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.. _compact:
=======
compact
=======
.. default-domain:: mongodb
.. contents:: On this page
:local:
:backlinks: none
:depth: 1
:class: singlecol
Definition
----------
.. dbcommand:: compact
Rewrites and defragments all data and indexes in a collection. On
:ref:`WiredTiger <storage-wiredtiger>` databases, this command will release
unneeded disk space to the operating system.
:dbcommand:`compact` has the following form:
.. code-block:: javascript
{ compact: <collection name> }
:dbcommand:`compact` takes the following fields:
.. include:: /includes/apiargs/dbcommand-compact-field.rst
.. warning:: Always have an up-to-date backup before performing server
maintenance such as the :dbcommand:`compact` operation.
.. _compact-paddingFactor:
paddingFactor
~~~~~~~~~~~~~
.. note::
Applicable for the MMAPv1 storage engine only; specifying
``paddingFactor`` has no effect when used with the WiredTiger
storage engine.
The ``paddingFactor`` field takes the following range of values:
- Default: ``1.0``
- Minimum: ``1.0`` (no padding)
- Maximum: ``4.0``
If your updates increase the size of the documents, padding will
increase the amount of space allocated to each document and avoid
expensive document relocation operations within the data files.
You can calculate the padding size by subtracting the document size from
the record size or, in terms of the ``paddingFactor``, by subtracting
``1`` from the ``paddingFactor``:
.. code-block:: none
padding size = (paddingFactor - 1) * <document size>.
For example, a ``paddingFactor`` of ``1.0`` specifies a padding size of
``0`` whereas a ``paddingFactor`` of ``1.2`` specifies a padding size of
``0.2`` or 20 percent (20%) of the document size.
With the following command, you can use the ``paddingFactor`` option of
the :dbcommand:`compact` command to set the record size to ``1.1`` of
the document size, or a padding factor of 10 percent (10%):
.. code-block:: javascript
db.runCommand ( { compact: '<collection>', paddingFactor: 1.1 } )
:dbcommand:`compact` modifies existing documents, but does not set the padding
factor for future documents.
.. _compact-paddingBytes:
paddingBytes
~~~~~~~~~~~~
.. note::
Applicable for the MMAPv1 storage engine only; specifying
``paddingBytes`` has no effect when used with the WiredTiger storage
engine.
Specifying ``paddingBytes`` can be useful if your documents start small
but then increase in size significantly.
For example, if your documents
are initially 40 bytes long and you grow them by 1 kB, using
``paddingBytes: 1024`` might be reasonable since using ``paddingFactor:
4.0`` would specify a record size of 160 bytes (``4.0`` times the
initial document size), which would only provide a padding of 120 bytes
(i.e. record size of 160 bytes minus the document size).
The following command uses the ``paddingBytes`` option to set the padding size
to 100 bytes on the collection named by ``<collection>``:
.. code-block:: javascript
db.runCommand ( { compact: '<collection>', paddingBytes: 100 } )
Behavior
--------
Blocking
~~~~~~~~
:dbcommand:`compact` only blocks operations for the database it is currently
operating on. Only use :dbcommand:`compact` during scheduled maintenance
periods.
You may view the intermediate progress either by viewing the
:program:`mongod` log file or by running the :method:`db.currentOp()`
in another shell instance.
Operation Termination
~~~~~~~~~~~~~~~~~~~~~
If you terminate the operation with the :method:`db.killOp()
<db.killOp()>` method or restart the server before the
:dbcommand:`compact` operation has finished, be aware of the following:
- If you have journaling enabled, the data remains valid and
usable, regardless of the state of the :dbcommand:`compact` operation.
You may have to manually rebuild the indexes.
- If you do not have journaling enabled and the :program:`mongod` or
:dbcommand:`compact` terminates during the operation, it is impossible
to guarantee that the data is in a valid state.
- In either case, much of the existing free space in the collection may
become un-reusable. In this scenario, you should rerun the compaction
to completion to restore the use of this free space.
Disk Space
~~~~~~~~~~
:dbcommand:`compact` has different impacts on available disk space depending on
which storage engine is in use.
To see how the storage space changes for the collection, run the
:dbcommand:`collStats` command before and after compaction.
WiredTiger
``````````
On :ref:`WiredTiger <storage-wiredtiger>`, :dbcommand:`compact` will rewrite
the collection and indexes to minimize disk space by releasing unused disk space
to the system. This is useful if you have removed a large amount of data
from the collection, and do not plan to replace it.
MMAPv1
``````
On :ref:`MMAPv1 <storage-mmapv1>`, :dbcommand:`compact` defragments the
collection's data files and recreates its indexes. Unused disk space is *not*
released to the system, but instead retained for future data. If you wish to
reclaim disk space from a MMAPv1 database, you should perform an
:term:`initial sync`.
:dbcommand:`compact` requires up to 2 gigabytes of additional disk space to run
on MMAPv1 databases.
Size and Number of Data Files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:dbcommand:`compact` may increase the total size and number of your data
files, especially when run for the first time. However, this will not
increase the total collection storage space since storage size is the
amount of data allocated within the database files, and not the
size/number of the files on the file system.
Replica Sets
~~~~~~~~~~~~
:dbcommand:`compact` commands do not replicate to secondaries in a
:term:`replica set`.
- Compact each member separately.
- Ideally run :dbcommand:`compact` on a secondary. See option
``force:true`` above for information regarding compacting the primary.
.. include:: /includes/extracts/fact-command-puts-secondary-into-recovering-compact.rst
Sharded Clusters
~~~~~~~~~~~~~~~~
:dbcommand:`compact` only applies to :program:`mongod` instances. In a
sharded environment, run :dbcommand:`compact` on each shard separately
as a maintenance operation.
You cannot issue :dbcommand:`compact` against a :program:`mongos` instance.
Capped Collections
~~~~~~~~~~~~~~~~~~
It is not possible or necessary to compact :term:`capped collections <capped
collection>` because they lack padding and their documents cannot grow.
As a result, they cannot become fragmented.
Index Building
~~~~~~~~~~~~~~
.. versionadded:: 2.6
:program:`mongod` rebuilds all indexes in parallel following the
:dbcommand:`compact` operation.
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