Database applications often insert new rows in the ascending order of the primary key. In this case, due to the layout of the clustered index in the same order as the primary key, insertions into an InnoDB table do not require random reads from a disk.
On the other hand, secondary indexes are usually nonunique, and
insertions into secondary indexes happen in a relatively random
order. In the same way, deletes and updates can affect data
pages that are not adjacent in secondary indexes. This would
cause a lot of random disk I/O operations without a special
mechanism used in
When an index record is inserted, marked for deletion, or
deleted from a nonunique secondary index,
InnoDB checks whether the secondary index
page is in the buffer
pool. If that is the case,
applies the change directly to the index page. If the index page
is not found in the buffer pool,
records the change in a special structure known as the
insert buffer. The
insert buffer is kept small so that it fits entirely in the
buffer pool, and changes can be applied very quickly. This
process is known as change
buffering. (Formerly, it applied only to inserts and was
called insert buffering. The data structure is still called the
Periodically, the insert buffer is merged into the secondary index trees in the database. Often, it is possible to merge several changes into the same page of the index tree, saving disk I/O operations. It has been measured that the insert buffer can speed up insertions into a table up to 15 times.
The insert buffer merging may continue to happen after the transaction has been committed. In fact, it may continue to happen after a server shutdown and restart (see Section 14.19.2, “Forcing InnoDB Recovery”).
Insert buffer merging may take many hours when many secondary indexes must be updated and many rows have been inserted. During this time, disk I/O will be increased, which can cause significant slowdown on disk-bound queries. Another significant background I/O operation is the purge thread (see Section 14.2.12, “InnoDB Multi-Versioning”).