This applies to all OSes, even (tho' it pains me greatly to say it) Windoze - every Windoze host has a C: drive, which, in this context, can be taken to be the equivalent of a volume name.

IMO. the question (to ask of annie06) ought to be: 'Are all volumes the same size on every host?'

My reading of the OP suggests that where ever i.e. on which ever host, vol2 exists, it is always 20g, so a simplified version of the lookup table based approach (similar to that outlined by kennethk) should suffice - something along the lines of i.e. untested ...

use warnings;
use strict;
use autodie;

#for each vol_size_entry in the volsize file do
#    add the vol_size_entry to the volsize lookup (keyed on vol name)
#done
open FILE, "<volsize";
my %volsize = map {
    local @_ = split;

    # $_[1] needs normalising (to chosen common units) and removal of 
+unit spec

    /_name/ ? () : ($_[0] => $_[1]);
} <FILE>;
close FILE;

#for each host_entry do
#    lookup the size of the vol for the host
#    add the vol size to the record for the host
#done
open FILE, "<hostfile";
my %hostsize;
while (<FILE>) {
    next if /_name/;
    local @_ = split;
    $hostsize{$_[1]} += $volsize{$_[0]};
}
close FILE;
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