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DOCS-8471: impact of indexes on write performance #2721

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DOCS-8471: impact of indexes on write performance #2721

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DOCS-8471: impact of indexes on write performance
steveren Aug 23, 2016
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DOCS-8471: impact of indexes on write performance
steveren Aug 23, 2016
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Merge branch 'DOCS-8471' of github.com:steveren/docs into DOCS-8471
steveren Sep 8, 2016
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32 changes: 23 additions & 9 deletions source/core/write-performance.txt
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Expand Up @@ -13,10 +13,29 @@ Write Operation Performance
Indexes
-------

After every insert, update, or delete operation, MongoDB must update
*every* index associated with the collection in addition to the data
itself. Therefore, every index on a collection adds some amount of
overhead for the performance of write operations. [#exceptions]_
Each index on a collection adds some amount of overhead to the
performance of write operations.

For each :dbcommand:`insert` or :dbcommand:`delete` write operation
on a collection, MongoDB either inserts or removes the corresponding
document keys from each index in the target collection. An
:dbcommand:`update` operation may result in updates to a subset of
indexes on the collection, depending on the keys affected by the update.
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Does the sparse/partial index blurb below need to be moved up here? Because if a collection only has partial index and the document being inserted/removed doesn't hit that index, that wouldn't be affected, yes?


.. note::

MongoDB only updates a :ref:`sparse <index-type-sparse>` or
:ref:`partial <index-type-partial>` index if the documents involved
in a write operation are included in the index.

On :program:`mongod` instances that use the
:ref:`MMAPv1 <storage-mmapv1>` storage engine, update operations may
cause a document to grow beyond its allocated space.
When a document outgrows its allocated space, MMAPv1 moves
the document to a new location on disk, and must
update each index on the collection to point to the new document
location. These move operations can be expensive but occur
infrequently.

In general, the performance gains that indexes provide for *read
operations* are worth the insertion penalty. However, in order to
Expand All @@ -28,11 +47,6 @@ For indexes and queries, see :doc:`/core/query-optimization`. For more
information on indexes, see :doc:`/indexes` and
:doc:`/applications/indexes`.

.. [#exceptions] For inserts and updates to un-indexed fields, the
overhead for :ref:`sparse indexes <index-type-sparse>` is less than
for non-sparse indexes. Also for non-sparse indexes, updates that
do not change the record size have less indexing overhead.

.. _write-performance-and-document-growth:

Document Growth and the MMAPv1 Storage Engine
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