diff --git a/README.rst b/README.rst
index aaa6ae1..13ca369 100644
--- a/README.rst
+++ b/README.rst
@@ -31,6 +31,14 @@ and `Pickling Class Instances(Python3) <https://docs.python.org/3/library/pickle
 This project is based on the achievements of `python-pqueue <https://github.com/balena/python-pqueue>`_
 and `queuelib <https://github.com/scrapy/queuelib>`_
 
+Slack channels
+^^^^^^^^^^^^^^
+
+Join `persist-queue <https://join.slack
+.com/t/persist-queue/shared_invite
+/enQtOTM0MDgzNTQ0MDg3LTNmN2IzYjQ1MDc0MDYzMjI4OGJmNmVkNWE3ZDBjYzg5MDc0OWUzZDJkYTkwODdkZmYwODdjNjUzMTk3MWExNDE>`_ channel
+
+
 Requirements
 ------------
 * Python 2.7 or Python 3.x.
@@ -193,6 +201,12 @@ The core functions:
 - ``ack``: mark item as acked
 - ``nack``: there might be something wrong with current consumer, so mark item as ready and new consumer will get it
 - ``ack_failed``: there might be something wrong during process, so just mark item as failed.
+- ``clear_acked_data``: perform a sql delete agaist sqlite, it remove the
+latest 1000 items whose status is ``AckStatus.acked`` (note: this does not
+shrink the file size on disk)
+- ``shrink_disk_usage`` perform a ``VACUUM`` against the sqlite, and rebuild
+ the database file, this usually takes long time and frees a lot of disk space
+ after ``clear_acked_data``
 
 .. code-block:: python
 
@@ -480,12 +494,6 @@ Contribution
 Simply fork this repo and send PR for your code change(also tests to cover your change), remember to give a title and description of your PR. I am willing to
 enhance this project with you :).
 
-Slack channels
-^^^^^^^^^^^^^^
-
-Join `persist-queue <https://join.slack
-.com/t/persist-queue/shared_invite
-/enQtOTM0MDgzNTQ0MDg3LTNmN2IzYjQ1MDc0MDYzMjI4OGJmNmVkNWE3ZDBjYzg5MDc0OWUzZDJkYTkwODdkZmYwODdjNjUzMTk3MWExNDE>`_ channel
 
 License
 -------
diff --git a/persistqueue/sqlackqueue.py b/persistqueue/sqlackqueue.py
index 45ea0f4..0e059c2 100644
--- a/persistqueue/sqlackqueue.py
+++ b/persistqueue/sqlackqueue.py
@@ -38,18 +38,18 @@ class SQLiteAckQueue(sqlbase.SQLiteBase):
                    '{key_column} INTEGER PRIMARY KEY AUTOINCREMENT, '
                    'data BLOB, timestamp FLOAT, status INTEGER)')
     # SQL to insert a record
-    _SQL_INSERT = 'INSERT INTO {table_name} (data, timestamp, status)'\
-        ' VALUES (?, ?, %s)' % AckStatus.inited
+    _SQL_INSERT = 'INSERT INTO {table_name} (data, timestamp, status)' \
+                  ' VALUES (?, ?, %s)' % AckStatus.inited
     # SQL to select a record
     _SQL_SELECT = ('SELECT {key_column}, data, status FROM {table_name} '
                    'WHERE status < %s '
                    'ORDER BY {key_column} ASC LIMIT 1' % AckStatus.unack)
-    _SQL_MARK_ACK_UPDATE = 'UPDATE {table_name} SET status = ?'\
-        ' WHERE {key_column} = ?'
-    _SQL_SELECT_WHERE = 'SELECT {key_column}, data FROM {table_name}'\
-        ' WHERE status < %s AND' \
-        ' {column} {op} ? ORDER BY {key_column} ASC'\
-        ' LIMIT 1 ' % AckStatus.unack
+    _SQL_MARK_ACK_UPDATE = 'UPDATE {table_name} SET status = ?' \
+                           ' WHERE {key_column} = ?'
+    _SQL_SELECT_WHERE = 'SELECT {key_column}, data FROM {table_name}' \
+                        ' WHERE status < %s AND' \
+                        ' {column} {op} ? ORDER BY {key_column} ASC' \
+                        ' LIMIT 1 ' % AckStatus.unack
 
     def __init__(self, path, auto_resume=True, **kwargs):
         super(SQLiteAckQueue, self).__init__(path, **kwargs)
@@ -65,9 +65,9 @@ class SQLiteAckQueue(sqlbase.SQLiteBase):
         unack_count = self.unack_count()
         if unack_count:
             log.warning("resume %d unack tasks", unack_count)
-        sql = 'UPDATE {} set status = ?'\
-            ' WHERE status = ?'.format(self._table_name)
-        return sql, (AckStatus.ready, AckStatus.unack, )
+        sql = 'UPDATE {} set status = ?' \
+              ' WHERE status = ?'.format(self._table_name)
+        return sql, (AckStatus.ready, AckStatus.unack,)
 
     def put(self, item):
         obj = self._serializer.dumps(item)
@@ -82,17 +82,17 @@ class SQLiteAckQueue(sqlbase.SQLiteBase):
         self.total = self._count()
 
