MySQL Online DDL Index Creation

I have a table with 25 million records and 10 columns on which I am trying to create indexes. According to the following documentation no table copy should occur but when I monitor disk usage it is showing that MySQL has high disk usage and data directory is increasing in size. Can anyone explain why this is happening.

http://dev.mysql.com/doc/refman/5.7/en/innodb-create-index-overview.html#innodb-online-ddl-summary-grid

This is working as intended. The indexes are objects written to the disk. The index creation is also written to the online logs. Creating indexes is expected to take up disk space and cause I/O operations.

StackExchange Database Administrators Q#141771, answer score: 2