It is sometimes useful to "mirror" a remote Pacman cache either to speed up installation or to control when updates appear by controlling when the mirroring happens. Pacman provides an automatic way of doing this called a mirror cache. Mirror caches work almost the same way as snapshots do except that mirrors are updateable. Let's try it out by mirroring a sample cache that comes with each copy of Pacman.
Go to a fresh working area and do
% pacman -mirror $PACMAN_LOCATION/democache -o test(the -o test.mirror is optional). When this is finished, you will find a directory test.mirror in your current working directory. This is the mirror cache. Just as with snapshots, this is self contained in the same sense, moveable, machine independent and accessible from other sites as well as locally. You can move the directory elsewhere and rename it as long as you keep the mirror suffix. As with snapshots, mirrors are caches in every sense. You can, for instance, do
% pacman -lc test.mirroror
% pacman -get test.mirror:Pythonand it all works quite a bit like it was a snapshot. Just as with snapshots, you can mirror any combination of caches including, source caches, snapshots, local installations and other mirrors.
The main point of mirrors is that they are updatable. To try this, modify one of the packages in $PACMAN_LOCATION/democache and do
% pacman -update-check test.mirror % pacman -lc test.mirrorNotice that this works the same way as updating an installation. Similarly, to re-check and accept updated packages, just do
% pacman -update test.mirror
Because mirrors are updateable, you can use them to control when installation see that an update is available. By installing from a mirror rather than directly, you may not only gain in speed, but it's also a mechanism to delegate the decision about when updates appears to the owner of the mirror cache (see tutorials).
Note that mirrors (like snapshots) can be used remotely via http, ssh or gsiftp just like other caches. Mirrors have the limitation that you can only update them locally. However, even at a remote site, you can use -update-check to find out if a remote mirror is up to date or not.