librenms-librenms/includes/polling/storage/freenas-dataset.inc.php

<?php

if (! is_array($storage_cache['dataset'])) {
    $storage_cache['dataset'] = snmpwalk_cache_oid($device, 'datasetTable', null, 'FREENAS-MIB');
    d_echo($storage_cache);
}

$entry = $storage_cache['dataset'][$storage['storage_index']];

$storage['units'] = $entry['datasetAllocationUnits'];
$storage['size'] = ($entry['datasetSize'] * $storage['units']);
$storage['free'] = ($entry['datasetAvailable'] * $storage['units']);
$storage['used'] = ($storage['size'] - $storage['free']);