Linux, iostat and device names

My favorite tools for looking at the I/O load of Linux boxes are iotop and iostat. Running “iostat -xm 5” is one of the first things I do whenever I have the feeling that a server might be I/O-bound. The output is perfectly fine and useful on your typical one-disk box, but once you throw in either Xen or DM-Crypt, then the output is not so intuitive any more as it is no longer clear what each of the dm-XX devices is actually holding.

So I whipped up the following quick perl script to translate them:

#!/usr/bin/perl -w
#
# Replace dm-x names in stdin with names from /dev/mapper, e.g.
# iostat -xm 5 | $0
#
# Otmar Lendl, 2012/08/24
#

use strict;

my %m;

foreach my $l (split(/\n/, `ls -l /dev/mapper`)) {
# lrwxrwxrwx 1 root root      8 May  3 18:49 vg1-abusehelper--swap -> ../dm-21
        if ($l =~ /\d\d:\d\d ([\w-]+) -> \.\.\/(dm-\d+)/) {
                $m{$2} = $1;
        }
}

while(<>) {
        s/(dm-\d+)( *) /substr($m{$1}. (' ' x 80),0,length($1.$2)).' '/eg;
        print;
}

The quotes in substitution line should be plain single quotes, not the typographic nonsense that wordpress insists on inserting.

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