     def _count(self):
-        sql = 'SELECT COUNT({}) FROM {}'\
-            ' WHERE status < ?'.format(self._key_column,
-                                       self._table_name)
+        sql = 'SELECT COUNT({}) FROM {}' \
+              ' WHERE status < ?'.format(self._key_column,
+                                         self._table_name)
         row = self._getter.execute(sql, (AckStatus.unack,)).fetchone()
         return row[0] if row else 0
 
     def _ack_count_via_status(self, status):
-        sql = 'SELECT COUNT({}) FROM {}'\
-            ' WHERE status = ?'.format(self._key_column,
-                                       self._table_name)
-        row = self._getter.execute(sql, (status, )).fetchone()
+        sql = 'SELECT COUNT({}) FROM {}' \
+              ' WHERE status = ?'.format(self._key_column,
+                                         self._table_name)
+        row = self._getter.execute(sql, (status,)).fetchone()
         return row[0] if row else 0
 
     def unack_count(self):
@@ -109,7 +109,7 @@ class SQLiteAckQueue(sqlbase.SQLiteBase):
 
     @sqlbase.with_conditional_transaction
     def _mark_ack_status(self, key, status):
-        return self._sql_mark_ack_status, (status, key, )
+        return self._sql_mark_ack_status, (status, key,)
 
     @sqlbase.with_conditional_transaction
     def clear_acked_data(self):
@@ -123,6 +123,11 @@ class SQLiteAckQueue(sqlbase.SQLiteBase):
                         max_acked_length=self._MAX_ACKED_LENGTH)
         return sql, AckStatus.acked
 
+    @sqlbase.with_conditional_transaction
+    def shrink_disk_usage(self):
+        sql = """VACUUM"""
+        return sql, ()
+
     @property
     def _sql_mark_ack_status(self):
         return self._SQL_MARK_ACK_UPDATE.format(table_name=self._table_name,

diff --git a/persistqueue/tests/test_sqlackqueue.py b/persistqueue/tests/test_sqlackqueue.py
index 2914eec..6d395db 100644
--- a/persistqueue/tests/test_sqlackqueue.py
+++ b/persistqueue/tests/test_sqlackqueue.py
@@ -212,6 +212,7 @@ class SQLite3AckQueueTest(unittest.TestCase):
         self.assertEqual(q.acked_count(), 100)
         q.clear_acked_data()
         self.assertEqual(q.acked_count(), 10)
+        q.shrink_disk_usage()
 
     def test_ack_unknown_item(self):
         q = SQLiteAckQueue(path=self.path)

ACK Queue: clear_acked_data() behavior
Playing around with the ```clear_acked_data()``` function, it seems to hang on to the last 1000 acked queue items. Why is that? I've already acked the data, yet disk space continued to be used.

Looking at the code in question:
```python
    @sqlbase.with_conditional_transaction
    def clear_acked_data(self):
        sql = """DELETE FROM {table_name}
            WHERE {key_column} IN (
                SELECT _id FROM {table_name} WHERE status = ?
                ORDER BY {key_column} DESC
                LIMIT 1000 OFFSET {max_acked_length}
            )""".format(table_name=self._table_name,
                        key_column=self._key_column,
                        max_acked_length=self._MAX_ACKED_LENGTH)
        return sql, AckStatus.acked
```

It seems that self._MAX_ACKED_LENGTH is a private member constant. Can this not be made tunable by the user (e.g.. a kwarg in __init__ for the class)? 

I opened my resulting sqlite data files and manually ran:
```sql
DELETE FROM ack_queue_default WHERE status = 5;
VACUUM;
```

Which reduced the file size by several GB. Unless there is some edge case, surely you'd want to do something more like this?

```python
    @sqlbase.with_conditional_transaction
    def clear_acked_data(self):
        sql = """DELETE FROM {table_name} WHERE status = ?""".format(table_name=self._table_name)
        return sql, AckStatus.acked

    @sqlbase.with_conditional_transaction
    def shrink_disk_usage(self):
        sql = """VACUUM"""
        return sql, None
```
persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_ack_and_clear persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_ack_and_clear
persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_ack_unack_ack_failed persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_ack_unknown_item persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_empty persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_multi_threaded_multi_producer persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_multi_threaded_parallel persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_multiple_consumers persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_open_close_1000 persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_open_close_single persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_protocol_1 persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_protocol_2 persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_put_0 persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_raise_empty persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_random_read_write persistqueue/tests/test_sqlackqueue.py::SQLite3AckQueueTest::test_resume_unack persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_ack_unack_ack_failed persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_ack_unknown_item persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_empty persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_protocol_1 persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_put_0 persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_raise_empty persistqueue/tests/test_sqlackqueue.py::SQLite3QueueInMemory::test_random_read_write persistqueue/tests/test_sqlackqueue.py::FILOSQLite3AckQueueTest::test_open_close_1000 persistqueue/tests/test_sqlackqueue.py::SQLite3UniqueAckQueueTest::test_add_duplicate_item