Metadata updates

Metadata is data about data, or all the overhead and attributes involved in storing the user data itself, such as the name of a file, the physical blocks it uses, modification and access timestamps, and so on.

The most expensive operation of a filesystem is in updating the metadata. This is because:

Almost all operations on the filesystem (even reading file data, unless you've specified the noatime option—see in the Utilities Reference) involve some metadata updates.