code: public void snapshot(String snapshotName, String columnFamilyName) throws IOException
      {
          assert snapshotName != null;
          boolean tookSnapShot = false;
          for (ColumnFamilyStore cfStore : columnFamilyStores.values())
          {
              if (columnFamilyName == null || cfStore.name.equals(columnFamilyName))
              {
                  tookSnapShot = true;
                  cfStore.snapshot(snapshotName);
              }
          }
  
          if ((columnFamilyName != null) && !tookSnapShot)
              throw new IOException("Failed taking snapshot. Column family " + columnFamilyName + " does not exist.");
      }
docs: Take a snapshot of the specific column family, or the entire set of column families
  if columnFamily is null with a given timestamp
  
  @param snapshotName     the tag associated with the name of the snapshot.  This value may not be null
  @param columnFamilyName the column family to snapshot or all on null
  @throws IOException if the column family doesn't exist