ohai should be more efficient when only displaying single attributes

It's impossible for ohai to save time because it doesn't know beforehand which plugins will be populating which attributes. Therefore it HAS to go through them all before selecting your attribute.

A valid workaround is to include only the plugins you'll need in your ohai configuration file, this can be done via the -f flag.

Sorry to be a pest, but "it doesn't know beforehand" contradicts "include only the plugins you'll need". The latter mechanism would satisfy the scope of the ticket, though I can find no documentation on how the "-f" flag works. The command-line help states only that -f is "A file to run Ohai against". I couldn't find a mention of this flag on the wiki either.

So if there is a mechanism that can be invoked via -f to only load a subset of plugins, it should also be possible to specify this option directly on the command line instead of via an external file. In fact, as in the example above, "ohai filesystem" should do just that: only load the "filesystem" plugin but nothing else.

The assumption was - that the user would know ( by looking at the source or reading the documentation? ) which plugins they'll need, as opposed to Ohai. Regarding the -f flag, it just reads in the file expecting regular ruby syntax, something like:

$ uname -morsv
Linux 2.6.32-33-generic #71-Ubuntu SMP Wed Jul 20 17:27:30 UTC 2011 x86_64 GNU/Linux
$ cat filesystem.rb 
require_plugin("linux::filesystem")
$ ohai -f /home/toomasp/filesystem.rb |head
{
  "filesystem": {
    "/dev/sda1": {
      "kb_size": "469311212",
      "kb_used": "176956920",
      "kb_available": "268514640",
      "percent_used": "40%",
      "mount": "/",
      "fs_type": "ext4",
      "mount_options": [

As you can see, there's no top-level filesystem plugin - there's also no top-level plugin which would do an require_plugin "#{os}::filesystem" - so Ohai uses all_plugins method which iterates over all the plugins available and loads them one by one. That's how you end up with the filesystem plugin being run. Ohai does not know which plugin will be providing the "filesystem" attribute, as there's no direct mapping between the plugin names/paths and the attributes that they might provide.

Granted, at one point we might end up with some mapping scheme that requires plugins to be named after the attributes they provide, but currently that's not in place. As some of the current plugins are in conflict with this naming scheme ( ip_scopes, language plugins like c, perl, php, ruby.. etc ) it'll break compatibility and obsolete a lot of cookbooks which depend on these attributes.

And no worries about being a pest, as questions like these cannot be answered unless asked in the first place. Hopefully you can see, if not agree with, my reasoning now.

We can do better.