Title: speeding up portage's metadata cache
Date: 2015-02-26
Lang: en
Tags: Gentoo

[Gentoo's](http://www.gentoo.org) portage keeps metadata about installed ebuilds in `/var/cache/edb`. Dependency info for all installed ebuilds is in a `dep` subdirectory which typically looks something like this:

    .
    ├── usr
    │   └── portage
    │       ├── app-admin
    │       │   ├── eselect-1.4.1
    │       │   ├── eselect-lib-bin-symlink-0.1.1
    │       │   ├── eselect-opengl-1.2.7
    │       │   ├── gamin-0.1.10-r1
    │       │   ├── logrotate-3.8.7
    │       │   └── perl-cleaner-2.16
    ...

When portage needs to process dependency info, it reads those files. On a normal system, you will have a couple of hundred packages installed. This means that dependency processing does quite a bit of file processing.

Now if that dependency info was stored in some kind of database, wouldn't that speed up dependency processing?

I stumbled over [a wiki page](http://gentoo-en.vfose.ru/wiki/Portage_SQLite_Cache) describing how to switch portage's metadata cache to sqlite. Even the portage man page talks about this - try running `man portage` and read the section about the `modules` file.

So I gave the sqlite metadata cache a try to measure if it really speeds up portage. After configuring the database and rebuilding the metadata cache, `/var/cache/edb/dep` looks a bit different now:

    .
    └── usr
        ├── portage
        └── portage.sqlite

Now let's get to the interesting part: does the database really speed up portage? To measure, I ran a couple of `emerge -vp` commands using the normal setup and again using the database. The results are quite disappointing, though:

The best improvement was about 6% with the metadata database. 

57% of the ebuilds did run slower with the metadata database, the worst increase was about 43%

So it looks like fiddling with portage's metadata cache is not really worth the hassle.