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FAQ: MongoDB Storage
.. default-domain:: mongodb
.. contents:: On this page
:backlinks: none
:depth: 1
:class: singlecol
This document addresses common questions regarding MongoDB's storage
Storage Engine Fundamentals
What is a storage engine?
A storage engine is the part of a database that is responsible for
managing how data is stored, both in memory and on disk. Many databases
support multiple storage engines, where different engines perform better
for specific workloads. For example, one storage engine might offer
better performance for read-heavy workloads, and another might support a
higher throughput for write operations.
.. seealso:: :doc:`/core/storage-engines`
Can you mix storage engines in a replica set?
Yes. You can have replica set members that use different storage
When designing these multi-storage engine deployments, consider the
- the oplog on each member may need to be sized differently to account
for differences in throughput between different storage engines.
- recovery from backups may become more complex if your backup
captures data files from MongoDB: you may need to maintain backups
for each storage engine.
WiredTiger Storage Engine
Can I upgrade an existing deployment to WiredTiger?
Yes. See:
- :doc:`/tutorial/change-standalone-wiredtiger`
- :doc:`/tutorial/change-replica-set-wiredtiger`
- :doc:`/tutorial/change-sharded-cluster-wiredtiger`
How much compression does WiredTiger provide?
The ratio of compressed data to uncompressed data depends on your data
and the compression library used. By default, collection data in
WiredTiger use :term:`Snappy block compression <snappy>`; :term:`zlib`
compression is also available. Index data use :term:`prefix
compression` by default.
.. _wt-cache-and-eviction:
To what size should I set the WiredTiger internal cache?
.. include:: /includes/extracts/wt-configure-cache.rst
How frequently does WiredTiger write to disk?
.. include:: /includes/extracts/wt-snapshot-frequency.rst
For journal data, MongoDB writes to disk according to the following
intervals or condition:
.. include:: /includes/extracts/wt-journal-frequency.rst
.. _faq-wt-reclaim-disk-space:
How do I reclaim disk space in WiredTiger?
The WiredTiger storage engine maintains lists of empty records in data
files as it deletes documents. This space can be reused by WiredTiger,
but will not be returned to the operating system unless under very
specific circumstances.
The amount of empty space available for reuse by WiredTiger is reflected
in the output of :method:`db.collection.stats()` under the heading
``wiredTiger.block-manager.file bytes available for reuse``.
To allow the WiredTiger storage engine to release this empty space to the
operating system, you can de-fragment your data file. This can be achieved
using the :dbcommand:`compact` command. For more information on its behavior
and other considerations, see :dbcommand:`compact`.
MMAPv1 Storage Engine
.. _faq-storage-memory-mapped-files:
What are memory mapped files?
A memory-mapped file is a file with data that the operating system
places in memory by way of the ``mmap()`` system call. ``mmap()`` thus
*maps* the file to a region of virtual memory. Memory-mapped files are
the critical piece of the MMAPv1 storage engine in MongoDB. By using memory
mapped files, MongoDB can treat the contents of its data files as if
they were in memory. This provides MongoDB with an extremely fast and
simple method for accessing and manipulating data.
How do memory mapped files work?
MongoDB uses memory mapped files for managing and interacting with all
Memory mapping assigns files to a block of virtual memory with a direct
byte-for-byte correlation. MongoDB memory maps data files to memory as
it accesses documents. Unaccessed data is *not* mapped to memory.
Once mapped, the relationship between file and memory allows MongoDB to
interact with the data in the file as if it were memory.
How frequently does MMAPv1 write to disk?
.. include:: /includes/fact-mmapv1-write-to-disk.rst
.. _faq-disk-size:
Why are the files in my data directory larger than the data in my database?
The data files in your data directory, which is the :file:`/data/db`
directory in default configurations, might be larger than the data set
inserted into the database. Consider the following possible causes:
Preallocated data files
MongoDB preallocates its data files to avoid filesystem fragmentation, and
because of this, the size of these files do not necessarily reflect the size of
your data.
The :setting:`storage.mmapv1.smallFiles` option will reduce the
size of these files, which may be useful if you have many small databases on
The ``oplog``
If this :program:`mongod` is a member of a replica set, the data
directory includes the :term:` <oplog>` file, which is a
preallocated :term:`capped collection` in the ``local``
The default allocation is approximately 5% of disk space
on 64-bit installations. In most cases, you should not need to resize the oplog.
See :ref:`Oplog Sizing <replica-set-oplog-sizing>` for more information.
The ``journal``
The data directory contains the journal files, which store write
operations on disk before MongoDB applies them to databases. See
.. _faq-empty-records:
Empty records
The MMAPv1 storage engine maintains lists of empty records in data
files as it deletes documents and collections. This space can be
reused for new record allocations within the same database, but MMAPv1
will not, by default, return this space to the operating system.
To allow the MMAPv1 storage engine to more effectively reuse space
from empty records, you can de-fragment your data. To de-fragment,
use the :dbcommand:`compact` command. The :dbcommand:`compact`
requires up to 2 gigabytes of extra disk space to run. Do not use
:dbcommand:`compact` if you are critically low on disk space. For
more information on its behavior and other considerations, see
:dbcommand:`compact` only removes fragmentation from MongoDB data files
within a collection and does not return any disk space to the operating
system. To return disk space to the operating system, see
.. _faq-reclaim-disk-space:
How do I reclaim disk space?
