librenms-librenms/includes/polling/mempools/hrstorage.inc.php

<?php

// HOST-RESOURCES-MIB - Memory Objects
if (!is_array($storage_cache['hrstorage'])) {
    $storage_cache['hrstorage'] = snmpwalk_cache_oid($device, 'hrStorageEntry', null, 'HOST-RESOURCES-MIB:HOST-RESOURCES-TYPES');
    $storage_cache['memorySize'] = snmp_get($device, 'hrMemorySize.0', '-OUqv', 'HOST-RESOURCES-MIB');
    $storage_cache['memorySize'] = $storage_cache['memorySize'] * 1024; // hrMemorySize is stored in KBytes
    d_echo($storage_cache);
} else {
    d_echo('Cached!');
}

$entry = $storage_cache['hrstorage'][$mempool['mempool_index']];

$mempool['units'] = $entry['hrStorageAllocationUnits'];
$mempool['used']  = ($entry['hrStorageUsed'] * $mempool['units']);
if ($device['sysObjectID'] == '.1.3.6.1.4.1.12325.1.1.2.1.1') { // bsnmpd based devices, like FreeBSD, opnsense, pfsense ...
    $mempool['total'] = $storage_cache['memorySize'];
} else {
    $mempool['total'] = ($entry['hrStorageSize'] * $mempool['units']);
}
$mempool['free']  = ($mempool['total'] - $mempool['used']);