Subtract ZFS ARC size from RAM usage

[waltmck]

No description provided.

[Jas-SinghFSU]

This will throw an exception if you don't have zfs as it won't be able to find that file - which will cause the ram calculation to fail in all cases unless zfs exists.

Most of this logic is sound, we can either:

1. Wrap this in a try catch with an empty catch (a bit smelly)
2. Move it out into a function and do a preliminary check to see if the path exists

I think approach 2 is cleaner so here we just do

            const arcSize = this._getZfsArcSize();

Then make a method down below

    private _getZfsArcSize(): number {
        const arcstatsPath = '/proc/spl/kstat/zfs/arcstats';

        if (!GLib.file_test(arcstatsPath, GLib.FileTest.EXISTS)) {
            return 0;
        }

        const [arcSuccess, arcstatsBytes] = GLib.file_get_contents(arcstatsPath);
        if (!arcSuccess || arcstatsBytes === undefined) {
            return 0;
        }

        const arcstats = new TextDecoder('utf-8').decode(arcstatsBytes);
        const arcSizeMatch = arcstats.match(/size\s+\d\s+(\d+)/);
        return arcSizeMatch ? parseInt(arcSizeMatch[1], 10) : 0;
    }

And we can reduce some nesting by using the early return pattern.

Also this is a bit wasteful since we're checking if the file exists on every ram calculation (every polling interval). Instead we can move hold a state int he constructor so we do that discovery on startup:

  constructor() {
      this._hasZfs = GLib.file_test('/proc/spl/kstat/zfs/arcstats', GLib.FileTest.EXISTS);
  }

Then we can just

      if (!this._hasZfs) return 0;