.. note::
You do not need to reclaim disk space for MongoDB to reuse freed
space. See :ref:`faq-empty-records` for information on reuse of
freed space.
For a secondary member of a replica set, you can perform a :doc:`resync
of the member </tutorial/resync-replica-set-member>` by stopping the
secondary member to resync, deleting all data and subdirectories from
the member's data directory, and restarting the secondary member. For
details, see :doc:`/tutorial/resync-replica-set-member`.
Dropping an unused database via :dbcommand:`dropDatabase` will also
delete the associated data files and free up disk space.
.. _faq-working-set:
What is the working set?
Working set represents the total body of data that the application
uses in the course of normal operation. Often this is a subset of the
total data size, but the specific size of the working set depends on
actual moment-to-moment use of the database.
If you run a query that requires MongoDB to scan every document in a
collection, the working set will expand to include every
document. Depending on physical memory size, this may cause documents
in the working set to "page out," or to be removed from physical memory by
the operating system. The next time MongoDB needs to access these
documents, MongoDB may incur a hard page fault.
For best performance, the majority of your *active* set should fit in
.. _faq-storage-page-faults:
What are page faults?
.. include:: /includes/fact-page-fault.rst
If there is free memory, then the operating system can find the page
on disk and load it to memory directly. However, if there is no free
memory, the operating system must:
- find a page in memory that is stale or no longer needed, and write
the page to disk.
- read the requested page from disk and load it into memory.
This process, on an active system, can take a long time,
particularly in comparison to reading a page that is already in
See :ref:`administration-monitoring-page-faults` for more information.
What is the difference between soft and hard page faults?
:term:`Page faults <page fault>` occur when MongoDB, with the MMAP
storage engine, needs access to data that isn't currently in active
memory. A "hard" page fault refers to situations when MongoDB must
access a disk to access the data. A "soft" page fault, by contrast,
merely moves memory pages from one list to another, such as from an
operating system file cache.
See :ref:`administration-monitoring-page-faults` for more information.
.. TODO consider removing manual padding question in v3.4
.. _faq-developers-manual-padding:
Can I manually pad documents to prevent moves during updates?
.. versionchanged:: 3.0.0
With the :doc:`MMAPv1 storage engine </core/mmapv1>`, an update can
cause a document to move on disk if the document grows in size. To
*minimize* document movements, MongoDB uses :term:`padding`.
You should not have to pad manually because by default, MongoDB uses
:ref:`power-of-2-allocation` to add :ref:`padding automatically
<record-allocation-strategies>`. The :ref:`power-of-2-allocation`
ensures that MongoDB allocates document space in sizes that are powers
of 2, which helps ensure that MongoDB can efficiently reuse free space
created by document deletion or relocation as well as reduce the
occurrences of reallocations in many cases.
However, *if you must* pad a document manually, you can add a
temporary field to the document and then :update:`$unset` the field,
as in the following example.
.. warning:: Do not manually pad documents in a capped
collection. Applying manual padding to a document in a capped
collection can break replication. Also, the padding is not
preserved if you re-sync the MongoDB instance.
.. code-block:: javascript
var myTempPadding = [ "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
db.myCollection.insert( { _id: 5, paddingField: myTempPadding } );
db.myCollection.update( { _id: 5 },
{ $unset: { paddingField: "" } }
db.myCollection.update( { _id: 5 },
{ $set: { realField: "Some text that I might have needed padding for" } }
.. seealso::
Data Storage Diagnostics
How can I check the size of a collection?
To view the statistics for a collection, including the data size, use
the :method:`db.collection.stats()` method from the :program:`mongo`
shell. The following example issues :method:`db.collection.stats()` for
the ``orders`` collection:
.. code-block:: javascript
MongoDB also provides the following methods to return specific sizes
for the collection:
- :method:`db.collection.dataSize()` to return data size in bytes for
the collection.
- :method:`db.collection.storageSize()` to return allocation size in
bytes, including unused space.
- :method:`db.collection.totalSize()` to return the data size plus the
index size in bytes.
- :method:`db.collection.totalIndexSize()` to return the index size in
The following script prints the statistics for each database:
.. code-block:: javascript
db.adminCommand("listDatabases").databases.forEach(function (d) {
mdb = db.getSiblingDB(;
The following script prints the statistics for each collection in each
.. code-block:: javascript
db.adminCommand("listDatabases").databases.forEach(function (d) {
mdb = db.getSiblingDB(;
mdb.getCollectionNames().forEach(function(c) {
s = mdb[c].stats();
How can I check the size of indexes for a collection?
To view the size of the data allocated for an index, use the
:method:`db.collection.stats()` method and check the
:data:`~collStats.indexSizes` field in the returned document.
.. _faq-tools-for-measuring-storage-use:
How can I get information on the storage use of a database?
The :method:`db.stats()` method in the :program:`mongo` shell returns
the current state of the "active" database. For the description of the
returned fields, see :ref:`dbStats Output <dbstats-output>`.