How and where to create a prefix key for memcache? I am up to here in drupal documentation :


  Start at least one instance of memcached on your server.
  Edit settings.php to make memcache the default cache class, for example:
    $conf['cache_backends'][] = 'sites/all/modules/contrib/memcache/memcache.inc';
    $conf['cache_default_class'] = 'MemCacheDrupal';
    $conf['memcache_key_prefix'] = 'something_unique';


Why do I need a memcache key prefix if have only one database? Anyway where and how do I set it?
All those settings are added to settings.php

usually sites/default/settings.php

$conf['cache_backends'][] = 'sites/all/modules/contrib/memcache/memcache.inc';
$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['memcache_key_prefix'] = 'k1';
// Use database for cache_form
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';


You only need a memcache key prefix if you're using memcache with more than one site on that server. Adding a short key, e.g $conf['memcache_key_prefix'] = 'k1'; is good practice.