diff --git lib/pstore.rb lib/pstore.rb
index 9fb0249..cc9e70d 100644
--- lib/pstore.rb
+++ lib/pstore.rb
@@ -9,7 +9,7 @@
 
 
 require "fileutils"
-require "digest/md5"
+require "zlib"
 require "thread"
 
 #
@@ -349,7 +349,7 @@ class PStore
   # Constant for relieving Ruby's garbage collector.
   EMPTY_STRING = ""
   EMPTY_MARSHAL_DATA = Marshal.dump({})
-  EMPTY_MARSHAL_CHECKSUM = Digest::MD5.digest(EMPTY_MARSHAL_DATA)
+  EMPTY_MARSHAL_CHECKSUM = Zlib.crc32(EMPTY_MARSHAL_DATA)
 
   #
   # Open the specified filename (either in read-only mode or in
@@ -406,7 +406,7 @@ class PStore
         size = empty_marshal_data.size
       else
         table = load(data)
-        checksum = Digest::MD5.digest(data)
+        checksum = Zlib.crc32(data)
         size = data.size
         if !table.is_a?(Hash)
           raise Error, "PStore file seems to be corrupted."
@@ -428,33 +428,12 @@ class PStore
     is_windows
   end
 
-  # Check whether Marshal.dump supports the 'canonical' option. This option
-  # makes sure that Marshal.dump always dumps data structures in the same order.
-  # This is important because otherwise, the checksums that we generate may differ.
-  def marshal_dump_supports_canonical_option?
-    begin
-      Marshal.dump(nil, -1, true)
-      result = true
-    rescue
-      result = false
-    end
-    self.class.__send__(:define_method, :marshal_dump_supports_canonical_option?) do
-      result
-    end
-    result
-  end
-
   def save_data(original_checksum, original_file_size, file)
     # We only want to save the new data if the size or checksum has changed.
     # This results in less filesystem calls, which is good for performance.
-    if marshal_dump_supports_canonical_option?
-      new_data = Marshal.dump(@table, -1, true)
-    else
-      new_data = dump(@table)
-    end
-    new_checksum = Digest::MD5.digest(new_data)
+    new_data = dump(@table)
 
-    if new_data.size != original_file_size || new_checksum != original_checksum
+    if new_data.size != original_file_size || Zlib.crc32(new_data) != original_checksum
       if @ultra_safe && !on_windows?
         # Windows doesn't support atomic file renames.
         save_data_with_atomic_file_rename_strategy(new_data, file)
@@ -484,11 +463,10 @@ class PStore
 
   def save_data_with_fast_strategy(data, file)
     file.rewind
-    file.truncate(0)
     file.write(data)
+    file.truncate(data.size)
   end
 
-
   # This method is just a wrapped around Marshal.dump
   # to allow subclass overriding used in YAML::Store.
   def dump(table)  # :nodoc:
@@ -511,7 +489,7 @@ end
 
 # :enddoc:
 
-if __FILE__ == $0
+if __FILE__ == $PROGRAM_NAME
   db = PStore.new("/tmp/foo")
   db.transaction do
     p db.roots