anchor.diff:  % estimated time to traverse all the volumes on a DDFS node.
   -define(GC_PROGRESS_INTERVAL, 30 * ?MINUTE).
   
  -% Tag cache expires in this many milliseconds if tag can't be fetched
  --define(TAG_EXPIRES_ONERROR, 1 * ?SECOND).
  -
   % Number of extra replicas (i.e. lost replicas recovered during GC) to
   % allow before deleting extra replicas.
   -define(NUM_EXTRA_REPLICAS, 1).
  
positive.code: 
  % Interval for checking available disk space in ddfs_node
  -define(DISKSPACE_INTERVAL, 10 * ?SECOND).
  
  % How often node should refresh its tag cache from disk
  -define(FIND_TAGS_INTERVAL, ?DAY).
  
  % /dfs/tag/ requests can be ?MAX_TAG_BODY_SIZE bytes at most
  -define(MAX_TAG_BODY_SIZE, (512 * ?MB)).
  
negative.code: % cache. It doesn't harm to have a long interval).
  -define(TAG_CACHE_INTERVAL, 10 * ?MINUTE).
  
  % Tag cache expires in this many milliseconds if tag can't be fetched
  -define(TAG_EXPIRES_ONERROR, 1 * ?SECOND).
  
  % How often node should refresh its tag cache from disk
  -define(FIND_TAGS_INTERVAL, ?DAY).
  
  % How often buffered (delayed) updates need to be flushed. Tradeoff: The
  % longer the interval, the more updates are bundled in a single commit.
  % On the other hand, in the worst case the requester has to wait for the
  