packages are now kept in a large "pool", structured according to the name of the source package. To make this manageable, the pool is subdivided by section (main, contrib, and non-free) and by the first letter of the source package name. These directories contain several files: the binary packages for each architecture, and the source packages from which the binary packages were generated.

You can find out where each package is placed by executing a command like apt-cache showsrc mypackagename and looking at the "Directory:" line. For example, the apache packages are stored in pool/main/a/apache/. Since there are so many lib* packages, these are treated specially: for instance, libpaper packages are stored in pool/main/libp/libpaper/.

The dists directories are still used for the index files used by programs like apt. Also, at the time of writing, older distributions have not been converted to use pools, so you'll see paths containing distribution names such as potato or woody in the "Directory" header field.

Normally, you won't have to worry about any of this, as new apt and probably older dpkg-ftp will handle it seamlessly. If you want more information, see the RFC: implementation of package pools.